Reality Conditions

One Year Later, the Belated End

2009-01-19T18:14:00.008Z

I think that when exactly one year passes without updating one's blog, it is a perfect moment to declare it officially dead. Of course, unoficially it has been dead for a long time, and probably none of my old readers is still checking this; but it is better anyway to have a proper closure, if not for anything else, at least to avoid random Google searchers thinking that the main focus of the blog was chess. So hereby I declare Reality Conditions officially defunct.

As a last bit of retrospective navel-gazing, and to ease the work of future searching and referencing, I am including here an index of my favourite posts.

Physics and Philosophy of Physics:

- Relational Quantum Mechanics (and the follow-up Taking back my words)

- Landscape chat

- On Price on the Arrow of Time (and the follow-up On Price (and Penrose) on Time Asymmetry in Quantum Mechanics)

- On not taking a stance

- Book Review: Paul Davies, Cosmic Jackpot

- Lagrangians, Hamiltonians, and Scientific Realism

- Report on the Quantum Gravity School: the discussion

- Quantum Mechanics in words of one syllable

- Loops 07: Conference report (part 3, including discussion session)

- Quantum Gravity Colloquium: the discussion. (QM vs. QG: the Grudge Match!)

Philosophy, Religion and Philosophy of Religion:

- Book Review: Thomas Nagel, The Last Word

- Are Evolution and Theism Compatible? (and the follow-up More on the compatibility of evolution and theism: Reply to Pruss)

- Chalmers, Dennett, and the Zombies

- Atheism, Religion, and Rationality; or, do you think that all those who believe in God are stupid?

- An examination of Dawkins’ “Ultimate 747” argument

Fun stuff:

- Weirdest Google Search leading someone to this blog (so far)

- What people search for here

- Mathematicians and computers

- Book review: Neal Stephenson, The Baroque Cycle

- Upstuff

In Memoriam Bobby Fischer

2008-01-19T16:40:00.000Z

It is sad that he has died unreconciled with the world and with sanity. But, hopefully, the future will remember him not for his madness but for his greatness. My favorite example of it is his famous "Brilliancy Prize" game against Robert Byrne in 1963. As a fitting homage, let us go over it:

Robert Byrne — Robert Fischer
1963 — 1964 US Championship

1. d4 Nf6 2. c4 g6 3. g3 c6 4. Bg2 d5 5. cxd5 cxd5 6. Nc3 Bg7 7. e3 O-O 8. Nge2 Nc6 9. O-O b6 10. b3 Ba6 11. Ba3 Re8 12. Qd2

The position looks almost symmetrical -who would have thought that White would be forced to resign after just ten moves and having made no obvious blunder?

12... e5! 13. dxe5 Nxe5 14. Rfd1 Nd3 15. Qc2 Nxf2!! 16. Kxf2 Ng4+ 17. Kg1 Nxe3 18. Qd2

Now everyone, Byrne included, was expecting 18... Nxd1, which leads to a favorable position for White. But Fischer pulled out of his hat the unexpected:

18... Nxg2!! 19. Kxg2 d4! 20. Nxd4 Bb7+ 21. Kf1 Qd7

And White resigned, to the surprise of most commentators and spectators who were still ranking his position as better. The white king is trapped in a mating net made of subtle, almost invisible threads, but from which there is no escape. For example: 22. Qf2 Qh3+ 23. Kg1 Re1+! 24.Rxe1 Bxd4 and mate in g2. As K. F. Kirby famously said, the progression from a seemingly equal position to this debacle seems "more witchcraft than chess".

You can play the whole game and read comments at Chess Games.

Finally Updating (Final Updating?)

2007-12-21T23:04:00.000Z

You were probably wondering why I wasn't posting anything. Or more probably, you realized that I was busy with the business of getting the thesis finished. Or even more probably, you didn't wonder at all. Whatever. This post is the belated Update On My Life:

I not only finished the thesis (complete with quote and all) but also submitted it yesterday! Hooray! Now on to packing, because...
I am leaving in a couple of days for my usual Christmas/New Year Argentinian holiday, and returning to Nottingham on January 11th. But what will I do here if I finished my thesis already? Well, I will be busy enough...
Arranging the details for my moving away from Nottingham, which will happen on the 31st; those will be three stressful weeks for sure, until I finally leave for...
Marseille! I got an ESF grant for a 6-month research visit to Carlo Rovelli's group at Luminy. The idea is to collaborate with research done on the new spin foam models that have appeared this year, especially in studying their semiclassical limit. This will be my first foray into actual quantum gravity research (from my usual quantum field theory in curved space), and I'm expecting to learn a lot and also hopefully come outwith a better-formed judgement on the value of the "LQG approach". After this, in August, I will come back to Nottingham for my viva. And after that, nobody knows yet...

So those are the news on my life. Now the news on the blog. I will almost certainly not write anything during my holidays, as I barely have time for computer using between meeting everybody and stuff. Later in January I will probably be too busy and/or stressed with the moving to be in a mood for writing, and I also expect to be quite busy my first days in Marseille. So this blog is, quite likely, coming to a halt until mid-February or so. We'll see then if I feel like going on or not.

Don't be too surprised or upset. You have surely noticed that the frequency of posts has declined a lot in the last months, reflecting not merely being busy with the thesis but also being a bit tired of the whole blogging business. On the other hand, I am sure that several of my readers will want to know what's going on in Marseille, and it is possible my enthusiasm will return and they'll get the reports on seminars and discussions, along with the usual philosophical disquisitions, book reviews, and the other staples of this blog. In any case, if I take the decision of closing the blog for good, I will make one last post to let you know, I promise. So for now, au revoir!

Searching for a Thesis Quote

2007-11-26T18:39:00.000Z

My PhD thesis is almost finished and hopefully I will submit it within a few weeks. The most important thing lacking, at the moment, is a suitable quote for the beginning. I hereby enlist the help of my readers for suggestions!

I guess I should say something about the topic of the thesis and what kind of quote I am looking for. The topic of the thesis is particle detectors in quantum field theory. I have given a nontechnical explanation in this old post, but if you don't care to go and read it, enough to say that it is about the possibility of defining the "particle content" of quantum fields operationally, by the energy transitions an interaction with the quantum field can produce on another quantum system such as an atom. If an atom interacting with a field gets excited, you can say that it has absorbed a field quanta or "particle". This is important because in a curved spacetime context there are usually no other "intrinsic" definitions of particles available. My work concerns more precisely the question of giving a rigorous definition of the transition rate of such a detector, which is not as simple as it sounds. You can read the details in my last paper.

For the beginning of the thesis, I do not want a prosaic quote from a physicist about these matters. My ideal would be a poetic, literary or philosophical quote that could, with an effort, be read as alluding to this topic (even though this was obviously not intended). As an example of the kind of thing I like, my undergraduate thesis concerned calculation of vacuum energy of quantum fields in a class of spacetimes. As the reality of quantum field vacuum energy means that there is no real vacuum in Nature, that anything that looks empty really has a "zero-point energy", I used a quotation from Parmenides, the Pre-Socratic philosopher that based his philosophy on denying the reality of Not-Being:

That things which are not are, shall never prevail, she said,
but do thou restrain thy mind from this course of investigation.
And let not long-practised habit compel thee along this path,
thine eye careless, thine ear and thy tongue overpowered by noise;
but do thou weigh the much contested refutation of their words,
which I have uttered.

The Spanish translation I used is much more poetical, for those that can read it:

Pues nunca dominará esto: que haya no ser. Aleja tú
el pensamiento de este camino de investigación,
y que la inveterada costumbre no te obligue, a lo largo
de este camino, a utilizar el ojo que no ve, el oído que
resuena, y la lengua; juzga con la razón la combativa
refutación que te he enunciado.

Besides the rejection of Not-Being, the quote was appropiate for talking about "investigación", which in Spanish means "research" besides "investigation", and is used everyday in scientific context. Also, the rejection of the senses in favour of reason can be seen (with a slant) as endorsing theoretical physics over experimental.

So this is the kind of thing I would like. At the moment, my best candidate is the following quote from Bertrand Russell's An Outline of Philosophy:

'Matter' is a convenient formula for describing what happens where it isn't. I am talking physics, not metaphysics; when we come to metaphysics, we may be able, tentatively, to add something to this statement, but science alone can hardly add to it.

Reasons why this quote is appropriate are that (although he was not exactly talking about the same thing) Russell seems to be endorsing the operational definition of particles my thesis is about; that the ending of the quote looks ironical as a preface to a hundred pages "adding to it" from a scientific point of view; and that An Outline of Philosophy is a very dear book to me, being the first real philosophy book I read. Rereading it recently I found it full of things I could not accept, either scientifically outdated or philosophically unsound; but its general spirit of approaching philosophy in a way closely related to and interwoven with science is one that I still admire. Reason counting against this quote is that it is a bit too prosaic; I would like something more dramatical and unexpected. Russell is so well-known in the scientific community that quoting him is only slightly less predictable than quoting Einstein or Feynman. But still for the moment this is the best I have found.

Any suggestions...?

Math Jokes

2007-11-18T17:20:00.000Z

Via the a link in a Quantum Pontiff thread, I found an excellent collection of mathematical humour, which (amazingly) includes many jokes I had never seen or heard before, and many of them good ones! (For highly nerdy values of "good".) For example, the hyperbolas joke made me laugh out loud:

Two hyperbolas were sitting on a plane.

The first hyperbola says to the other "I sure wish I could oscillate."

The second one replies, "Holy crap! A talking hyperbola!"

Fooled you there, didn't I? I bet you were expecting some atrocious mathematical pun instead of a variation of the Great Muffin Joke. Me too, and that's why I laughed.

On a more conventional note, the site includes great lists of "...walks into a bar" jokes, of dubious proof methods (also here), and my favourite math joke ever:

The cocky exponential function e^x is strolling along the road insulting the functions he sees walking by. He scoffs at a wandering polynomial for the shortness of its Taylor series. He snickers at a passing smooth function of compact support and its glaring lack of a convergent power series about many of its points. He positively laughs as he passes x for being nondifferentiable at the origin. He smiles, thinking to himself, "Damn, it's great to be e^x. I'm real analytic everywhere. I'm my own derivative. I blow up faster than anybody and shrink faster too. All the other functions suck."

Lost in his own egomania, he collides with the constant function 3, who is running in terror in the opposite direction.

"What's wrong with you? Why don't you look where you're going?" demands e^x. He then sees the fear in 3's eyes and says "You look terrified!"

"I am!" says the panicky 3. "There's a differential operator just around the corner. If he differentiates me, I'll be reduced to nothing! I've got to get away!" With that, 3 continues to dash off.

"Stupid constant," thinks e^x. "I've got nothing to fear from a differential operator. He can keep differentiating me as long as he wants, and I'll still be there."

So he scouts off to find the operator and gloat in his smooth glory. He rounds the corner and defiantly introduces himself to the operator. "Hi. I'm e^x."

"Hi. I'm d / dy."

The Ruin of the Roman Republic: Recent Reading & Reviewing

2007-11-12T00:15:00.000Z

This is a post on a subject close to my heart that I’ve wanted to write about for a long time. Now, with my working hours full with agonizing over thesis rewritings, postdoc applications and spin foam studying, I have finally wrote it in a semi-cathartic way. I hope some of my loyal readers are at least half-interested in the subject; otherwise, pass over it as a narcissistic exercise on self-indulgence (which is after all the whole point of blogging, isn’t it?)

In the last few months I’ve read no less than three books, one nonfiction and two fiction, about my favourite historical period, the late Roman Republic: Tom Holland’s Rubicon, Colleen McCullough’s Antony and Cleopatra, and Robert Harris’s Imperium. I can find endless fascination in reading versions and perspectives on Roman history between years (say) 100 and 30 B.C. And I am not the only one: it is one of the time periods most visited by historical fiction and film (and television; I am still to finish watching the excellent recent Rome series). I think there are two intertwined reasons that explain this fascination:

1) It is a world very ancient and different from our own, but also surprisingly modern in many respect –and surely the closest thing to the modern world that existed before, say, the seventeenth century at least. There were large-scale democratic politics, a complex government system involving checks and balances between different kinds of magistrates, heated electoral campaigns, political opposition between conservatives and progressives, vast and complex financial enterprises that enriched a few and impoverished many, and an intricate legal system of justice that combined sensible and fair principles with an often corrupt practice. These very recognizable features combine with others that seem alien to us, such as the huge importance of a military career as a way to fame, riches and political power, the almost quotidian occurrence of massive warfare (civil or not), the horrors of the slave economy system, the normality of gladiator fights as entertainment, and the enshrining of superstitions like reading the future by augurs as part of the political constitution. Moreover, it is a period in which the contradictions and tensions within the system become greater and greater and ultimately unsustainable, leading to the collapse of the Republic and the emergence of the Empire, which looks rather less “modern” and more akin to other ancient-world civilizations. One could say that the Roman Republic was an early and clumsy attempt by the secret gods that write human history of creating a modern world, that failed because many of the crucial ingredients were misplaced or omitted altogether.

2) It is an extremely well-documented period, with lots of surviving primary sources, some of them written by the main protagonists themselves (Cicero, Caesar). The leaders of this historical drama are known with enough detail that they come out as fully-fleshed human beings with complex personalities; we know about them not only the battles they fought and the laws they passed but also lots of little juicy titbits and anecdotes. It is an era that produces an unusually large number of “Great Men”: Marius, Sulla, Pompey, Crassus, Cicero, Caesar, Clodius, Cato the Younger, Brutus, Mark Antony, Octavian/Augustus … Each of them has a recognizable personality and a particular kind of achievement in which he reigns supreme; e.g. for Cicero rhetoric, for Crassus moneymaking, for Clodius demagoguery, for Octavian political craftiness (his portrayal as an trusting fool in I, Claudius notwithstanding) and for Cato either integrity and incorruptibility or stubborn, dogmatic close-mindedness, according to the leanings of the writer. There are also several “Great Women” in the background: Servilia, Clodia, Livia, and of course that quintessential femme fatal Cleopatra.

Before moving on to the reviews I can’t resist sharing my favourite anecdote about this period, to give you a taste of the treats you can find reading about this period. It comes from Plutarch’s Life of Cato and Life of Brutus. The date is December 5th, 63 B.C. The consul Cicero has discovered incriminating evidence against several important senators and noblemen who have been plotting with Catiline to take over the government; Catiline himself has left the city some time ago, hounded out by Cicero’s powerful rhetoric, and is now an enemy of the state. There is a meeting of the Senate to decide what to do with the captured conspirators. Cicero, the consul elect for next year Silanus, and most other senators speak in favour of the immediate death penalty. Gaius Julius Caesar (at this time only an up-and-coming politician, who has a long way to go before conquering the Gauls, defeating Pompey in a civil war, and becoming master of Rome) is the only one against it: he reminds everyone that it is against the law to put to death Roman citizens without a trial, and of how terrible a precedent it would be to violate this sacred principle, even with patently guilty men. Many are persuaded by his speech, but the arch-conservative Marcus Porcius Cato is unmoved. These are not times for constitutional scruples, he argues; Catiline is still in arms against the Republic and if we let his followers live, perhaps to escape and join him, we are risking the very existence of the state. Executing them immediately is an act of sheer self-defence, and if Caesar is not afraid of letting them live… well, perhaps he knows all too well that he has nothing to fear from the conspirators if they succeed! (This was a clear accusation of Caesar being involved in the plot, something for which there was no evidence but that plenty of historians both ancient and modern have suspected.) Caesar defends himself from the personal attacks, and an impassioned debate between the two men begins. At the most heated moment, a messenger enters the Senate with a note for Caesar. Cato interrupts his speech to cry out: Look, Caesar is shamelessly receiving treasonous letters from his fellow conspirators as we discuss in the Senate! I demand that he reads aloud that message exposing his guilt!

Caesar gives the letter to Cato without saying a word, and upon reading it with the eyes of the whole Senate fixed on him, Cato sees that it is no letter from the Catiline conspirators… but a love letter. A scandalously passionate letter (Plutarch calls it “wanton” and “unchaste”, but unfortunately does not transcribe its contents) from one Servilia Caepionis, who belongs to the crème of the Roman aristocracy, is the wife of the elect consul Silanus… and the half-sister of Cato himself.

Can you imagine a more deliciously ironic situation? (Compounded, of course, by Cato being an old-fashioned moralist for whom his sister’s behaviour must have been a huge embarrassment.) Cato crumpled the note and hurled back to Caesar with an insult, and resumed the debate as is nothing had happened. He even managed to win the argument, and the conspirators were executed; Catiline was defeated in battle and killed a few months later. (A few years down the road, the illegality of the decision would come back to haunt Cicero, who was exiled from the city for a couple of years by manipulations of his political and personal enemy Clodius on the charges of having put to death Roman citizens without a trial.)

Oh, and by the way: this Servilia, lover of Caesar, had a son from an earlier marriage, at this time about twenty years old, whose name was Marcus Junius Brutus and who was in the future to wield a knife against Caesar on a certain day by the middle of March. It seems unlikely for the affair between Servilia and Caesar to have lasted for two decades and more, and started when Caesar himself was a teenager, so most historians dismiss the rumours you are thinking of… but now you know what the apocryphal words “you too, my son” are supposed to mean.

I must repress the urge to go own telling you more stories about these people, in particular my second favourite one: how Clodius cross-dressed to sneak in unnoticed into a secret ritual to which only women were allowed. The consequences of this fascinating and twisted tale include both the Cicero-Clodius feud I mentioned above, and our use of the phrase “Caesar’s wife” for a woman who is or should be above suspicion. But I must go on to the reviews. You can read the full story of Clodius’s sacrilege, among with essentially everything else that you should know about this period, in Tom Holland’s book Rubicon. (The only exception is the story of Servilia’s letter, which Holland unaccountably leaves out. Servilia’s love affair with Caesar and the Cato-Caesar debate over the conspiracy are both mentioned, but separately and without bringing in the letter incident.)

Rubicon is one of the best books of popular history I have ever read. It tells the complete story I outlined at the beginning of the post –how the Republic became “out of control” and drifted inexorably into the Empire- with careful factual accuracy but the rhythm and pace of a thriller. The language and style are more journalistic than academic, making the book extremely accessible and easy to read at a quick pace, but at the same time Holland has a thorough knowledge and control of all the historical sources and never falls into anachronism. If you ever thought Roman history was boring, this is the book that will change your mind (unless my post has already done it). I have two only quibbles with the book: one is a slight bias present towards the “Republicanism” of Cicero and Cato, which I regard as more short-sighted and dogmatic than Holland presents it; although it must be said that it is difficult for writers on this period to avoid both the Republican bias and the opposite Caesar hero-worship bias, which pervades for example McCullough’s Masters of Rome series. The second one is that it focuses much more on the political and military story (the dealings of the cast of characters I listed at the beginning) than on the social and economic background, and is exclusively told from the perspective of the high class (the common people of Rome are ”the mob”, rarely positive actors with legitimate interests of their own). This makes the story more exciting to read and I don’t regret that the book is written like that, but it would be nice to contrast it with a more, let’s say, “Marxist” perspective.

Antony and Cleopatra is the Shakespearean title of the last book in Colleen McCullough Masters of Rome series. The very existence of this series is a godsend to those who like me are fascinated by this period: seven huge door-stopping novels telling the complete story of Rome between the years 110-27 B.C. McCullough’s scholarship rivals that of Holland or any other historian, and she brings to life the political and personal struggles of the era as no other writer has done. This last book covers the events between the battle of Philippi and the death of Brutus in 42 B.C and the final death of the Republic with Octavian, rechristened Augustus, becoming the first emperor in 27 B.C. The central characters are well-known: the weak-willed and pleasure-loving Mark Antony, the seductive and capricious Egyptian queen Cleopatra, and the master of politics and propagandistic spin Octavian, who uses the romance of the other two as launching board for justifying to the Senate and People his war against them -as Antony has ceased to be a “true Roman”- and hence his emergence as the sole leader of the world. The story does not fail to be thrilling and page-turning even if the ending, like in all tragedies, is known from the beginning. However, this book is lacking in comparison to previous entries in the series. There is a feeling of tiredness, of efficient “writing by the numbers” lacking inventiveness, that gives the suspicion that McCullough wanted to be finished with the series once and for all. None of the characters, except perhaps Octavian at some of his best moments, are as compelling and interesting to read about as Marius and Sulla in the first two books of the series (The First Man in Rome and The Grass Crown), or Pompey in the third.(Fortune’s Favourites). One big disappointment was the portrayal of young Livia Drusilla, Octavian’s wife, who in older age is the unforgettable villainess of I, Claudius. I knew that Robert Graves had taken some liberties with history, and that the scrupulous McCullough was unlikely to make Livia a cold-blooded poisoner; but I was hoping that she would try to rise up to the antecedents and give us a compelling version of her. Sadly, she comes out as bland and mostly uninteresting. Overall, I recommend the novel only to those who have followed the series up to it (who are unlikely to need my advice to rush to buy it.) Others would do better starting from the beginning.

Robert Harris’s Imperium is a very different book from McCullough’s detailed and realistic chronicles. It tells the story of Cicero’s early years in politics, up to his election as consul. The novel is structured around three key episodes: the prosecution by Cicero of the corrupt governor Verres, the unexpected success of which makes him the leading advocate and orator in the city; the political manipulations to ensure Pompey receives a special commission to fight the pirate menace, with Caesar and Cicero main actors behind the stage; and the election itself in which Cicero faces Catiline for the first time. (An announced sequel will surely deal with the conspiracy and its aftermath.) The style is that of a political thriller, with conventional tropes used, and rather successfully, for generating suspense. Characters are painted with broad strokes and only Cicero himself attains some complexity. The political complexities are much simplified in comparison with McCullough or even Steven Saylor, who also writes thrillers situated in this period but with a much more careful display of research. There are a couple of rather jarring anachronisms, such as calling a consular candidate a “religious fundamentalist” (something utterly meaningless in the context of Roman religion) or Pompey’s reaction to the threat of the pirates as a caricature of George W. Bush’s war on terror, complete with the words “those who are not with us are against us” put in his mouth. Imperium is a quick and easy read which may appeal to readers generally put off by history or historical novels; I enjoyed it well enough despite its faults, and am likely to read the sequel when it comes… if only to see how the story of Servilia’s letter is played!

In summary: I recommend Rubicon to anyone who has a pre-existing interest in the period, or knows nothing about it and wishes to learn in an enjoyable but reliable way; Imperium to those who enjoy thrillers with historical touches, and the whole of McCullough’s series (not Antony and Cleopatra standing on its own) for those who want to really immerse themselves in Roman history.

Yet more Shameless Self-Promotion

2007-10-31T10:32:00.000Z

Transition rate of the Unruh-DeWitt detector in curved spacetime

Jorma Louko, Alejandro Satz

Abstract: We examine the Unruh-DeWitt particle detector coupled to a scalar field in an arbitrary Hadamard state in four-dimensional curved spacetime. Using smooth switching functions to turn on and off the interaction, we obtain a regulator-free integral formula for the total excitation probability, and we show that an instantaneous transition rate can be recovered in a suitable limit. Previous results in Minkowski space are recovered as a special case. As applications, we consider an inertial detector in the Rindler vacuum and a detector at rest in a static Newtonian gravitational field. Gravitational corrections to decay rates in atomic physics laboratory experiments on the surface of the Earth are estimated to be suppressed by 42 orders of magnitude.

#1 in Google meme

2007-10-24T22:38:00.000+01:00

Yet another meme that has been going around to replace what could have been a thoughtful post with some navel-gazing fun. This time the idea is to find five search keywords or phrases that retrive your blog as first result in Google. I found several of these after some playing around (trying to avoid obvious things like post-titling phrases unlikely to appear elsewhere). My 5 favourite examples:

1. reality conditions

Yes, an obvious one, but considering that it is a common phrase in mathematics and mathematical physics and also the title of a book, I am rather proud to be the first one (the first two, actually).

2. evolution and theism compatible

I get both results #1 and #2 for this one! If that doesn't make me a Neville Chamberlain evolutionist, I don't know what can make it ;).

3. review thomas nagel

4. review lee smolin

5. review cosmic jackpot

My book reviews seem to be rather popular with Google. I am also #2 (after Amazon) for the simple string "cosmic jackpot". Curiously enough, I am #1 for "review neal" but not for "review neal stephenson".

And as bonus, the result that indulges my vainity the most: I am the #10 google result for the search "alejandro", thus appearing in the first page displayed, just a few lines below Peru president Alejandro Toledo, filmmaker Alejandro Amenabar, and Alexander the Great (who is Alejandro Magno in Spanish).

Book meme

2007-10-10T23:09:00.000+01:00

This has been going around the Internets in the past weeks, and it is as good a way as any of making a post when I have nothing interesting to post about. The rules are:

1. Bold what you have read
2. Italicise what you started but couldn't finish

The second seems to be the main point –apparently this is a list of the books more often left unfinished or something like that. To make it slightly more interesting I will add a couple of comments here and there, on all the unfinished books and some of the others.

Jonathan Strange & Mr Norrell
Anna Karenina
Crime and Punishment
Catch-22
One Hundred Years of Solitude
Wuthering Heights
The Silmarillion
Life of Pi: a novel
The Name of the Rose
Don Quixote

Well, almost –perhaps I should have italicized. When I had to read it for high school I did finish it, but cheated and skipped some of the chapters which are unrelated sub-stories told by characters within the novel. I have the firm intention of rereading it without cheating some day.

Moby Dick

This was the second novel I read in my life –the first being Robinson Crusoe- or so I thought for most of my childhood. Later I discovered that the version I had read was a heavily abridged edition, perhaps a fourth of the length of the original novel. (Robinson I had read unabridged, honestly!) I only caught up with the whole book some four or five years ago. Great stuff.

Ulysses
Madame Bovary

This one is high on my list of Great Books that Suck –okey, I exaggerate, but it had little or nothing that appealed to me. I like Stendhal much better than Flaubert.

The Odyssey

Tried it right after finishing The Iliad with no problems, but got distracted with other stuff and never picked it up again. Needless to say I fully intend to read it some day.

Pride and Prejudice
Jane Eyre
A Tale of Two Cities
The Brothers Karamazov
Guns, Germs, and Steel: the Fates of Human Societies
War and Peace
Vanity Fair
The Time Traveler's Wife
The Iliad
Emma
The Blind Assassin
The Kite Runner
Mrs. Dalloway
Great Expectations
American Gods
A Heartbreaking Work of Staggering Genius
Atlas Shrugged
Reading Lolita in Tehran: a memoir in books
Memoirs of a Geisha
Middlesex
Quicksilver

See my review of the whole Baroque Cycle. I fully understand why this one is on the list; it took me about half a year to go through it. The other two volumes are quite more difficult to put down.

Wicked : the life and times of the wicked witch of the West
The Canterbury Tales
The Historian: a novel

I’m starting to read this one just now! Looks promising, so it will soon become a bolded item.

A Portrait of the Artist as a Young Man
Love in the Time of Cholera
Brave New World
The Fountainhead
Foucault's Pendulum

This one and The Name on the Rose were very high on my list of Favourite Books Ever for several years (roughly the second half of my teens). I can’t understand what happened to Eco after this; The Island of the Day Before and Baudolino were miles below the previous novels in quality. I have not read The Mysterious Flame of Queen Loana, but my hopes are not high.

Middlemarch
Frankenstein
The Count of Monte Cristo

Read it in a children’s edition which must have been about one twentieth of the size of the original book… so no, I don’t even count it as unfinished. Definitely want to read it some day.

Dracula
A Clockwork Orange
Anansi Boys
The Once and Future King
The Grapes of Wrath
The Poisonwood Bible: a novel
1984
Angels & Demons
The Inferno
The Satanic Verses
Sense and Sensibility
The Picture of Dorian Gray
Mansfield Park
One Flew over the Cuckoo's Nest
To the Lighthouse
Tess of the D'Urbervilles
Oliver Twist
Gulliver's Travels
Les Misérables

Yes, I read the complete edition of this one, including the description of the battle of Waterloo, the philosophy of convents and the disquisition on criminal jargons. And loved it, by the way.

The Corrections
The Amazing Adventures of Kavalier and Clay

Read this one on recommendation by my cynical friend, which reminds me that it's been a while since the last time we had one of our little blogfights -maybe he will come and make some sarcastic comments on my readings to enliven things a bit

The Curious Incident of the Dog in the Night-time

??? How can many people leave this book unfinished? It must be the easiest to read great novel ever written. Maybe they were put off by discussions of prime numbers and the Monty Hall problem?

Dune
The Prince

I assume it is Machiavelli. It was long ago that I tried, and I’m not sure why I didn’t finish it. Probably I just didn’t find it interesting.

The Sound and the Fury
Angela's Ashes: a memoir
The God of Small Things
A People's History of the United States: 1492-present
Cryptonomicon

Yes, I got this one and read it in the last months, after finishing The Baroque Cycle. It is better crafted as a novel and mostly more exciting to read, but the ending is more disappointing and it didn’t have as many little fascinating things as TBC. Also, I am somewhat less interested in twentieth-century geekery than in seventeenth-century geekery. Leibniz beats Turing all the way.

Neverwhere
A Confederacy of Dunces
A Short History of Nearly Everything
Dubliners
The Unbearable Lightness of Being
Beloved
Slaughterhouse-five
The Scarlet Letter
Eats, Shoots & Leaves
The Mists of Avalon
Oryx and Crake : a novel
Collapse: How Societies Choose to Fail or Succeed
Cloud Atlas
The Confusion

Notice that the first two parts of The Baroque Cycle are on the list, but not the third. Anyone who has invested the time of going through the first two will not leave the last one unfinished and make the whole previous effort pointless.

Lolita
Persuasion
Northanger Abbey
The Catcher in the Rye
On the Road
The Hunchback of Notre Dame
Freakonomics: a Rogue Economist Explores the Hidden Side of Everything

I read most of this one standing at the WH Smith shop of the Heathrow bus terminal, once that I got stuck waiting there a couple of hours for a bus and had finished off my reading material on the plane.

Zen and the Art of Motorcycle Maintenance : an Inquiry into Values
The Aeneid

Made a half-hearted attempt as a teenager. Didn’t get too far.

Watership Down
Gravity's Rainbow
The Hobbit

I'm not surprised about The Silmarillion, but it's perplexing that The Hobbit is on the list and Lord of the Rings is not. I know many people who tried it and left it at the prologue or shortly after.

In Cold Blood : A True Account of a Multiple Murder and Its Consequences
White Teeth
Treasure Island

Another surprise. How can you pick up this book and (assuming you like pirate stories, and if not why would you pick it up?) not read it complete? What’s wrong with you?

David Copperfield
The Three Musketeers

See here for some intensely personal comments on Dumas.

Overall: 32 read and 5 unfinished, out of 106 in total. Or if you insist on counting the Quixote as unfinished, then I count The Historian as finished to balance –I am reading it now and liking it, so it is just a matter of days till it can be added to the list. How did you do?

Spinny Foamy talk

2007-10-06T14:26:00.000+01:00

Last Tuesday, Johnathan Engle gave a talk for the International Loop Quantum Gravity Seminar. The talk was on the new spin foam models that have been proposed this year; namley, this one by Engle, Pereira and Rovelli and this one by Freidel and Krasnov. We have been having some discussions on both papers here at the Nottingham group as well. The two things that seem to me more important are the possibility of getting a better understanding of the role of the Immirizi parameter, which for first time is appearing in spin foam models, and the possibility of checking which is the correct dynamics via semiclassical calculations.

The audio and the slides of Engle's talk are available at the above link. If you are interested in getting an idea of the current state of research in the LQG/spin foam community, I encourage you to listen to the full audio. About the last half of it is an hour-long discussion between (mostly) Rovelli and Freidel. For me it gave me a strong feeling of how little really is known, and how basic are the disagreements that are still possible between people follwing essentially the same research program. It is something what does not see so directly in papers or even in most conference talks.

Quantum Gravity Colloquium: the discussion. (QM vs. QG: the Grudge Match!)

2007-09-21T00:17:00.000+01:00

This is a continuation of the previous post. The discussion session on “Are the foundations of Quantum Mechanics relevant for Quantum Gravity”, informally chaired by Anna Gustavsson, was the ending of the colloquium and one of its high points.

The two main opposing positions (basically “No” and “Yes”) were championed by Frank Hellmann and Jamie Vicary respectively. I took what started as a compromising position and was driven later to stand on Frank’s side when Jamie became more and more iconoclastic. Anna sided with Jamie, but I think few of the rest did, though I can’t really remember any other of the specific opinions voiced.

Frank outlined the “conservative” position: Quantum mechanics works extremely well and has convincingly passed experimental tests in a very large range of scales. Granted, we don’t fully understand its ontological consequences, and thinking about them is a legitimate issue; but there is no reason not to be confident in applying the formalism to the problem of quantum gravity. There are specific technical problems that arise in this application of quantum principles that do not arise in others (e.g. the problem of time and the “partial observables” formalism Frank himself has worked in) but in his opinion, these problems can be examined “orthogonally”, so to say, to the philosophical/interpretational problems.

I am much less convinced that both kinds of problems can always be neatly separated. At first glance, making sense of the quantum formalism in a “timeless” context would seem to require re-examining the notions of measurement and “collapse” in the usual formalism, which are strongly time-asymmetric. The opinion I voiced in the discussion was that, perhaps you don’t need to solve the problem of the foundations of QM to do QG, but at least you will have to worry about it. In the context of normal applications of QM, we have proof that the various “interpretations” (naïve Copenhaguen, Everett, Bohmian…) are not distinguishable in practice and only differ philosophically; for example, decoherence ensures that even if the wavefunction never really “collapses”, it would appear as if having collapsed after a measurement. In the context of QG, we lack a proof that all interpretations give the same answer to questions, or that questions about observables can be framed in a way that is neutral between interpretations. This is simply because we do not know what QG is! So when building up your theory of quantum gravity, you should think about which interpretation you are endorsing, and what do “states”, “measurements” and other thorny terms mean in your theory.

Frank sort of agreed with me on this, but said that his paper with Rovelli, Perez and Mondragon proved that the meaning of these terms in generally covariant quantum theory is as “interpretation-independent”, from a practical point of view, as in ordinary quantum theory. So the worry I raised was legitimate, but has been addressed and solved already. I am unconvinced. The paper considers a particular way of defining the quantum observables in the background-independent case. It applies to quantum theories which are canonical quantizations of a classical theory and where the quantities parametrizing the classical configuration space are promoted to kinematical quantum observables. The problem of what happens with the collapse for several measurements “at different times” is considered, and the answer is given (as far as I understand) that one needs to consider the measuring apparatus of the first measurements as quantum as well, and only apply the formalism to calculate the probability for an outcome in the “final” measurement, where the system and the apparatuses that measured it previously are observed simultaneously. This is clever, but I don’t think it solves the problem as conclusively as Frank implied. Can we be certain that the correct theory of quantum gravity will have the form described above? Can’t the correspondence between classical and quantum observables be more complicated and subtle that in this kind of theory? Just to pick an example, does the formalism outlined apply to a theory of quantum gravity that is defined, as string theorists claim it can be done, by an holographic correspondence between the spacetime and a very different theory on its boundary? How are the observables of such a formalism to be understood? I think it is quite rash to claim, at the present level of ignorance, that we know for certain that quantum gravity can be developed without worrying any more about the philosophical interpretation.

Anyway, this disagreement with Frank played a very short part in the discussion, compared to the more basic argument between both of us and Jamie. Apparently relishing the role of contrarian, Jamie made some strong “radical” claims against the “conservative” position. He said that not only we do not understand the meaning of our present quantum theory; its basic assumptions go also largely unquestioned and are moreover very likely to be false in the realm of quantum gravity; therefore we ought not to try to keep its basic structure, but get rid of it and devise new kinds of theories to attack the problem of QG. When asked what were the “unquestioned basic assumptions” he mentioned a smooth, continuum spacetime, the use of complex numbers, the basic concept of “probability” as a positive real number, and perhaps some other I have forgotten. The rest of us pointed out that the quantum formalism by itself does not require a smooth spacetime and that in many QG approaches spacetime is discrete while the standard rules of QM are preserved; but Jamie seemed to think there is some inconsistency in this. For example, he said that probabilities would not be real numbers unless there was a real physical continuum –but as Frank said, discrete systems such as a qubit can be in an arbitrary superposition state parametrized by real numbers, and besides even if the actual probabilities existent in nature were all combinatorial and measured by rational numbers that would hardly a fundamental difference to the standard situation meriting a whole different formalism. Jamie (and I think someone else as well, but I can't remember whom) also talked about the standard idea of probability as presupposing the possibility of repeating the exact same experiments, which is not possible if spacetime is dynamical. It was answered that, by that reasoning, quantum electrodynamics would require the assumption of a constant background electromagnetic field, which is not true; and that spacetime being dynamical does not preclude the possibility of setting local regions of spacetime in a desired local state –e.g., to do repeatable graviton scattering experiments, if it was technically possible (as black hole production is in large extra dimension models). Another possible answer, which didn’t come to my mind during the discussion, is that probability does not need to be defined by frequencies in repeated experiments; it can be defined by degree of belief as in Bayesianism or by physical “propensities” inherent in particular systems. (A philosopher who specialized in these subjects told me once that the frequentist interpretation of probability is completely discredited and nobody endorses it anymore. This may be an exaggeration, but physicists tend to take too much by granted that probabilities just "mean” frequencies in repeated experiments.)

Anna sided with Jamie, arguing that QM has worked well for the other three forces but not for gravity, which would indicate that something different is needed in this case. Jamie was firm on the idea that different mathematical frameworks need to be explored, and are more likely to work than the old quantum one. Frank and I answered that on the contrary, the most reasonable way to do research in absence of experimental information is to pay maximum respect to ideas that have worked exceptionally well in tested areas, and that Jamie’s method would lead to completely arbitrary new theories, most likely with no relation to reality. Historically, new paradigms are developed replacing old ones when empirical evidence makes it clear that the old ones will not work and gives pointers to new possibilities; trying to develop them in vacuo, in a purely philosophical way, does not lead to anything. (Einstein’s development of GR is perhaps an exception, but it is a unique case. The 200 years of unsuccessful efforts to replace Newton’s action at distance by some kind of mechanical model of gravity is a more likely comparison to what Jamie was proposing, at least in my opinion.)

I think it was Anna who suggested at some point that “quantizing general relativity” was perhaps the wrong track for research; perhaps what we would call “quantum gravity”, in the sense of being the microscopic structure of spacetime, has nothing to do with a quantization of Einstein’s GR. I agreed on this but I thought that this is independent of whether the QM formalism needs modification; string theory would seem to satisfy Anna’s description while being “orthodox” with respect to quantum foundations, as far as I understand it.

This is all I remember of the discussion. It is very likely that my memories are partial, incomplete and downright inaccurate. I beg the participants to join in the comments to give their own account, correct my misrepresentations, add things I forgot, and of course… continue the discussion! People who where not present at the moment are also invited to this last thing, of course.

Quantum Gravity Colloquium: the talks

2007-09-17T22:13:00.000+01:00

Last Friday I went to London for a quantum gravity student colloquium at the Imperial College. The idea was to meet up with postgraduates working in quantum gravity in UK and other European countries, and have some give talks explaining their research, with ample time for questions and discussion, in a very informal setting. Keeping it student-only makes it easier to dare to ask potentially stupid questions and voice one's opinions. We had already had one such meeting in Cambridge several months ago, and this second experience was as good as or better than the first.

This time we had four seminar-like talks, plus an open discussion session on the topic "Are the foundational problems of quantum mechanics relevant for quantum gravity?" Starting with the talks: Leron Borsten talked on the entropy of black holes in supergravity theories, and intriguing connections it has with entanglement in quantum information theory. There is no clear picture yet in which to view the black hole entropy as entanglement entropy, but there are some identities or isomorphisms between the mathematical description of both concepts that do not look as a coincidence. Yousef Ghazi-Tabatabai talked on an approach to interpreting quantum mechanics which seemed related to the ideas pushed forward by Rafael Sorkin in his plenary talk at Morelia.

Frank Hellmann (who also must be given the lion's share of the credit for organizing the colloquium) talked on "Partial Observables", an approach to defining quantum observables in generally covariant theories and, potentially, solving the problem of time. It has been long championed by Carlo Rovelli, with whom Frank worked before coming to Nottingham. The talk explained how observables that are evolving in a conventional framework can be recast as Dirac observables when the dynamics is written in a generally covariant way. These are fit to answer the question "If the system is in physical state Rho, what is the probability of seeing the correlation (x,t)?" (where x,t are the variables of the classical configuration space). The answer to this question is Tr (Rho P(x,t)), where P(x,t) is the operator that projects states into the physical state which is itself the projection, onto the physical Hilbert space, of the kinematical state corresponding to correlation (x,t). Sadly little time was left by the end of the talk for Frank to discuss the thorny case of multi-time measurements, which is the real centrepiece of his paper.

Eugenio Bianchi gave an excellent talk on Perturbative Regge Calculus and Loop Quantum Gravity. It was a version of the talk he gave at Morelia, but with the math replaced by the concepts, which was much better! I think this stuff is extremely important and I am hope to start working into it in the future, so I will summarise the talk in more detail than the previous ones. I would be extremely happy to receive comments discussing it or pointing out mistakes in my exposition.

Loop Quantum Gravity is an essentially non-perturbative theory. Any attempt to find a "semiclassical limit" and connect it to established physics is complicated by the fact that semiclassical physics is essentially perturbative; so there is the problem of even mathematically connecting the two frameworks, before a concrete calculation to see if they agree can be done. One way of doing this connection is the boundary amplitude formalism introduced by Rovelli. Take a kinematical semiclassical state, a kinematical state given by a superposition of spin networks which is a Gaussian peaked on on a classical spacetime (and choose this to be flat space). Fix this as the state on the boundary of a region, and you can compute correlations of observables, measured in the boundary, due to the dynamics in the interior. Use a spin foam model to specify the dynamics: for example, the Barrett-Crane model. You can then calculate, based on a nonperturbative theory, semiclassical correlations of your dynamical variables, which are spins jmn. You obtain results. However, you don't know if your initial theory that defines the boundary state (LQG) is correct, nor if the spin foam model you have chosen to encode the dynamics is correct, and besides that as the whole conceptual and calculational framework looks very different from things used in other areas of physics, you would really really want something to compare your results to as a check.

Enter Perturbative Regge Calculus. Regge Calculus is an approximation scheme to GR in which the curved manifold is replaced by a skeleton triangulation, with the geometry encoded in the discrete edges and vertices. As Eugenio stressed, it can also be thought of as exact (not approximated) GR but with piecewise flat metrics instead of continuous metrics. Choose a triangulation that discretizes flat space; it is described by the connectivity C and a set of edge lengths Li. Now add small perturbations to the edge length variables, and quantize these perturbations. You are now doing Quantum Perturbative Regge Calculus, which is a straightforward background-dependent, perturbative quantum theory, in which all the standard rules of the game apply. Change your variables from edge lengths to face areas, and calculate the quantum area-area correlations on the boundary of a region. Compare them with the fluctuation in spins jmn calculated from the non-perturbative theory. Are there equal?

The answer is that, for the Barrett-Crane model dynamics, all quantities compared between both calculations up to the 3-point function match exactly, provided that one identifies the spins used as variables in LQG with the areas used in Regge calculus, up to the factor 8 Pi G b, with b being the Imirizi parameter! This is an independent and nontrivial check of the famous LQG spectrum area, which was derived at a purely kinematical level. Here the dynamics is necessary to ensure the right correspondence between areas and spin variables. For example, if one uses a non graph-changing Hamiltonian to define the dynamics, the correspondence is not recovered. The calculation with the new spin foam model recently proposed by Rovelli, Pereira and Engle, which cures certain problems with the Barrett-Crane model, is still not completed and its results are eagerly awaited.

It should be stressed that, though nontrivial, this check does not amount yet to confirming that LQG has the "correct" semiclassical limit. The only perturbative theory of quantum gravity that has right to be acknowledged as "correct" (because it uses uncontroversial quantum field theory principles in an unobjectionable way) is the Effective Field Theory approach popularized by Donoghue. It is not known if quantum perturbative Regge calculus is a sort of discrete equivalent of this; understanding the connection between them would be a key step forward. But the calculation Eugenio talked about has great importance by itself, because it shows that a fully nonperturbative approach to quantum gravity, when used in conjunction with a semiclassical state, can give the same answers as a better understood perturbative approach.

As this post is getting longish, I will break it here and post in one or two days about the discussion on the relation between foundations of QM and QG. Stay tuned.

Answering my Readers: the Catechism of the Eastern Orthodox Church edition

2007-09-05T23:55:00.000+01:00

Alana has asked me to give my opinion, from a scientific point of view, on an argument for the existence of God that she copies from the Catechism of the Eastern Orthodox Church. The Catechism seems to be a sort of FAQ about the basic doctrines of this church, and according to this website it has non-official status. Its author is one one Rev. Constas H. Demetry. The non-official status is fortunate, because as a good “Neville Chamberlain atheist” I do not wish to offend anybody’s faith, least of all that of a commenter who was nicely asking a question, and I must say that I will be highly critical of Demetry’s level of both scientific knowledge and philosophical acumen. I understand that one cannot expect very complex and precise philosophical arguments from a FAQ, but one should at least avoid elementary mistakes.

The paragraph Alana quotes (adding the conclusion of the argument, which she didn’t quote) says:

Q. About subtopic (a), How is it proved from the existence of the universe that there is a God apart from it?

A.
1.) The universe, (the earth and the heavenly bodies) could not come into being of itself because it consists of matter, which is inert. (A body is called inert, when it of itself, without external influence, cannot change its state.) Therefore there must be a personal Power apart from it, which gave it its beginning. And this personal Power is God.

The conception the Reverend has of matter seems taken straightly from scholastic philosophy, with no contamination from modern science. It is unclear what is to be understood by the “state” of a body, but on any natural reading, the assertion that matter is inert in the defined sense is false. As a very simple example, take an lump of an unstable element, such as uranium; left to itself and without external influence, it will emit spontaneously (at times not predictable except in a statistic, probabilistic way) a radiation of particles (radioactivity) and its atoms will transform gradually into different ones. Contrary to what I hastily wrote in a comment to the previous post, this also can happen to elementary particles: the muon, for example, transforms spontaneously into an electron, a neutrino and an antineutrino, even though it is currently believed to be elementary (not composed of more basic particles). There are hundreds of other examples one can give of material systems that change their state without external influence, and one does not need fancy complicated physics: how about a digital clock, continuously updating the display? An ordinary alarm clock, suddenly ringing? A volcano erupting? And so on…

But leaving aside all this, the argument is still an obvious non sequitur. Substituting the definition of “inert”, the structure of the argument is:

(1) Matter cannot change its state without external influence.
(2) Therefore, it could not come into being of itself.
(3) Therefore, there must be a personal Power apart from it, which gave it its beginning.

How does (2) follow from (1)? (1) is only about how matter can change, while (2) is about how it can begin to exist. Okay, perhaps we can allow for “coming to exist”, as a subspecies of change, though it sounds strange. But (3) certainly doesn’t follow from (2) and (1). No argument is made to show that matter could not have existed forever. Even if such an argument could be made, it still wouldn’t follow that it was created by a personal power. “Personal” qualities (consciousness, intelligence, purposefulness, etc.) are empirical features of the world that we see exemplified in only a few beings on the surface of a tiny planet. To attribute them, without any argument, to the unknown First Cause of the universe is to make a gigantic leap in reasoning that cannot be left unjustified.

I see this as the main stumbling block of cosmological arguments for a deity, even those that are much more sophisticated than this one. I do not find the case for a First Cause or a Necessary Being persuasive (I think I can conceive perfectly well an infinite regress of causes or explanations), but even if there is an Ultimate Explanation, maybe it takes a “formal” nature more akin to a physical or mathematical principle than to a Being; and even if it is a Being of sorts, I have never seen any good reason to suppose it to be a personal one. Most of the arguments for this that I have seen depend on a spurious dichotomy between Matter and Persons (or in other words, between mechanical and teleological explanations), together with a spurious conception of Matter as “inert” and Persons as “active” (or in other words, of mechanical explanations as always needing further ones, while teleological ones get a free pass as acts of will.)

All these are concepts and distinctions that fit perfectly into the scholastic, medieval view of the world and very badly into the modern scientific one. I’m not saying that modern science can prove medieval philosophy to be “wrong”, except in a few peripheral matters. One can always start from a philosophical system, declared to be a priori known, and interpret the whole of world and science according to it. But I find it more reasonable to begin with modern science as a solid starting point (there is much more agreement among scientists that among philosophers, after all!) and then try to build up a philosophy that squares well with it. And according to such a naturalistic philosophy, purposes and intentions are not free-standing fundamental metaphysical categories but just features of the way we describe and explain at a high, emergent level the behaviour of some very complex material systems –us.

I have sidetracked, however, discussing more general cosmological arguments and my opinions on them, rather than concentrating on Rev. Demetry’s arguments. This is sensible, because Demetry’s next arguments are unbelievably silly. The Introduction to the document does not give any dates, but one can infer from it that Demetry lived in the twentieth century, and not too long ago. Were it not for this, this argument would make me think he was writing in the sixteen hundreds. His scientific knowledge has not surpassed that stage:

2.) The Universe, according to the astronomers, moves, and moves regularly, and in circles (rotates). This rotating movement needed a power apart from the universe to produce this motion, and, in order that the power should not be exhausted or become larger or smaller, a Personal and Omnipotent Power is needed to renew the power which is lost on account of the friction of the motion, and to regulate it so that the motion might always be uniform.

The Universe rotates??? God renews the power lost in the friction???

*shaking my head in disbelief, as if having seen a living fossil in the wild.*

Demetry seems to be taking a leaf from Isaac Newton, who argued that God’s action was needed to keep the solar system perpetually moving because otherwise the perturbations that some planets cause to the orbits of others would make it unstable. But even Newton (and Kepler before him) knew that the planets do not move in circles but in ellipses. And the idea of the whole universe rotating is even more obsolete: it only makes sense in the geocentric cosmology that Copernicus and Galileo overthrowed. It is not only the Reverend’s philosophy that is medieval… Of course, even in Newton’s days his argument was attacked on philosophical grounds by Leibniz, who said it was diminishing of God’s perfection to have to tinker continuously with his creation like a clumsy watchmaker. The point was rendered moot when later it was proven that Newton’s calculations were wrong and that the Solar System can survive by itself without any external tinkering. Nowadays arguments for God from misunderstandings of physical cosmology invoke more usually the Big Bang; their philosophy is not much better (see the link for my comment), but their science is at least updated.

I will not go on with Demetry’s next arguments for the existence of God, which are not much better than the ones above. I will, however, quote a passage that made me jump in my seat when I saw it, from the section on the Trinity. We read:

Q. Can we understand the Holy Trinity?

A. No, because it is a mystery.

Q. What is a mystery?

A. A mystery is a truth which we cannot understand.

So far so good; I would certainly accept that there are many truths we cannot understand at present, and maybe there are some that we cannot even understand in principle (though this would depend on how we understand “truth”; deep philosophical waters await there). Of course I would reject that the Trinity is one such truth, and moreover I would reject that we can ever have warranted belief in a truth if we believe we cannot even in principle understand it; one could also wonder what is meant exactly by believing something you do not understand, beyond repeating the words in an empty way. But certainly there is material there for a thoughtful philosophical or theological discussion. Look, however, how Rev. Demetry tries to argue this matter:

Q. Is it right that we should reject everything which we cannot understand?

A. No, because there are many things which we do not understand, but which exist, and which we use continually; for example, magnetism, electricity, gravity, etc.

*blinks and stares*

Snarky reaction: “Well, the fact that you don’t understand them doesn’t mean anyone else can’t, after just a couple of years of basic physics courses!”

More serious reaction: We do understand magnetism, electricity, and gravity; I can write, in just a couple of lines, the equations that these phenomena satisfy in all conditions that have been studied so far. Of course, this understanding is not complete: we don’t know for sure if these same equations hold in other conditions we haven’t tested yet; indeed, we have reason to believe they don’t hold –for example- at very small length scales, and we do not understand what equations replace them. But exactly to the degree to which we don’t understand these phenomena, we lack as well any warranted beliefs about them. To the extent that we don’t understand gravity at the quantum scale, we do not hold the belief that gravity “exists” at that scale, and in that sense we “reject” it, or at least we remain agnostic. So as an analogy to the Trinity, this is absolutely dreadful and backfires completely against the Reverend: following the analogy would be a good reason for me to not believe in the Trinity. Even knowing next-to-nothing about the theological conception of the Trinity, I am sure I could write a better defense of it than this one, in the spirit of Sean's recent post.

I hope I have answered Alana’s question without being too aggressive or dismissive. I do not think that religious people are necessarily stupid or even irrational (see here), and I think some theological arguments deserve to be taken seriously. But I'm sorry to say that I cannot take seriously those of Rev. Constas H. Demetry.

By the way: Readers asking questions or petitioning for posts on subjects are a good way of making me overcome my natural blog laziness. So, is there any other topic you would like to see discussed here?

Quick Links

2007-08-19T22:03:00.000+01:00

Aaron Bergman is guestblogging at Uncertain Principles and promises to answer all your questions about String Theory

*Peter Woit gets criticized for scientism by Richard for dismissing the "simulation argument", in what seems to me an example of people with different concerns talking past each other. Richard is right that science, in the narrow sense of empirically testable theories, does not exhaust the realm of rational discourse; Peter is right that the simulation argument is not science nor is even close to being science, as some versions of the anthropic principle may be. If philosophers want to have rational discussions of the simulation argument they may do so, and they may even come to illuminate some matters in epistemology; but this is not connected to anything scientific, and to pretend otherwise would be pseudo-science. (I also think that philosophical discussions with no scientific or practical connections will tend towards the sterile and the scholastic, but that's a separate discussion.)

*Via this discussion I found an intersting blog to add to my blogroll, Overcoming Bias. Its ultra-Bayesian-rationalistic approach to everything under the sun looks to me very limited and a distorted view of actual rational practices; but many of the posts there can be extremely thought-provoking when abstracted from this framework in which they are posed.

*Windmills of my Mind, an excellent film blog, is dedicating this month to exhaustive analysis of Steven Spielberg's films: one post per day, one per film.

*Even though he hates the books and I am a certified fan, I am enjoying Mike Smith's snarky chapter-by-chapter review of Harry Potter and the Deathly Hallows. I liked the book a lot, though there were a couple of disappointments. And I was right on target with one of my main predictions -and wrong with several others. Best of the dozens of online discussions I've read are at Pandagon and In Medias Res. Also unmissble are the summary of the book in lolspeak and the roundup of naughty double entendres.

Not dead. Just crippled.

2007-08-10T23:02:00.000+01:00

I will probably post [even] less than usual in the following weeks. I had a bad fall this week and as a consequence I'm wearing a sling on my left arm. Typing is [obviously] not impossible but it is a good deal more annoying than normally, and I will save my efforts at it for emails and thesis writing. I may do some "quick links" post, but nothing substantial for a while. Meanwhile you can have fun with some new links in the sidebar, including one to Xkcd which is the best webcomic ever.

Loops 07: Conference report (part 3, including discussion session)

2007-07-29T17:04:00.000+01:00

Saturday 30/07

There were only two plenary talks this morning, followed by a discussion session. The first was by John Stachel, who is a specialist on philosophy and history of physics (with special reference to Einstein and relativity) . He introduced a general philosophy called "measurability analysis", which is based on analyzing and defining possible measuring processes and abstracting from them the quantities that need to be quantized (transformed into non-commuting operators). His analysis of GR suggests, to him at least, that the projective and the conformal structures of spacetime geometry are "what needs to be quantized" in quantum gravity. The second one was by Michael Reisemberger, who sketched with admirable clarity a canonical formalism for GR in which initial data are on two null intersecting hypersurfaces. The plus is that null initial data are free, not subjected to constraints. He provided a definition of the Poisson Bracket in this formalism and suggested that quantization leads to area discretization, though this is not yet solid ground.

In the discussion session, Carlo Rovelli followed the same procedure used in the Zakopane lectures and read some selected questions from a notebook that had circulated among the audience the previous days for people to write them. Obviously there were dozens of questions written and Rovelli, given the time constraints, had to select only a handful of them for people to discuss. These are the ones that made the cut:

1) Do we expect topology change in Quantum Gravity?

Oriti said that he expects a general framework to allow for topology change, but probably the classical limit can only be recovered on a sector that disallows it. Ashtekar was of the opinion that that canonical LQG framework must allow macroscopic topology change (if lots of spins become trivial, we have macroscopically a "branching" spacetime)

2) What is the relation of Quantum Gravity to the foundational questions in Quantum Mechanics?

Obviously, a question that provokes a lot of discussion. Thiemann and Rovelli are conservatives who think that QG and foundations of QM can be treated separately -for Rovelli's reasons see what he said in Zakopane. Bianca Dittrich thinks that we need to develop our understanding of relational observers. Lucien Hardy said that GR is at least as radical as QM, so it is unlikely that it can be treated with the standard QM framework. John Donoghue disagreed: according to him, effective field theory shows that GR breaks down at high energies, so the sensible thing is to modify it and keep QM. (As I said in a previous post, this overlooks the fact that in this context the what is meant by GR is not the exact Einstein theory, but the conceptual fact that spacetime is dynamical and not fixed.)

3) Could there be experimental consequences of fluctuating causal structure?

Sabine Hossenfelder mentioned the possible consequences for arrival time of photons, but stressed that this comes only from a phenomenological model with no relation to underlying theory. Hardy said that a "fluctuation", as a superposition between two classical states, would need some kind of interference experiment to observe, which is very difficult to be realizable in practice. I think Ashtekar got into a discussion with him here, but I couldn't follow it well enough to take notes -anybody remembers? Martin Reuter said that causal structure may be different for different observables used to probe it, and especially the scale of these observables.

4) What is finite in spin foam models?

Alejandro Perez gave a rather technical answer, of which the only notes I managed to take say: "some models (in 4D) are finite, some are not". Whoa, that's informative. Sorry.

5) Do we expect the fundamental theory to be combinatorial, or to be embedded in a pre-existing manifold?

Rovelli pointed out the conflict between Thiemann's new "Algebraic Quantum Gravity" approach, which is purely combinatorial, and Smolin's program to recover matter from graph braiding, which requires graphs to be embedded. Thiemann said that matter can be included in the algebraic approach, just as a part in the complete Hamiltonian. (Obviously, it would be more appealing if we could derive matter instead of putting it by hand -but can we?) José Antonio Zapata said that the basic thing we need to do is to understand how to build up a quantum theory on a differential manifold (one not previously equipped with a metric structure, I gather).

And now the last question. It asked, to all plenary speakers, to say they "dream for Loops '17"; that is, on their most optimistic possible view, what is the title and abstract of the talk they imagine themselves presenting within ten years?

Many of the answers were predictable and variations of a basic template: abstracts saying "we present a complete theory of quantum gravity with testable (or, in the most ambitious cases, confirmed) predictions." Ashtekar said something like this, adding that his estimated probability for this scenario was 0%. (But he also gave the in my opinion rather optimistic figure of 50% for the probability of having some experimental evidence to start resolving ambiguities.) Reuter had one of the most concrete dreams: "It is shown that LQG is equivalent to Asymptotic Safety, and that that the quantuization ambiguities in it are finite in number and equivalent to the dimensionality of the Non-Gaussian Fixed Point." And finally, there was an extremely amusing exchange between Thiemann and Alejandro Perez, which is a fitting conclusion to this series of posts:

Thiemann (reading his dream abstract): "We present quantum gravity corrections to the electron fine structure, and find that they are in agreement with experiments carried out by the author"

[laughter from the audience]

Perez (reading his dream abstract): "We show that Thiemann's calculations are totally wrong."

[hysterical laughter from the audience]

Loops 07: Conference report (part 2)

2007-07-25T11:40:00.000+01:00

This is the second part of my conference report on Loops 07. The first part was here. Remember that you can download the slides or the audio for most of the talks from the conference webpage.

Wednesday 27/06
Plenary talks

Moshe Rozali started by giving an excellent talk about background independence in string theory, a topic that has been subject of legendary long discussions on Cosmic Variance and other blogs. The main points of his talk were: a) Perturbative string theory is in fact background independent, being a generalization of GR in a background field gauge; it's just that the perturbative framework makes the background independence non-manifest. b) Holographic dualities provide a way of archiving background independence in a more explicit way. In AdS/CFT, a gauge theory on the boundary can be manifestly diffeomorphism invariant and be equivalent to quantum gravity in the bulk of AdS. Rozali stressed that only the asymptotics of AdS (i.e. a particular negative value of the cosmological constant) need to be fixed; the interior geometry is completely dynamical. Ashtekar seemed to disagree about the extent of this statement and tried to press for a discussion in the question session, but it was interrupted for lack of time.

Klaus Fredenhagen talked about QFT in curved spacetime as a route quantum gravity. He extolled the virtues of Algebraic Quantum Field Theory and the techniques of microlocal analysis to provide a sound axiomatic foundation to QFT in curved spacetime, and explained some recent results proven in this area. Then he discussed the application of this formalism to the graviton field treated as a perturbation around a classical background, and wondered about its relation to the methods of effective field theory.

My namesake and compatriot Alejandro Perez (with whom I have been confused a couple of times for those two reasons, though we look nothing alike) gave a rather technical talk involving strings, BF theory and path-integral sum over topologies. I wish I could say more about it, but I got lost soon after the introduction.

Martin Reuter gave an exceptionally clear and compelling presentation on Asymptotic Safety in Quantum Gravity. It covered more or less all the ground that he had covered in the Zakopane lectures in March, which I will not summarise again (click on the link), but also a few tantalizing new implications for cosmology. If the results of the "Einstein-Hilbert truncation" are accepted as approximately true, then the physical cosmological constant "runs" with the scale in the following way: it is constant (at its currently observed tiny value) at lengthscales larger than 10^(-3) cm, and then starts growing as the fourth power of momentum (inverse length) until the Planck scale is reached, and from there on it grows quadratically. This means that in the early universe it was much larger than in the present but decreasing as the universe increased its scale. This provides a natural mechanism for inflation without any driving field. The inflation was driven by the same cosmological "constant" that we see today, and was due to the intrinsic running with scale of this parameter. Reuter had some calculations that seemed to show his model gives good results for the entropy of the universe, as well as a scale-invariant perturbation spectrum.

This is obviously the kind of thing that is either brilliantly right, or completely wrong. The "dark matter + small cosmological constant + inflation" model that is accepted in conventional cosmology gives predictions of extraordinary accuracy for many different observations (at least with respect to its first two elements). A lot of care would be needed to examine if Reuter's model can really emulate all the confirmed predictions, and whether it can make new ones that are testable. But if Reuter is right, then his talk was by a large margin the most important in the conference.

There were no parallel sessions on Wednesday afternoon, which was a free afternoon.

Thursday 28/07
Plenary talks

Daniele Oriti talked about Group Field Theory (GFT). According to him, GFTs (nonlocal field theories on group manifolds) can be interpreted as "second-quantized quantum gravity". They can be used as a general framework in which to rewrite discrete quantum gravity approaches such as LQG and spin foams. Oriti hopes that the elusive semiclassical limit of these theories may be more tractable with GFT methods. Instead of studying e.g. coherent semiclassical spin network superpositions, take a hugely populated "multi-particle state" of the GFT. The techniques of statistical field theory, used for the semiclassical limit of quantum mechanics in condensed matter theory, are suited to be applied to GFTs. By this way one may hope even to define notions of "temperature" and "phases" as they apply to quantum spacetime. One interesting result that he mentioned by the end, without much explanation, is that GFTs must be Fermi-quantized in the Lorentzian case and Bose-quantized in the Riemannian. Can anyone explain to me what he meant by this?

By this time I was feeling ill and with a bit of temperature (I had been warned against the local food, but...), so I went back to my hotel room to have some medicine and rest an hour or so. I thus missed David Rideout's talk on supercomputers and came back for Martin Bojowald's on effective field theory applied to LQG, on which I had put high expectations. Bojowald rewarded these expectations by dedicated one slide of his talk to quoting this blog…

…well, not exactly. The idea of the talk was to replace exact equations for quantum states by semiclassical, effective equations for a finite number of moments of a state (expectation value, fluctuation, etc.) This method is applied successfully to quantum cosmology. He hinted at the end at possible observable consequences in the inflation perturbation spectrum and at computable corrections to the Newtonian potential (meaning the 00 component of the metric in FRW cosmology). These do not seem to match those computed in Donoghue's ordinary effective field theory, but I'm not sure if this isn't because this is a different meaning of "Newtonian potential".

I kept feeling ill and missed almost all the other talks of the day, and didn't take notes in the few I attended. These included talks in the parallel sessions by Sundance Bilson-Thomson and fellow blogger Yidun Wan on models in which spin network braids are standard model particles. Next day would see Lee Smolin champion the same idea in a plenary talk. I returned early to rest in my hotel room and watch Argentina beat USA by 4-1 at football.

Friday 29/07
Plenary talks

As I was still not feeling perfectly well, I slept till late and attended only the last two morning talks. The first was by blogfriend Sabine Hossenfelder, on Phenomenological Quantum Gravity. She has written up the introduction to the talk in this post, so I can do nothing better than recommend you to read it. The rest of the talk examined the generic predictions made by models such as Minimal Length, Generalised Uncertainty Principle and Deformed Special Relativity. According to her, the main problem with all these models is an insufficient connection with fully developed fundamental theories.

Lee Smolin, as I said, talked on braided QG structures as elementary particles. He started making the point that for LQG and related models of quantum spacetime to work, it is needed to explain how low-energy excitations (gravitons, photons, etc.) can propagate through the spacetime foam without decohering with it. That is, one needs to identify "noiseless subsystems" and a ground state on which they propagate coherently, protected by an emergent symmetry. Then he presented the main result: a class of spin network models exists whose simplest coherent excitations (braided, embedded framed graphs) match the quantum numbers of Standard Model 's first generation of fermions. Higher generations can can be included, at the cost of some exotic states. Interactions can be included. (But he did not say the crucial thing: if these "interactions" match, or can be made to match, the U(1)xSU(2)xSU(3) gauge structure of the Standard Model.) Open problems are to include symmetry breaking and masses (all these degrees of freedom are massless), find momentum eigenstates and conservation laws.

I can understand Smolin's excitement about these ideas, but for the moment I remain highly skeptical about them. The Standard Model is a lot more than a table with quantum numbers, and without much more development it will be hard to convince me that the behaviour of some pretty knots can reproduce the rich mathematical structure of Quantum Field Theory.

Parallel sessions

I chose to go to the sessions centred on black holes. William Donelly gave a talk on entanglement entropy of spin networks, and its use in calculation of black hole entropy. Ashetekar expressed skepticism, saying that those calculations did not include the fact that the surface used is a black hole horizon; Donelly answered that he assumes that any surface will have entropy for some observer accelerating in a way so that the surface is a horizon to him. Daniel Termo talked on how the bulk entropy of a graph scales with its boundary, hoping to identify a "holographic regime" of LQG. The conclusion is that LQG will not be holographic, unless the Hamiltonian constrain reduces dramatically the allowed graph complexity. Bad news, I guess. Yidun Wan talked a second time, this time giving the talk of his colleague Mohammed Ansari who couldn't make it to the conference. It was on an alternative framework to the "isolated horizons" one for dealing with quantum black holes. By a reasoning I could not follow, macroscopic corrections to Hawking radiation were predicted; Ashtekar was again skeptical. Another talk worth mentioning was Jacobo Diaz-Polo's on the old problem of the black hole area spectrum in LQG. Jacobo and his collaborators did exact numerical calculations of the area degrees of freedom, without the approximations used for analytical calculations. They obtain, as usual, the Bekenstein-Hawking entropy as leading term (up to a choice of the Imirizi parameter) and a universal logarithmic correction with prefactor -1/2. The number of states as a function of the area has an interesting structure with evenly spaced peaks of degeneracy. If as a first approximation one considers only the states on the peaks, one gets an equidistant area spectrum and the Bekenstein – Mukhanov effect. Of course, all of this is purely kinematical (Jacobo himself stressed it) and the question of how to incorporate the dynamical constraint seems to remain as elusive as always.

This will be enough for today. My next and last post on the conference will describe the last day's two plenary talks and the discussion session that closed the conferemce. As always, if anyone has anything to add to my summaries, thinks I forgot something important, or wants to correct some egregious mistake, they are more than invited to do so.

My next three days

2007-07-18T17:07:00.000+01:00

I apologize to all eager readers who will have to wait a few days more for the second part of mu Loops 07 report. After some hesitation, I have decided to go the Low-Energy Quantum Gravity Workshop at York on Thursday and Friday. And then Saturday will obviously be dedicated to reading Harry Potter and the Deathly Hallows. Quantum gravity blogging will probably not resume until Monday or Tuesday. After (or perhaps before) finishing off the Loops 07 posts I may say something about the York workshop if there was material of bloggable interest in it.

By the way, #1 : The slides of my talk are now available at the conference webpage, so you can check by yourself whether my handwriting is really illegible.

By the way, #2: Pandagon and Matthew Iglesias and this excellent article by Michel Berube discuss the "backlash" against the Harry Potter books by "serious" literary critics. I think there is a point missed in many of the discussions. Quite beyond the literary quality of the books themselves, there is genuine cultural value in sharing an experience with millions of other people. It is the reason I am not ashamed of eagerly waiting to read the next Harry Potter and then discuss it with people and see all the online fans' reactions to it, while I am blissfully ignorant of many other fantasy series which are probably better written. In the same way that I watch every football match I can catch during the World Cup and then forget almost completely about the sport for four years.

(Yeah, Alejandro, nice one. Trying to pretend that you have forgotten about football just after Agentina has been ignominiously defeated 3-0 by Brazil. Very convenient!)

Loops 07: Conference report (part 1)

2007-07-17T14:20:00.000+01:00

This will be a very long post, or more likely, the first of a series of very long posts. So let me skip quickly over the praises for the quality of the conference and the people present (I met many old friends, both from the real and the virtual worlds) and go directly into the physics. Remember that you can go beyond my comments and get both the slides and audio for most of the talks at the conference website.

Monday, 25/06
Plenary talks

Lucien Hardy talked on the causaloid formalism for quantum gravity. It was actually a foundations of quantum mechaincs talk, based on a "operationalist" philosophy: data are recorded, and physics tries to predict probability correlations among data. These probabilities behave different if data are from "causally connected regions" or not; this allows a definition of what is meant by causal connection in background-independent theories. I found the talk interesting but think that concrete progress in quantum gravity is unlikely to come from such an extremely "top-down" approach. As a matter of philosophical principle, I am suspicious of theories motivated by philosophical principles.

Rafael Sorkin on "anhomomorphic logic". Another foundations of QM talk. Sorkin favours a quantum logic interpretation, in which propositions describing unobserved microevents (e.g. "the particle passed through the lower slit") are assigned truth values that behave according axiomes different from classical logic. Besides what I said above on Hardy, I was especially suspicious of this approach because it "adds sturcture" that is not present on the bare quantum mechanics formalism, for reasons that I find unmotivated.

John Donoghue talked next on Effective Field Theory of General Relativity. This was a much-expected talk, and it was also referenced by many of the following speakers. It was an introduction to effective field theory and the way it provides a consistent perturbative theory
of quantum gravity, for sub-Planckian energy scales. Donoghue emphazised that scattering amplitudes and physical results such as the first quantum correction to the Newtonian potential can be calculated unambiguously and independently of the high-energy completition of the theory, and that any theory that pretends to provide this completition (such as LQG) must recover these corrections as well as the classical "zeroth-order" theory. According to Donoghue, the Problem with capital P is not "reconciliating GR and QM" but finding the fundamental high energy theory that completes quantum GR at the Planck scale. I think, however, that when most people in what is loosely called the "LQG community" talk about reconciliating GR and QM, they understand it as implying much more than what is provided by effective field theory. What is wanted is a quantum theory in which spacetime is fully dynamical, and the EFT results (while important, and truly a nontrivial check for any proposed theory) are still very far from this, as they are based on the perturbative framework of QFT.

Parallel sessions

My own talk "The transition rate of an Unruh detector in a general spacetime" was scheduled for one of the Monday afternoon sessions. It went quite well, with only a brief question by Jerzy Lewandowski during the presentation, and no questions afterwards (though a couple of persons came to talk to me expressing interest later). I suspect many people couldn't understand much, between my ilegible handwriting in the transparencies (I promise to use software next time!), the bad quality of the projector, and the high speed of my speaking due to nerves. I was feeling uncommonly nervous, both before and after the talk, and I didn't take many notes on other talks that afternoon. I have some notes on Rodolfo Gambini's talk, about how quantum mechanics is modified when instead of an abstract time variable we use a physical clock, subjected to decoherence, in the Schroedinger equation; of course, "unitarity" in this time variable is lost. Then he argued that there are fundamental limitations to any clock a the Planck scale, and therefore quantum mechanics would need modifications there. I think that a "timeless" formalism of QM (as Rovelli, Oeckl and others have tried to build) is needed before one can assess these arguments. Guillermo Mena-Marugan and Iñaki Garay talked about quantizations of restricted classical solutions of GR, the Gowdy model and Einstein-Rosen waves respectively; Iñaki had some nice plots of quantum solutions exhibiting both classical and non-classical behaviour. Garrett Lisi then talked on his ambitious "theory of everything" that attempts to describe the whole Standard Model, gravity included, with a single Lie group, E8. I thought when hearing it that it was just a formal game and was surprised to see Lee Smolin ask interested questions, and even more when I saw that John Baez had wrote a whole TWF column on this theory.

Tuesday, 26/06
Plenary talks

This was "the big LQG day", with talks by heavyweights Thiemann, Ashtekar and Rovelli. There was also a talk by Jan Ambjorn about the discrete sum over histories approach, but I missed it.

Thomas Thiemann gave a summary of things known and unknown in Loop Quantum Gravity. For me it added little to to what he had covered in the more complete series of lectures in Zakopane. "Secured land" includes the kinematical framework, the LOST theorem, the area operator spectrum, and kinematical coherent states. "Uncharted territory" includes his more recent Master Constraint Operator (M) to define physical states and the checking of its good semiclassical behaviour. "Open problems" are whether 0 is in the spectrum of M or whether there are anomalies; the resolution of quantization ambiguities in the definition of M; a systematical calculational framework for physical states; a connection with quantum field theory in curved space, with perturbative theory, and a definition of gravitons and Feynman graphs; and conceptual issues related to the problem of time and relational observables. In response to a question by the audience, he admitted that little or none work had been done to connect LQG with the effective field theory results. I think that everyone came out of the conference with the agreement that this is an extremely important thing to do.

Abhay Ashtekar gave a summary of results in symmetry reduced models: loop quantum cosmology and "loop quantum black holes". He started arguing that while results in symmetric models do not prove generic validity, they cannot be dismissed a priori either; witness the example of the hydrogen atom spectrum predicted correctly from symmetric model, against the complexity of solving full QED. He next summarised the by now familiar results of LQC: the Big Bang singularity is replaced by a bounce, both in the zero and positive curvature cases. An important feature is that the correct semiclassical limit heavily constrains how ambiguities in the Hamiltonian are resolved. Similar bounces avoid the singularity in black hole spacetimes, showing that there is no information loss and that evolution is deterministic throughout the quantum regime into a new classical region. It is also known that these bounces are stable against small perturbations.

Carlo Rovelli asked a question at the end of Ashtekar's talk, one that has worried me for a long time, that I discussed briefly here about a year ago, and that has recently been discussed at Cosmic Variance (see previous post here for the link; I can't access CV now). In our universe, the Big Bang was a state of uncommonly low entropy; this ensures the existence of an arrow of time because entropy has naturally grown since then. If there was a collapsing phase and a bounce before the Big Bang, what was happening to entropy in it? Symmetry seems to demand it to decrease –but "naturally" a gravitational collapse increases entropy to a maximum, as in a black hole. The collapsing universe would need to be extremely fine-tuned for entropy to decrease in it.

I couldn't follow Ashtekar's answer to Rovelli, but later I found an oportunity to pose the question again to him in a coffee break. He said that while matter entropy is very difficult to analyze in the simple models that have been studied so far, gravitational entropy –the "likeliness" of the gravitational state- does indeed seem to behave symmetrically in the bounce models. The quantum regime near the singularity is a very special, intrinsically low-entropy state. I have been convinced by Frank (fh) in the discussion here a year ago that if this is so, the most natural description of the situation is not "a previously collapsing universe with decreasing entropy followed by an expanding universe with increasing entropy" but "a low entropy state that expands, increasing entropy, in both two time directions". In other words, it seems more natuural to define "positive time direction" at each of the two stages by the increase of entropy, even if this gives two different results and time is no more a "line" but a "double arrow". Surely, if there were observers in the (from our point of view) "collapsing"phase, they would take themselves to live in an expanding universe, if as it seems almost certain the psychological arrow of time is tied to the thermodynamical one. Ashetekar however, didn't seem to think much of this point of view (probably dismissing it as too philosophical). For him the scalar field that serves as "internal time" in these quantum cosmology models is the true "clock", and it is monotonically increasing.

I am still puzzled, however, about what happens with entropy in the closed universe model (postive curvature without dark energy). This one becomes under quantization cyclical, expanding and contracting again at regular rate. What happens when the apex of the expansion is reached? does entropy reverse itself suddenly, as in Gold's old cosmology? but how can this be, if the moment of maximum expansion is completely classical and localized systems should follow ordinary mechanical and thermodynamical laws without knowing about the cosmological turnaround? I find this very perplexing. A possible way out is that the existence of dark energy with its actual value, which accelerates the expansion and ensures that the universe is not cyclical, is somehow not an accidental but a necessary feature of the universe, so that the cyclical model will ultimately be shown to be inconsistent. But this is only a personal hope. See also my old review of Price's book on the arrow of time for more discussions of these questions.

Going on with the conference: Carlo Rovelli talked next about the new spinfoam vertex, an improved model that pretends to replace the Barrett-Crane one. He discussed at length the graviton propagator calculation he and his collaborators did a couple of years ago, explaining that since then the nondiagonal terms of the propagator had been computed and found to be wrong –but only because the Barrett-Crane model was used! Using the new model the problem is solved. The key difference is that second class simplicity constraints are imposed weakly rather than strongly. In the improved model the bondary states of spin foams match exactly the spin network states of canonical LQG, and intertwiner degrees of freedom remain free. (There were some technicalities about all these that I couldn't follow, but if you are interested download the slides and audio; it was a very clearly delivered talk.) The conclusion was optimistic: Carlo believes that this model may be the key for reconciliating the "canonical" LQG approach and the "covariant" spin foam one.

Paralell sessions

The talks I attended to this afternoon were mostly about highly technical aspects of LQG and spin foams, and I don't want to bore neither me nor you by writing much about them. I will comment only on two of them which were of special importance, to me at least. Kristina Giesel talked of the work she did with Thiemann on Algebraic Quantum Gravity, a new version of LQG which is defined in a purely "combinatorial" way; spin networks are abstract graphs and not embedded in any pre-existing manifold. Semiclassical analysis, however, can be done by specifying a 3-manifold and a classical phase space point in it, and constructing coherent states peaked on that geometry. The zeroth-order and first-order in hbar of the expectation value of the master constraint in these states come out correct; what is unknown is whether there are anomalies in M or whether 0 is in its spectrum. The second talk I want to remark upon was Eugenio Bianchi, on work related to the graviton propagator calculations. He showed computations of large scale area correlations in spin foam models, for boundary states peaked on a classical geometry, and showed that they agree exactly with those computed in perturbative Regge calculus. The point is that correlations calculated in a semiclassical state of the full, nonperturbative theory are here compared with correlations in the vacuum state of the perturbative theory around a corresponding classical solution. Finding agreement is a nontrivial check for the spin foam model. In this case the model was Barrett-Crane, but Eugenio thinks the results still hold in the "improved" model Rovelli had talked about.

And this is enough for today. The rest of the conference will be covered in one, or perhaps two, following post(s). As usual, stay tuned!

Loops 07: pics and links

2007-07-05T16:30:00.000+01:00

I am adamant in my decision of not writing physics posts while on holiday, so the report about the talks will have to wait. But my self-imposed ban does not cover pictures and links about the conference:

The cathedral of Morelia, just a block away from the university centre were the conference was.

Ileana and her unicorn-shaped balloon.

Frank expresses his ignorance -of what, I don't know.

Cecilia "la Madonna" Fiori.

Bee has already posted on the conference, including a photo of yours truly. My detailed report will start coming in about one week. Meanwhile you can see the slides and audio of many of the talks clicking around from here. My slides are not there yet (I used transparencies and didn't get around to scan them yet). Tangentially related is this Cosmic Variance post, on a subject that fascinates me and that came up during the conference. More on this later.

Away again

2007-06-21T16:03:00.000+01:00

Tomorrow I wil be heading off to Loops 07, in Morelia, Mexico. I will write my usual conference report afterwards, but that will not be until mid-July. I'll be some time on holiday in Buenos Aires after the conference, and holiday includes holiday from writing work-related posts. There will be other bloggers present at Morelia (Bee, Yidun, and perhaps others I am not aware of) so maybe you'll get multiple perspectives on the meeting.

An examination of Dawkins’ “Ultimate 747” argument

2007-06-11T00:26:00.000+01:00

Richard Dawkins’ master argument against the existence of God is spelled out in the fourth chapter of The God Delusion, which is confidently titled “Why there almost certainly is no God”. It may be summarised as follows:

1- God is supposed to be an ultimate explanation for everything there is in the universe, and especially (according to design arguments) for its organized complexity and its friendliness to life.

2- But any God that was responsible for consciously designing the Universe must have a huge degree of complexity itself –at least equal to that of the whole universe.

3- Things with large degrees of complexity are statistically unlikely and stand in need of explanation.

4- So God is an inadequate “ultimate explanation” for the universe –the only possible explanations of complexity are those which, like natural selection or anthropic selection, build up complexity from simple underlying natural laws. (Cranes, not skyhooks, in Dennett’s motto.)

It is obvious that the whole argument rests upon the notion of “complexity”. Just exactly what sense of this concept does Dawkins have in mind, for which very complex things need explaining?

Can it be mathematical complexity as defined in information theory (Shannon information)? But in this sense, a set of particles organized as a living organism would have less complexity than the same particles in a random, uncorrelated state, because you would need more information to specify the latter state than the former. This does not seem right. Can it be the inverse of Shannon information, then –a measure by which random, uncorrelated states have low complexity and highly correlated, low-entropy states have high complexity? But then Dawkins argument fails completely, because physics tells us that the initial state of the universe had very low entropy, and entropy has been growing since. While Dawkins would like to say that in the history of the universe complexity is built from simplicity via the mechanisms of natural selection.

I think there is no rigorously definable sense of “complexity” that has the properties Dawkins requires of it. Ironically, it seems close to the “specified complexity” of Dembski and other creationists! A concept which, as Mark Chu-Carroll’s and others’ criticisms have made clear, has no more technical definition than “something that looks designed”; or, in Behe’s more pedantic but no more precise phrase, “purposeful arrangement of parts”. We can accept the notion, however, as something we understand intuitively and recognize in living beings but cannot at the moment define rigorously, and go on examining the argument. (Accepting this concept as meaningful does not entail accepting Dembski’s creationist arguments, which pretend to prove pseudo-mathematically that complex things in this sense cannot arise by natural selection; because they can.)

Next, let us examine whether this concept of complexity applies to God. Many theists dig their heels here, asserting that the God they believe in is simple. I don’t think this is the best strategy for them. Whether or not there is some metaphysical sense in which their concept of God is simple, I think it is clear it is not the one that Dawkins has in mind. If God is all-knowing, then he must contain at least as much information as the universe, and not only information in the mathematical sense that applies also to random sequences, but information that contains organization, purposeful patterns, and all the marks of “complexity” in the sense we are interested in. A human mind is hugely “complex” in this sense, and God’s mind must be incommensurably more complex.

No, the key premise that must be examined critically is the third one. First, let us clear off that confusing talk of “statistical unlikeliness”. Something “complex” is also “statistically unlikely” only if it appears as result of a stochastic process whose underlying laws do not single out that particular result among many others. For example, the proverbial tornado blowing through a junkyard and assembling a Boeing 747 would be miraculous, because the “complex” outcome results from a process that could also produce billions of other outcomes which are not complex, and we know that the laws governing the behaviour of tornados do not discriminate among those outcomes singling out the complex result as more likely. Just as throwing a coin a thousand times and obtaining always heads is surprising only if we thought that neither the coin nor the way of throwing it were biased. But in the case of God we do not have this knowledge of an underlying process with laws blind to the results; in fact, it would be absurd to postulate such a process, because God is supposed to be an ultimate explanation.

This is really a key point. Dawkins says, in effect: “Look at the typical ID examples of “complexity”; be it the eye of an organism, or the fine-tunedness of laws of nature. They are very statistically unlikely, and so they need an explanation. Design is useless as an explanation because the designer would itself be in need of explanation for the same reason. So “cranes” that build them up from simple things, such as natural selection, are the only possible answer.” But the sentence I have bolded is by no means obvious in absence of context and underlying laws that make the complex thing really “statistically unlikely”. It is certainly not an a priori truth. Think of an Aristotelian view of nature, for example: imagine that species are fixed, each has a “form” or essence which all individuals of the species embody better or worse, and the organisms come to exist and develop by teleological laws, striving to fulfil their potentiality as befits their essence. This is a consistent way the world could have turned out to be, and in it “complex” things do not need any special explanation: the basic laws and principles of the universe make reference to them.

Of course, the real world turned out not to be like this. In the real world the basic laws as far as we know them do not make reference to complex beings, but are completely “blind” to them. Or at least, the whole tendency of the past four centuries leads towards such a conclusion. It is only in the context of this worldview where the apparent design of living organisms raises a problem. Aristotle (to my knowledge) did not use design arguments for the existence of God. Even in the eighteenth century, Hume in his Dialogues still feels free to counter the Design argument by pointing to causes acting in nature such as “generation” and “vegetation”, that are different from design but can produce complexity. But once the post-Newtonian view of the basic laws of the universe as purely “mechanical” (devoid of all teleology) became dominant, how living organisms can come to exist became a pressing problem: for they are really “statistically unlikely” given blind, mechanical underlying laws. Both Paley and Darwin tried to address this problem –only the latter doing it successfully, in a way fully consistent with mechanism and borne out by the evidence.

What Dawkins is doing is to take this story and elevate it to the cosmological or even the metaphysical level. The problem I have with him is not so much taking this step; it is more not acknowledging that it is a step, that it needs to be justified, and that it may reduce the certainty of the conclusion. Taking as a given that complexity is statistically unlikely and needs explanation, Dawkins thinks that God as ultimate explanation is extremely unlikely; I think the “almost certainly” qualifier in the title of the chapter refers really to the infinitesimal probability that something as complex as God may exist “by chance”; hence the “Ultimate 747” moniker. But as I argued, to discuss statistical unlikeliness in absence of underlying laws is vacuous. So Dawkins should have instead argued: “In our experience, and according to the scientific knowledge we have gained in the last centuries, complex things are explained by cranes that build them up from simple tings and laws of nature that are ultimately blind to the complex things. In our experience, complex things stand in need of explanation. So to postulate a very complex thing (such as God) as ultimate, unexplained explanation to the universe is unlikely to be true.”

I think this is a reasonably strong argument, one that may justify someone in calling himself an atheist instead of an agnostic. I am willing to endorse it myself. But it is much less powerful that Dawkins thinks his argument is. The “unlikely” in the conclusion is purely epistemic, not statistical as Dawkins’. The argument makes an inductive leap extrapolating from the current picture we have of how the universe works to say that it is unlikely that its trend will reverse at the cosmological or metaphysical level. If the appearance of teleology is reducible to mechanism or explainable from it in the biological world, then it is most unlikely than teleology (in the form of the intentions of a God) can be an ultimate explanation at a metaphysical level.

This argument assumes, of course, that all instances of teleology are reducible, including its more sophisticated offshoots at a human level, such as consciousness, intentionality and morality. In other words, the argument assumes the naturalistic program in philosophy and stands or falls with it. It must be admitted that this program has not been fully completed yet, but I see the whole course that science has taken since Darwin (if not since Galileo) as pointing towards the plausible truth of naturalism. As long as we don’t have a complete naturalistic account of these higher and more complex forms of teleology, however, I will see theism as a “reasonable” stance someone can take: mind, meaning and morality are pervading in such a way in all our human life, and they come so naturally and irresistibly to our prescientific imaginations as “basic” things, things that can explain things but require no explanation, that it is perfectly understandable that most people think they must be part of the ultimate cause of the universe, instead of late newcomers ultimately built upon blind mechanism, as I am confident they are.

Inquiring minds want to know

2007-06-02T18:21:00.000+01:00

Google Analytics is way more powerful that Sitemeter. With its help I have uncovered some interesting questions that some people have typed into Google reaching this blog as a consequence. In loose order of increasing weirdness:

are any two things completely identical

world religions influenced the way you think about religion?

if you wake up at a different time and a different place would you be another person

human beings cannot survive without communications, discussi

how does a panda use it's body parts to survive it's life

how was carl gauss teased

what would happen if there was no gravity in school

why my imagination doesn't affect reality

I really like the last one. It brings to mind Philip Dick's dictum "Reality is that which, when you stop believing in it, doesn't go away". In which case, the answer to the question is: "By definition!

Also, it is really strange that my blog is the top Google hit for the panda question, given that I have never blogged about pandas.

Recent Reading

2007-05-29T22:32:00.000+01:00

Some notes on books I have read in the past two or three months

-Philip Roth, The Plot Against America. First book I read by him, this an alternative history in which famous aviator and anti-Semite Charles Linbergh was elected president of the United states instead of Roosvelt in 1940. Told from the point of view of an American Jewish child growing up in a climate of increasing fear and persecution, the story is remarkably well-written and the political setting chillingly believable. The largest problem is a deus-ex-machina ending which solves rather too simply all the problems created in the story

-Denis Guedj, The Parrot's Theorem. A popularization history of mathematics thinly disguised as a thriller, in the line of what Jostein Gaarder did for philosophy in Sophie's World, but less successful. The mystery plot is too contrived and implausible too keep the attention of the reader, and the mathematics is very dumbed down (there are almost no equations). If you are a layman wishing to learn some mathematics, stick to nonfiction, like Simon Singh. Fiction can be excellent if it embodies mathematical ideas, as in some Borges stories for example, but it is a poor medium for explaining mathematics in a pedagogical way.

-Laurie M. Brown (ed.), Renormalization: from Lorentz to Landau (and beyond). Collection of essays on the historical progression of ideas on renormalization in quantum field theory, including an excellent essay by Tian Yu Cao on the "new philosophy" of effective field theory. Recommended.

-Kate Fox, Watching the English. An amusing anthropological study on the cultural customs of that strange tribe, the English. With all the rules to understand pub discussions, ironic humour, awkward introductions, obsessive queuing, subtle linguistic class distinctions, and much more. A must read if you live here or are planning a visit.

-Ursula K. Le Guin, The Dispossessed. First I read by her, a story in classic SF style about two contrasting alien societies, one anarchistic and one capitalistic, with the protagonist being raised in the former and then moving to the latter. Some insightful social philosophy compensates for a somewhat slow plot. Overall I liked it and I'm likely to try more by her.

-Terry Pratchett, Darwin's Watch. Once more, this is the first book I've read by him. (Seriously!) Our universe, Roundworld, is kept in a one-foot long glass globe in the meta-universe of Discworld, where the wizards of the Unseen University have to find a way to interfere with human history to ensure that a certain Charles Darwin writes the "right" book instead of the one he has written in the present timeline, a vindication of Paley's design argument by the title of The Theology of Species. (In this alternative timeline, the true theory of evolution was not discovered until a hundred years later by one "Rev. Richard Dawkins" -Rev. because only theologians could be biologists after the success of Darwin's book. I loved that detail!) The book mixes comic fantasy with scientific popularization chapters written by Ian Stewart and Jack Cohen. Pratchett's style of humour reminds me of Douglas Adams', a particular kind of British humour which is engaging at first and becomes tiring after longer exposure. I have not become tired of Pratchett's yet, though, so I am likely to read some more by him. I know there is a huge literature on Discworld: any particular recommendations?

-Karen Armstrong, The Great Transformation. A serious book on history of religion, to compensate for the one that follows on this list. Armstrong traces the development of religious and philosophical views in China, India, Israel and Greece in the so-called Axial Age, ranging from 900 to 200 BC, the time of Buddha, Confucius, Laozi, Isaiah, Jeremiah, Pythagoras, Socrates, and many other key figures that created ways of thinking about the universe, mankind and our ethical duties that are still living with us. Fascinating reading about a period that has interested me since I read Gore Vidal's novelistic account in Creation. The only thing that put me a bit off was Armstrong patronizingly saying that the Greeks only entered halfways in the deep spirit of the Axial Age; to my mind inventing scientific rationalism, tragedy and democracy is a much more impressive achievement than inventing ethical philosophies which, no matter how sublime and insightful, few people ever followed to a full extent. But we are all entitled to our biases, aren't we?

-Richard Dawkins, The God Delusion. Yes, I after all that fuss and reading reviews and reviews of reviews and commenting on them, I finally read the book. It was less substantial than it seemed by skimming through it at the bookshop, and more or less what the reviews would have lead me to expect: a sort of Atheism 101 textbook. The level is the same as your average Pharyngula comment thread: some insightful, learned or witty commentary mixed together with a lot of trite arguments, simplifications, and even childish jabs. I love the Flying Spaghetti Monster, for example, but we must admit that to use it in a (anti)theological argument is equivalent to losing all pretensions of academic seriousness. That is not to say that I decry the writing of this book: there are probably a lot of people who have been indoctrinated with dogmatic faiths for whom reading it could have a liberating effect, and the outreach of this kind of book is far larger than that which a scholarly refutation of the latest argument by Plantinga could have. I would much prefer to read the latter, but then, I became an atheist long ago already, and partly by reading, reflecting on and re-elaborating by myself arguments that were at first no better than the ones Dawkins champions. If Dawkins was only intending to write a simplified "atheistic primer for the masses" I would be less annoyed than I am when I see that he thinks his arguments can challenge the views of serious theologians. The main point at which this happens is at his famous "Ultimate 747 Argument"; I will probably dedicate a post soon to analyze it and show, contra Dawkins, that is doesn't prove that "there is almost certainly no God" -but that an argument related to it can be used to argue that there is "probably" no God, though this "probably" is much more qualified and uncertain than Dawkins'.

Obligatory Harry Potter predictions post

2007-05-22T23:11:00.000+01:00

(Warning: this post contains huge spoilers for all the already published Harry Potter books. And perhaps for the seventh as well, though I’d rather wish not, and be surprised…)

Well, it’s now less than two months’ time to the publication of Harry Potter and the Deathly Hallows, and everybody and their mother are posting their theories and predictions. And though I have arrived lately to the fun (I didn’t think much of the early books when I read them for the first time, and only became a true fan after the sixth), I don’t want to miss the chance of embarrassing myself with confident predictions that will be shattered to pieces when we read that “the Giant Squid is actually the world's largest Animagus, which rises from the lake at the eleventh hour, transforms into Godric Gryffindor and...” (as Rowling has whimsically said)

So, let’s go. There are two main points of controversy among fans: whether Harry is or not a Horcrux, and whether Snape is Good or Evil.

On the first, I am quite sure that he is. Dumbledore’s theory that the snake Nagini is the sixth Horcrux is an obvious red herring; it struck me as dodgy even the first time I read it. Harry being a Horcrux explains too many things:

1) The strange and unnatural scar is likely a mark left by the Horcrux-creating spell, and probably the precise locus where the soul fragment is enclosed.

2) The telepathic connection with Voldemort, which is natural and expected if they share parts of the same soul.

3) The transmission of Parselmouth power to Harry.

4) The fact that Harry could destroy so easily Riddle’s diary in CoS, while the much wiser and abler Dumbledore lost a hand trying to destroy the ring Horcrux. It is likely that the curses with which Voldemort protected the Horcruxes would not act against the same soul they harboured.

5) The fact that Voldemort couldn’t sustain possession of Harry at the end of OoTP. It is likely that something goes screwy if you try to put together again the two disjointed soul fragments.

6) The fact that in HBP Voldemort orders the Death Eaters not to kill Harry, as we know courtesy of Snape.

The way I see things, Voldemort had planned to use the murder of baby Harry as means to create his last Horcrux, as Dumbledore surmised. The spell went wrong thanks to Lily’s sacrifice and Voldemort’s original body was killed (what happened to it, by the way? It was not found. Did Pettigrew dispose of it? It’s difficult to believe he would have cared to.) but the Horcrux was created anyway. Voldemort didn’t realise this at first, and that’s why he implied to the Death Eaters in the graveyard scene in GoF that he had not archived immortality yet; he thought he needed one more Horcrux to have the magical number of seven soul-parts. He still hadn’t discovered the truth by the end of OoTP, when he tried to kill Harry with an Avada Kedavra. He probably figured it out after the failed possession attempt, and that’s why in HBP he switches his attention to getting Dumbledore killed and gives explicit orders that the Death Eaters not kill Harry.

So, will Harry eventually have to sacrifice himself to destroy Voldemort? I’m afraid it is very likely. But Rowling has also said in an interview (upon asked about her religious beliefs) that she is a Christian but that “if she talked too freely about that any intelligent reader, whether 10 or 60, will be able to guess what's coming in the books”. Putting this clue together with the previous line of reasoning I conclude that we are heading for a resurrection scene. Of course, it may be that she was thinking only that Harry sacrifices himself to save the world, but I don’t really think so. It would be too gloomy an ending, and also contradict the Prophecy. It is more likely that he will intend to sacrifice himself, but somehow only the fragment of Voldemort’s soul is killed while Harry miraculously survives, perhaps after passing by dead for some time. I also think (hope?) it is unlikely that Ron, Hermione or Ginny will take a bullet, although the future looks very dark indeed for Hagrid.

Now, about Snape. The first thing here is that the “Evil Snape” scenario that Harry and his friends accept at the end of HBP does not hold water for a second. There are too many indications that Dumbledore had arranged with Snape that he should kill him if the situation came to a crisis in which there was no other way out. Let’s go through the main arguments:

a) Hagrid overheard a discussion in which Dumbledore ordered Snape to do something Snape seemed to be reluctant to do. This did not receive any explanation by the end of the book. The most reasonable theory is that the order was to kill Dumbledore if it came to be necessary to preserve Snape’s status as spy, and indeed his life, since he had made an Unbreakable Vow.

b) Dumbledore obviously knew everything about the Unbreakable Vow and Draco’s mission; this is made clear both by his nonchalant reaction to Harry’s information about the conversation he overhead between Snape and Draco, and by his conversation with Draco himself in the tower. He knew it from Snape (from who else?) Snape would not have told him about all this if he was really Voldemort’s servant.

c) Dumbledore’s plea “Severus… please…” only makes sense as a plea to kill him. Dumbledore would not have begged for his life, and the alternative theory that he was begging to Snape not to kill him, for the sake of Snape himself, is still unconvincing. If Dumbeldore had believed up to that moment Snape that Snape was on his side, why would he plea not to kill him? He would have expected him to tackle the Death Eaters instead.

d) Snape’s extreme reaction against Harry calling him “coward”, which makes sense if he has just done something very brave.

e) Snape only taunting Harry but not hurting him with any serious curse when he has him at his mercy.

f) We still haven’t discovered the reason why Dumbledore trusted Snape’s loyalty (it is not, as Harry assumed, just remorse on being cause of the death of James and Lily. Dumbledore never said that nor even implied it, if you read carefully.) We must be explained that in Book 7, and from a storytelling standpoint it would be all wrong if we were explained this point that was a key secret for the whole series in a context of “ha, that fool Dumbledore believed this!”

These arguments and other ones have convinced the majority of fans that Snape is “good”, and I have seen many an essay that portraits him as a loyal agent of Dumbledore, courageous and selflessly dedicated to the task of vanquishing Voldemort. I have difficulty picturing him in this heroic role. As he is portrayed in the books, he is petty, jealous, unfair, vindictive, and often sadistic, not only with Harry but with all his students except a few Slytherins. This ambiguity is, of course, the reason that makes him so fascinating.

My view of Snape is that he is an essentially self-centred person who does not live by ideals of good and evil, but simply tries to survive. He may care for a few other people (his concern for Draco seems genuine and not exclusively motivated by his Vow) but he does not care for saving the world from evil or anything like that. I would speculate his strongest desire is to be free from teaching stupid kids and be able to study the Dark Arts at leisure. Not for desire of power, but just because they fascinate him. Snape is not a Dark Arts freak; he is a Dark Arts geek. After signing up with the Death Eaters and seeing what it was all about he changed his mind, because no matter how much he hated and craved revenge on a few people like James Potter and Sirius Black, he did not have a taste for having absolute power over everyone or making everyone suffer. He just wanted everyone to leave him alone. And he was intelligent enough to realise that under Voldemort’s thumb he would be even less free that under Dumbledore’s.

So he sought Dumbledore’s protection to try to get out safely of the Death Eaters gang, and he found he was trapped. Because he was in an ideal position to spy for Dumbledore, and could not refuse his insistence to do so; besides, he could not openly leave the DEs or they would have killed him. So he was forced since then to live as a double agent, a role he didn’t play out of heroism but out of necessity and for which, as an Occulmens, he was eminently suited. In HBP he was under terrible pressure from all sides (Voldemort, Dumbledore and the Vow he made to Narcissa) and finally had no option but to kill Dumbledore, which was where all three sources of pressure pressed towards.

Dumbledore didn’t care to continue living once he had put Harry on the track to destroy the Horcruxes, and thought Snape’s life was much more important than his to provide “inside help” to Harry in his mission. But Snape did not want to be put in this situation. He lost his protector, and is forced to fight alone now playing his role with Voldemort and having to hide from the rest of the world who sees him as a murderer. Moreover, he has (at least, on Dumbledore’s orders) to assist Harry and provide him with the help he requires to play his role as Chosen One and destroy Voldemort. And Snape sincerely and utterly loathes and despises Harry. He has been forced by circumstances and Dumbledore’s plans into a position he never wanted to be in, to play a role which he hates but sees no other option than performing. He is not a “born hero” as Harry is: he has been thrust into a hero’s place against his will. That’s why he could feel sincere hatred and revulsion when he cast the Avada Kedavra at Dumbledore, and that’s why he got into a frenzy at being called coward. He is now unstable and unpredictable. He may still overcome himself and help Harry as Dumbledore wanted, but he may also snap and try to play his own game against both Voldemort and Harry, or even switch sides to Voldemort at the last moment.

As you see, even after having reconstructed his whole story and (I believe!) successfully puzzled out the riddle of his character I am still unable to predict what role will he play on the final denouement and what his end will be. I used to think he would die a noble and “redeeming” death sacrificing himself for Harry, but I realised later that this role or a very similar one is already reserved for Peter Pettigrew. At the moment I am inclined to think he will grudgingly do the right thing, stand on Harry’s side, and that after destroying Voldemort both will be forced at the end to revise their view of each other, admit their respective virtues, and thus take a large step in growing up. (For all his cleverness, Snape is very immature in many ways –witness his petty taunts against Sirius in OoTP, and his behaviour towards Harry in the whole series just on behalf of his father.) This –Harry coming to terms with adulthood in forgiving and accepting a matured Snape- could make an excellent ending if it is played well. But perhaps I am being too optimistic -this would be an ending too rosy for the darkness that has encompassed the series lately, wouldn't it?

Well, I have many more ideas, but you really must be tired of hearing them. What are your own predictions?

Link and Notes

2007-05-17T23:39:00.000+01:00

I've been a Bad and Lazy blogger lately, and I am likely to remain so for some time. The reason is that I've started to write a new paper, and when I pass most of my working hours writing I don't feel like writing for fun in my free time. Writing for fun works better as a rest after a day spent calculating. (What, you say I should also consider the possibility of getting a life? What is that?)

So here a few interesting links I have collected lately, all of them on the kind of subjects you might have seen at some time discussed in this blog:

-The FQXI Community webpage for fundamental questions in physics looks very cool. I recommend especially Matt Leifer's post "Is the world made of wave-vectors?", an extremely clear and concise summary of the pros and cons of "ontological" and "epistemic" interpretations of quantum mechanics.

-I don't intend to become any sort of chronicler of the new papers appearing in the quantum gravity community (the guys at Physics Forums take care of that) but given that this new spin foam model appeared today and seems possibly important, I thought I might just as well link to it. Also there is this recent one by Ashtekar, based on his "LQG FAQ" talk at MG11, which I blogged about last year.

-At the n-Category Cafe, a fun quiz to give your probability estimates for whether a number of quasi(?)-mythological(?) figures (ranging from Gilgamesh to Robin Hood) actually existed. The discussions in the comments involve both what it means for X "to have existed" (descriptive vs. causal theories of proper names) and what it means to assign a probability to it (Bayesianism vs. others).

-Via Thoughts, Arguments and Rants I found this interesting paper, which describes a recent mathematical result which may be of unexpected relevance for the centuries-old philosophical problem of induction. Would old David Hume ever have guessed that there is a rule which, given any function on the real numbers and its values up to (but not including) a certain T, can predict its value at T correctly, for almost all values of T? I'm sure not! What I'm not sure is how relevant this is for the traditional problem -especially given that the theorem only assures the existence of a rule, without explicitly providing one.

-Talking about Hume, Brandon has been putting up some notes on reading and interpreting his Dialogues Concerning Natural Religion. This is one of my favourite philosophy books, so I am following this series of posts with great interest. The posts are: Part one and two, Part two and three, Part four and five.

Quantum Mechanics in words of one syllable

2007-05-08T23:29:00.000+01:00

Some think Quantum Mechanics is impossible to explain in simple terms. To refute this idea I have composed this piece, which is inspired both by the “Theory of Relativity in words of four letters or less” and by “Philosophy in words of one syllable”. I have not used any kind of dictionary or thesaurus.

In this piece we will talk of small things and how they are. How small, you ask? Well, close to the scale of Planck’s h. This will mean the bits of stuff the world is made of: the bit of charge, the bit of light, and so on. These small bits are quite weird, not at all like the large stuff we are used to.

If you have a set of those bits, at each time it will be in a state. We write the state as a ket, as Paul taught us. The ket for state A looks like this: IA>. The ket has in it all one can know of the small bits we talk of. The state will change in time, of course. To say how much it will change per sec (its rate of change) we have a rule Er told us: i times the rate of change of the ket is H times the ket. H is a key thing; it keeps track of how much stuff is there is, the mass of each bit of stuff, which is the force that acts on each bit, and so on.

So we know how states change, fine. How do we get from this to a claim on what we will see in the lab? Here is where things are not like we are used to. In the large world we are used to, when we know the state of a thing we know that if we look at the thing we will see it in that state. But here, if we know the state we can’t tell in which state will we see it. We will have a chance to see it in a new state. If it was in IA>, it may be that we see IB> when we look. We can’t tell for sure. But we can tell what chance we have to see each state.

For that we use a rule that a guy called Max Born gave us. To get the chance to see state B when you look at some stuff that is in state A at that time, do some math called "ket A dot ket B", or IA> . IB>. What comes out of this math has to do with the chance to see B if the state is A. But it is not quite that, ‘cause the chance must be real and what we have now turns out to have i in it. Darned math! But just take the square of the real part and add to it the square of the part with i (that is, take the norm of what you have) and you will get the chance to see B if the state is A.

There are lots of things one can want to look at when one looks at stuff in the lab: Mass, charge, where bits are, how fast they move, and so on. For each of these things there are some states called “self states” of the thing. When the state is a self state of a thing, we can tell what we will see if we look at that thing. But in most states we can’t. As a case, if we add two self states IA> and IB> of a thing the new state IA> + IB> will not be a self state for that same thing. If this is the state, when we look at that thing we may see A or B, each with a chance of one half. Once we have looked, the state will be A or B; but not till we look.

This leads to the well known case of Er’s cat. If the state of some bits of stuff is IA> + IB>, and we put those bits in a box with a cat, and make things such that state A will in due time kill the cat, while state B lets it live, then the cat will not live nor die till we look in the box! Weird, huh?

I know you must want to say: “No way! For sure, the state was A or was B all the time, we just did not know which till we looked!” No such luck. If the state is IA> + IB>, then it may be that the A part and the B part “mix” and we can tell that the state was not “A or B” with a look at some things in which you see that mix. It is hard to do for a large thing, like a cat, ‘cause you would have to keep track of each bit of stuff; but it has been done for small things, and there is no clear line that breaks small from large.

Old Al did not like all this stuff. He was sure God did not play dice with the world. He and two pals came up with a thought to show all this must be wrong. Take two bits of stuff, say two light bits. Let the state be one in which both bits must have a thing not the same; say, one of them has “spin up” and one “spin down”, but the state makes not clear which has which spin. (Spin is like a turn ‘round that the bit may have; this turn may point up or down for each bit.) Let the two bits move a lot one each on its way, so they come to be far. Now look at the spin of the first bit. If you see it “up”, the spin of this bit has changed from an “up plus down” state to “up”, and the spin of bit two has changed at the same time from state “down plus up” to “down”. But how can the state of bit two change when we look at bit one, which is far from bit two? It makes no sense.

But it does. A smart guy called John Bell came up with a slick way to test this stuff, and it turned out that Old Al had been wrong for once. One is forced to grant that bits of stuff can “know” what goes on far, far from them, or else that in a sense things are not “out there” till we look at them! More and more weird, I say. It may be that some day we will make sense out of this. By now, guys are still not of one mind on what one should say. But, at the same time, with all this we can work out, know and grok lots of stuff. So it must be true. So when one starts to ask a lot, like “what does it mean”, some guys say: “Shut up and work!”

Coming next: Quantum Field Theory in words of one syllable. Quantum Gravity is much easier; one just needs to write: “What?”

Teleportation: why you survive

2007-04-30T23:49:00.000+01:00

This is a belated follow-up to a post I wrote almost two months ago, in which I polled my readers as to whether teleportation (understood as destroying a body after recording its exact structure, sending the information somewhere else and re-constructing it there) would kill the original person and create a different with false memories, or whether the original person would survive as the teleported one. (If you missed that post you would do better reading it and the comments discussion before this one.) The results were 5-3 in favour of survival (6-3 including myself) but my friends Merrick and DrNitro voted for death, and this post is an attempt to convince them and other skeptics of why I would not be afraid of using such a machine, in the extremely unlikely case that it becomes technologically possible in the course of my life.

Let us first assume without discussion that the “self” does not reside in an inmaterial soul. I think my friends agree with this assumption. Their certainity that the teleportation experiment kills you is grounded in the implicit belief that you are essentially your body, and destroying your body destroys you. But this is not completely exact, because they would probably agree with me that you could survive the destruction of any part of your body, or all of them, with the exception of your brain. If my brain is kept functioning (perhaps in a vat, as in the old Matrix-like philosophical scenario) then all the rest of my body can be destroyed and I will surely survive. So the only question is: Is my survival tied to the survival of my concrete, material brain, or only on the survival of a pattern of brain structure, enconding my personality traits and memories, such that if a new brain comes to exist with this same pattern I continue living in it?

I will give now two arguments (actually, two thought exeperiments or “intuition pumps”) for the second option.

First argument: Suppose we perform the teleportation experiment with a guy –call him Ernest- while he is in coma or deeply anaesthetized. Suppose that we don’t destroy the original Ernest, but only scan his body and send the information to create a duplicate Ernest. (Technical note: as commenter Dmitry remarked, this is physically impossible if the teleportation must copy the exact quantum state of the original, as that is impossible without destroying it. I am assuming that the features of brain structure responsible for the self do not depend on the quantum state.) While both are still unconscious, we lie them side by side and randomly mix them up, so nobody can tell which is the original (not even us, if the randomization and arrengement is made by a computer that doesn’t save the information). Now we prepare to kill one of the copies and awake the other one.

According to my friends, it matters a great deal which copy we destroy. If we kill the original, we are murdering Ernest, and the awoken copy will be just an imitation with faked memories; while if we destroy the copy before awaking it, nothing bad has happened and Ernest goes on living normally. But how can this be? Both bodies lying there are completely identical in every detail that matters. Setting aside random influences that may have changed them since the duplication (which would have affected any of them with no preference) they will react in the same way if awakened, remember the same things, feel the same way. Which body was the original and which the copy is what Daniel Dennett would call an “inert historical fact”. It is a fact purely about the past, that cannot affect anything anymore in the present or the future. How can whether Ernest lives or dies depend on this fact?

To make this intuition stronger, imagine that the experiment is done with the brains. We first destroy Ernest’s body keeping alive in a vat his unconscious brain; we all agree that this doesn’t kill him. Now we duplicate the brain and mix up the two copies. We have in front of us two brains in two vats, exactly alike in all neronal details; any of them if awoken would claim to be Ernest. Unless we believe that there is an invisible soul mystically attached to the original brain of Ernest, how can it matter for his survival which of the two we choose to awake? If thought and consciousness are just brain activity, and the same brain activity would go on in both brains, then any of them we choose to awake will continue Ernest’s life equally well.

This was my first argument. My second one involves a different series of modifications we can make to poor Ernest’s brain. Let’s take anaesthetized Ernest and perform on him any of the following operations. (Assume they are technically possible –even if they are not, I don’t see any impossiblity in principle going on here, surely not more than in the teleportation experiment):

a) “Freeze” his brain so all living signs are completely disappeared, and then “relive” him.

b) Same as above, only that while the brain is “frozen” we cut separate a small part of the brain and then “patch” it together with the rest in exactly the same way it was. (We can decide how large to cut this piece, from a very small one to as much as half of the whole brain).

c) Same as above, only that instead of cutting only a part we cut his whole brain in many little pieces, and then put them back together again.

d) Same as b), only that we duplicate with our teleportation machine the separated piece and use the duplicate instead of the original to reconstruct the whole brain.

e) Same as c), only that we duplicate all the pieces and make the complete brain using the duplicates, or perhaps some of the duplicates and some of the originals.

Now, question for those who believe teleportation is death: In which of all these cases is the brain/person that awakes after the operation Ernest, and in which it is not Ernest but a mere copy and Ernest has died? For me, this is a non-question; the person that awakes has in all cases Ernest’s personality, memories, etc, so he is Ernest and it doesn’t matter which parts of the brain are originals and which not, or whether the brain has had a continuous uninterrumpted existence as a whole object. But for those who disagree with me there has to be a difficult question in deciding where to draw the line: they should say that Ernest has died in case e) (which is very similar to the original teleportation scenario) and that Ernest is still alive in case a), but what of the intermediate cases? Exactly how much of the original brain needs to have persisted for Ernest to survive? Any answer seems to be arbitrary. The simplicity and non-arbitrariness of my answer to these questions seems a good argument for my position.

Thoughts…?

More Polish pictures

2007-04-26T18:51:00.000+01:00

The webpage of the Quantum Gravity and Quantum Geometry School has put up some good photos from the conference, chosen as a selection from all the pictures taken by participants.

Science and Philosophy links

2007-04-24T15:55:00.000+01:00

A few links on topics intersecting philosophy and science:

* Matt Leifer argues against the many-worlds interpretation of quantum mechanics from a Quinean philosophical standpoint.

* Lubos points to a paper by Hartle and Srednicki criticizing sharply the way the anthropic principle and related "typicality" ideas are used in cosmology. The gist is that, if we do not know that we were selected as random observers by a physical process, we should not calculate as if we were.

* David Corfield discusses two cultures within mathematics: the theory-builders and the problem-solvers.

* Richard Chapell discusses whether philosophy needs science. He thinks only perhaps in practice, not in principle. I disagree and explain my reasons in a comment there.

* In the latest Philosophers' Carnival there is a special entry featuring Dan Dennett's article "Higher order truths about chmess". I had read it some time ago, and I definitely recommend you to read it as well; it criticizes certain aspects of contemprary philosophy, in a way which could also be applied to contemporary theoretical physics. (It sits well with a remark made by Carlo Rovelli in the talks blogged about previously, that we should focus on problems of physical interest and not in technical investigations about unphysical situations that are good for publishing quick papers. It also links somehow with my comment at Richard's blog.)

Report on the Quantum Gravity School: the discussion

2007-04-16T23:47:00.000+01:00

Carlo Rovelli's talk on "Where are we on the path to Quantum Gravity" was scheduled to have two parts, one on the evening of each of the free days we had. He chose a different format: the first day he talked, and the second day he read a list of questions from a sheet of paper that had circulated among the audience on previous days, and gave his opinion on them encouraging people to give theirs and discuss. I think this was an excellent method, and the only problem with it was that we didn't have unlimited time; the discussion session started at 6 pm and the dinner at the hotel closed at 8. As the list had no less than 23 questions set for discussion (some of the first of them raising heated opinions, like the quantum measurement problem!), many of the last got a very short time, and the last six didn't get any discussion at all.

As I think my notes from this discussion session will be of more interest than those of the first talk, I limit this post to sharing these with you. Besides, Francesca has already a brief summary of some points raised in the first talk at this Physics Forums post.

Here, a list of questions written by people and the discussion they got, loosely organised by subjects in the way Rovelli read them:

1) Is there a relation between quantum gravity and the measurement problem in quantum mechanics?

Rovelli mentioned Penrose and t'Hooft as important voices on the affirmative. His own position is that, while regarding the foundational questions in QM as still open, they have no specific relation to quantum gravity; one may try to develop a quantum theory of gravity without needing to solve the philosophical problems. Someone objected that the problems overlap at quantum cosmology, where many people think you have to use the Everettian interpretation or some variation thereof because there is no "external system" to the universe. Rovelli answered distinguishing two meanings of "quantum cosmology" and "the wave function of the universe"; it might mean "a quantum mechanics that is self-contained, including no external observer", and then it relates to the foundations problem, but it has no specific connection with actual cosmology and with gravity; or it can mean "quantum theory of cosmological degrees of freedom", and then you need quantum gravity, but you don't really need to have a philosophy of QM for doing that, as you can have an observer within the universe measuring its cosmological properties and use naive "Copenhaguen" interpretation. I agree that this sounds sensible, but see the next question.

2) Has there been work using decoherence with relation to the classical limit in LQG (or spin foams or the other models we learnt about)?

This was the first of the three questions asked by yours truly. I was inspired to ask it by my conversation with Eugenio Bianchi about the graviton propagator calculation, in which I suggested that instead of fine-tuning the boundary state to ensure it remained semiclassical one could perhaps introduce an environment to keep it classical by constantly "measuring" it. Rovelli said he couldn't recall any work on this, and he didn't seem to think it was a very important idea. Relating it to the previous question, I think it may be important in the following sense: I agree with Rovelli that decoherence does not solve the philosophical problems of quantum theory, but in my understanding it does provide an excuse to forget about them when doing concrete calculations. For example, if you are an Everettian who thinks that the wave function "really" never collapses, decoherence tells you that in practice all results will be indistinguishable from those obtained by assuming a true collapse in measurements, so you can go on using "naive Copenhaguenism" and forget about the philosophy. But decoherence provides this only provided you have an environment, with statistical dissipation and especially a time asymmetry given by a direction of growth of entropy. Otherwise you could never get the appearance of time-asymmetric collapse out of unitary evolution of a time-symmetric theory. Therefore if you are trying to do quantum gravity in a fundamental, "timeless" way, then you may need to think carefully which philosophy of quantum mechanics you endorse and assume in your theory, because you don't have decoherence to provide an "effective equivalence" of the different interpretations. Either that, or you may try to introduce an environment and some kind of decoherence describing it with the "timeless" framework you are using. My question was pointing to this kind of thing. After the discussion someone approached me and told me that there has been work done with decoherence in quantum cosmology to model the passing from the quantum regime to a classical universe. In fact, I knew about this work before, but had forgotten about it.

3) How are singularities dealt with in quantum gravity?

Rovelli summarized briefly the results in Loop Quantum Cosmology that show avoidance of Big Bang singularity, and similar results for black holes. He also mentioned there is a lot of work done in string theory on the same issues. Someone asked whether there was a formal definition of singularities in QG, and Rovelli said that for the moment there wasn't and the usual criterion used for singularity avoidance was boundedness of expectation values of curvature operators.

4) The quantum theory of gravity, unlike the classical theory, has a preferred energy scale given by the Planck mass. Can this result in a broken or deformed diffeomorphism invariance at the level of an effective, semiclassical theory?

Rovelli seemed slightly dismissive of the idea, saying there weren't any reasons to think so. This question was posed by two friends of mine who read this blog, so hopefully they will argue for and defend their idea in the comments ;) .

5) What is the fundamental meaning of symmetry?

I have no notes on the discussion, except an "Unclear", and can't remember anything about it. Comments...?

6) Can Hawking radiation be derived from QG?

Not in the framework used for the famous calculations of entropy from counting area states, which assume a static black hole. Perhaps something will be possible in the dynamical horizons framework.

7) What can kill an approach to QG, aside from experiment?

An interesting question, not only for quantum gravity but for general philosophy of science, though of course the lack of experiments in QG makes it more pressing. Rovelli suggested two possibilities: An approach may die because a different approach is having greater successes, and then most people switch to work in it. Or people may simply become discouraged if fundamental problems persist unsolved for a long time, and the approach fades away.

8) Are there any experiments to test theories of QG?

Rovelli mentioned two possibilities: the "quantum gravity phenomenology" program to test Lorentz invariance at the Planck scale, and predictions from quantum cosmology which possibly may be observable in the fluctuations of the cosmological microwave background.

9) This, Rovelli said before reading it, was the MAIN QUESTION asked in the sheet of paper. Pause for suspense, and then:

"Is there any job for any of us in the future?"

Ha! Laughs (perhaps somewhat nervous laughs?) from the audience. Rovelli granted it might be difficult, but not so terribly difficult, and gave some statistics from his own students: about 60 percent of them had found positions. (But I reckon the statistics for those with recommendation letters from Rovelli may be positively biased!)

10) Has there been any work trying to use the background independent approaches and formulations we have heard about to attempt a background formulation of string theory?

Another question written by myself. I wanted to know if the divide between the sting and the LQG communities is really as large and insurmountable as the blog discussions often make it seem. Rovelli mentioned the Maldacena conjecture and its offshoots as an attempt to provide some kind of background independence in string theory, and string field theory as a previous program that wasn't very successful; but these ideas are not influenced by the other approaches than string theory. He also mentioned some work by Lee Smolin trying to merge stringy and loopy ideas, with little success, and the "loop quantization" of the bosonic string by Thiemann, which wasn't trying to be anything else than a toy model. In conclusion, almost nothing. Perhaps (if I dare to tread in the perilous waters of the String Wars (TM)) I will write some day a post with some more thoughts on the string-LQG divide. Meanwhile, if you haven't read yet the review of the Director's Cut of The String Kings you should do so immediately. Hilarious.

11) Concerning Asymptotic Safety, has Reuter's program been put to test in well understood theories like pure QED or the Electro-Weak interaction?

For some reason I don't have notes for this discussion. Anyone can contribute anything?

12) How do the results of the Asymptotic Safety program cohere with those that indicate a fundamental discreteness at the Planck scale? The fixed point that Reuter finds is obtained following the flow of the effective action all the way to k -> infinity, to much higher scales that the Planck scale, which doesn't seem to play any special role in it.

This was the third and last question I had asked, and it came out of some big discussion I and my friends had after Reuter's lectures. It seems that our discussion had been mirrored by some parallel discussion between Reuter, Rovelli and other "big shots" themselves. Rovelli agreed that it was a very important question, that could perhaps be studied by doing Reuter-style calculations in 3 dimensions, where quantum gravity is well understood. He also said that if there is fundamental discreteness at the Planck scale, this would appear in the effective action as nonlocal terms becoming dominant after that scale. (You are insisting in treating the situation as if there was a continuum, and then you get "nonlocal" dynamics because one discrete element of geometry affects the next one at a Planck scale distance.) This is another topic in which I invite my friends who understand Asymptotic Safety better than I do to contribute in the comments.

13) How does LQG treat spatial distribution of curvature?

Answer: Spin networks are discrete states of 3-geometry; from a spin network state, the 3-curvature can be in principle found and calculated.

14) Are there ambiguities in LQG, and what is their status?

Here I think Rovelli eluded somewhat the question. I would have been very interested in hearing a detailed exposition of exactly how many quantization ambiguities are there in LQG, and whether they are expected to be solved by finding some principled mathematical argument, or by requiring the right semiclassical limit, or by experiment (ie not solved at all). But instead he repeated something he says in his book, namely that, given the apparent inconsistency of general relativity and quantum theory, our immediate goal should not to find the unique and correct quantum theory of gravity, but just to find a consistent theory that includes both GR and QM. Once we have a consistent and well-developed theory we can worry about its uniqueness. [It is impossible to resist thinking that with this philosophy, a member of the "LQG community" loses the right to be snarky about the string landscape...]

15) Are there any problems with the Master Constraint operator?

Thiemann: "I am not aware of any."
Rovelli: "A real Thiemann answer!"

(More seriously, a bit of discussion lead to the acknowledgement of some technical issues which Thiemman trusts will be solved soon.)

16) and 17) Both questions were variations on "the problem of time".

Rovelli gave an answer which will be familiar to those who have read his book Quantum Gravity, and which I find persuasive. The motto is that physics is about relations and correlations of observables between many variables, and it is not possible in general to single out one variable as "time" and discuss the change of all the rest of the variables with respect to this one. One should first learn to do both classical dynamics and quantum mechanics with this "relational" way of thinking, and then apply it to quantum gravity. All this is discussed at length in the book.

Here was where the time constraint of dinner forced us to conclude the discussion. Rovelli read quickly the remaining six questions, which didn't get any discussion time. I write them below in case people wish to discuss them here:

18) Will the fundamental theory of quantum gravity be purely combinatorial, or will it use continuum differential geometry concepts? [This is a very interesting question. I am currently trying to read Thiemann's papers on Algebraic Quantum Gavity, which seem to be the farthest LQG has gone into the combinatorial direction.]

19) Is 3D quantum gravity equivalent to a spin foam model? [Rovelli said "Yes", and went on reading the next question.]

20) What is the heuristic explanation of the relation between LQG and Spin Foams?

21) What is the status of breaking of Lorentz invariance in LQG?

22) What are the observable effects of DSR or violations of Lorentz invariance?

23) Is the Barrett-Crane model correct?

And now, let the comments begin. We have no dinner constraint on this blog, so hopefully we can go on discussing till we find satisfactory answers to all the 23 questions...

Upstuff

2007-04-13T18:49:00.000+01:00

I had a long post on the discussion session chaired by Carlo Rovelli almost completely written and saved in Blogger's dashboard, or so I thought. But when I tried to sign in today to finish it and post it, Blogger forced me to upgrade to the new version using a Google account -and after going through that, I found my post was almost completely gone from the memory, with only the first paragraph remaining. I don't know if it is somehow Blogger's fault and related to the account switching, or if I was really absent-minded enough to not save everything I had written and still be completely convinced that I had saved it. But anyway. This means that you won't be seeing the post on the discussion session today, as I feel too pissed off and tired to rewrite it now; I promise to do it in the weekend. My apologies.

Meanwhile, to keep the "reports from the QG school" series alive, I write a short post informing you of an addition to the English language we came up with during the school. The neologism we coined is upstuff. It has a very simple and beautiful definition:

Upstuff is what you are making when you make up stuff.

So it has a similar field of meaning that "bullshit" or "crap" (applied to talk). But these are completely negative words, while "upstuff" is neutral or even positive in evaluation. Sometimes it is fun to make up stuff and to listen to people who make up stuff. If you have friends who are slightly geeky (and share a taste in hunour with some of my own slightly geeky friends), you might be familiar with a situation in which an informal question or demand for information is answered with an inspired, lenghty explanation randomly made up in the moment. This is a perfect example of good upstuff. A concrete case in point is this wonderful post at the Volokh Conspiracy. (For my Argentinian readers, "fruta" as in "mandar fruta" is a good translation for "upstuff", although perhaps less positive in connotation.)

I think this is a word and concept that the English language sorely needs, as was proved by our very frequent use of it in the days after coining the word. I rely on you Dear Readers to use it and propagate it widely.

The serious reports on the physics will resume, as promised, after I find time for rewriting the report on the discussion session, which you can expect to see it posted on Sunday or Monday. I promise it will not be upstuff.

Report on the Quantum Gravity School: the lectures (2 – Asymptotic Safety)

2007-04-10T01:33:00.000+01:00

I had promised to write about Martin Reuter’s lectures on Asymptotic Safety. These were the “surprise hit” of the school; Reuter had only five lectures slots assigned, less than many other speakers, but it was the results presented by him which arose most excitement and discussion. I would have been happy with one or two more lectures on the subject. The time spent answering interested questions forced him to leave out of the program the topic of fractal dimension of spacetime at high energy scales.

Let me attempt a brief summary of the Asymptotic Safety approach to quantum gravity, as I understood it. (There may very well be inaccuracies and even gross mistakes; I expect you to point them out if you see them). The cornestone of the program is the hypothesis, first proposed by Weinberg, that the renormalization group flow for gravity might have a non-Gaussian fixed point when examined nonperturbatively. That would mean that the quantum theory would be well-defined despite not being perturbatively renormalizable.

Consider a “theory space” formed by all possible diffeomorphism invariant action functionals of the spacetime metric. You can “coordinatize” it by the values of dimensionless coupling constants of different terms, where the dimensionless couplings are constructed from the dimensional ones dividing them by the energy scale k at which you are thinking the theory (to the relevant power). For example, you would have the Einstein-Hilbert action with couplings G and Lambda (suitably rendered dimensionless), plus terms with any power of the curvature scalar, and of the square of the Ricci tensor, and so on. You can define on this space the Exact Renormalization Group Equation (ERGE) which determines the flow of the effective action for gravity in this space. The effective action is the action from which all interactions at scale k can be calculated accurately at tree level. Varying the scale k, the couplings start “running” and some may be turned on or off. If the flow of the effective action, for k going to infinity, reaches a fixed point at which the couplings are not all zero, this is a non-Gaussian UV fixed point and the theory is said to be assymptotically safe. If in addition the flow towards the fixed point is attractive in only a finite number n of dimensions in theory space, the “bare action” you find at infinite k will have only n free parameters, and the exact quantum theory will be as predictive as a perturbatively renormalizable theory with n adjustable parameters is.

Of course, solving the ERGE exactly is out of the question; it is an infinite system of coupled differential equation. The strategy Reuter uses is “truncation” –arbitrarily decide to consider only actions with a given number of terms of a given kind. For example the first and most brutal truncation is the Einstein-Hilbert one: consider only the flow of the two terms of the EH action, with couplings G and L (the L is supposed to be read as "Lambda" and represent the consmological constant). It would be an abuse of language to call this an “approximation”, because a priori there is no reason to believe that the results of the exact flow will be close to those of the truncated flow, or that taking more terms will yield better and better approximations, as long as there are still an infinite amount of neglected terms. To a skeptical mind, this renders the whole program worthless. But Reuter managed to convinced many of us nevertheless that the program was being highly successful. I will explain now the results with which he archivied this effect.

First: the flow of the EH-truncated renormalization group does have a non-Gaussian UV fixed point. The trajectories flowing back from it with decreasing k spend a lot of “time” in the regime where the dimensionless couplings are small. In these region the flow looks “classical”: the dimensionful G and L are constant.

Second: There is a cutoff scheme involved in the calculation. For the exact ERGE, the results are independent of the cutoff scheme, but this is not guaranteed to happen in the truncated calculation. However, Reuter and his collaborators find that the results they have obtained are in fact independent to a high degree of the cutoff used. They take this as partial evidence that a similar fixed point exists in the full, nontruncated theory.

Third: The “next order” of including in the action a third term, proportional to R^2, has been carried out. It must be stressed that there is no reason at all to suppose that the results with this term included would resemble those without it. Instead of two coupled differential equations we have three now, so the situation is much more complicated. However, surprisingly enough, essentially the same fixed point is found! The values of dimensionless G, L on it are almost exactly the same than those at the fixed point found in the EH-truncation, and the coupling of the added term is very small at the critical point.

Fourth: There is very recent work which extends the previous results to actions containing all powers of R up to R^6. The system of 7 coupled diffential equations is now hugely complicated, and if the exact theory did not have a fixed point it would be “magical” that the flow leads to the same point as the previous truncations did. But it does! The GL-projection of the flow near the fixed point is still essentially the same as the one found with the original EH-truncation. Moreover, the dimension of the “attractive hypersurface” is only 3, which means that the bare action has only 3 independent parameters instead of 7 within this truncation. This gives hope that the exact theory may be predictive.

These results had us all very excited! Reuter also talked about some implications for cosmology that arise if we assume the EH-truncated flow to be a good approximation. One important one is that, if the RG trajectory realised in Nature has a “long” classical regime at all, then the physical cosmological constant L is automatically constrained to be much smaller than the physical Newton’s constant G. Thus the smallness of L poses no extra “naturalness” problem beyond the mere existence of a classical regime. Another one is that Reuter expects the truncation to break down as an approximation in the infrared, at length scales much larger than the “classical” regime. Nonlocal terms would presumably begin to act there. A dimensional argument shows that the scale at which this should happen is the scale of the physical cosmological constant! Reuter therefore makes a (very tentative) prediction of new physics at the Hubble scale, and even speculates on a relation to alternative MOND-like theories to dark matter.

Much more work would be needed to see if there is anything substantial in this last speculation, and in the approach as a whole. The exciting thing is that it is a little explored path, which despite being fully "background independent" uses techniques familiar from ordinary QFT, and may be able to make contact with it more easily that models which introduce discrete physics like LQG or spin foams. Of course, even if the exact renormalization group flow has a fixed point with all the required properties and quantum gravity exists as a theory by its own right, this doesn’t mean that this is realised in Nature! String theory provides a very different UV completion to gravity, and if it is true I guess it would render the UV fixed point of “pure gravity” physically irrelevant. But if one of the main motivations for string theory in the first place is the non-renormalizability of pure gravity, then Reuter’s results are making this motivation rather shaky.

Report on the Quantum Gravity School: the lectures (1)

2007-04-08T02:18:00.000+01:00

I will not copy or summarise the program of lectures we had, as you can read it here. From my point of view, there were three sets of lectures that were the “main course” of the school, which were: Thomas Thiemann on Loop Quantum Gravity, Martin Reuter on Asymptotic Safety and Laurent Friedel and Etera Livine on Spin Foams (these last two sets of lectures can be seen as the first and second part of one set, as Friedel covered mostly 3D spin foams and Livine talked about 4D models). The other lectures, though sometimes very interesting, covered more peripheral or mathematical topics.

In the Spin Foam lectures, Friedel defined the basic structure of spin foam models, introduced the diagrammatic notation used for them, and discussed 3D quantum gravity extensively as an example, both without and with matter. It was only by the end of the last lecture that he reached the connections of the latter case with an effective non-commutative field theory with deformed Poincare invariance, which is probably the most exciting ofshoot of this research. Livine discussed four-dimensional models, which arise from writing gravity as a constrained BF theory. He centered on the Barrett-Crane model, and sketched the calculation of the 10j symbol, showing that from it the Regge discretization of gravity arises naturally but with extra “bad” terms. He them talked about the calculation made by Rovelli and his collaborators of the graviton propagator from spin foams, which eluded this problem by introducing a boundary semiclassical state with a phase that, when the calculation is carried out, cancels exactly the “bad” terms of the 10j symbol.

I may add some personal remarks here. When I first heard about the graviton propagator calculation I was excited, but I was unsure as to whether that phase had not been but “by hand” to ensure a nice result which otherwise the theory refused to provide. That is often the problem when you don’t actually work in a field (and you lack the experience and smartness required to check claims by yourself if you don’t work in it): you read a paper and if it gives you this kind of suspicion, you have no way of deciding whom to trust. This is one reason why it is so great to travel to conferences and get an opportunity to discuss face to face with people who don’t mind answering your questions even if they are naïve. In this case it was a dinner table conversation with Eugenio Bianchi that reassured me that the calculation was sound. The phase of the boundary state is not arbitrary or chosen by hand: it is the correct phase to use for the state to be really semiclassical, being peaked not only in the “configuration variables” that specify the classical boundary geometry but also in the “momentum variables” conjugate to them. In fact, in the LQG lectures Thiemann constructed a precise mathematical definition of “semiclassical states” and, according to Bianchi, the state that Rovelli, he and the rest used was the same kind of state, independently found by physical ansatz instead of rigourous mathematical definition. (All this was in fact actually explained in their paper; but reading it after a personal explanation is so much clearer!)

Going back to the lectures, the most intensive ones were Thiemann’s twelve lectures on canonical Loop Quantum Gravity. I certainly learnt a lot from them, and not only abouth LQG per se but also about more general things like constrained systems and how a quantum algebra is constructed from a classical one. The Master Constraint program for dealing with the Hamiltonian (which replaces the constraint at each point of space by the integral over all space of the square of the constraint) was introduced, and after some discussions of the subtleties and ambiguities to be solved, Thiemann enunciated (without proving –that would have probably another 12 lectures!) an important theorem: A particular “quantum Master Constraint operator” was defined in such a way that satisfies all desired properties, including finiteness, dipheomorphism invariance, and most importantly, good semicalssical behaviour. By this Thiemann meant that given a classical field configuration (a pair A, E of Ashtekar variables) one can find a semiclassical state (built as a superposition of spin networks which peaks on it) that makes the expectation value of the MC operator agree with the classical value of the MC to within any desired accuracy. (It is important for this that the classical state and the semiclassical one that approaches it are not physical states, in which the constraint vanishes).

After this, Thiemann also defined in a formal way the scalar product in the physical Hilbert space, which suggestively can be seen as the “time integral” of a transition amplitude. He commented on the possibility that spin foam models might benefit from using the master constraint instead of ordinary constraints, something that has not been tried (and I have no idea how it could be tried). He also described a way to compute expectation values of the volume operator to arbitrary precission in semicalssical states, and mentioned (unfortunately briefly) his more recently developed "Algebraic Quantum Gravity". After the lectures ended I imprudently tried to ask him (in personal conversation) a question concerning “the unsolved problem of the classical limit of LQG” and cut me by saying that the problem was already solved! What emerged from our follwing conversation (which on my side had a rather bewildered face) was this: the “problem of the classical limit” consists in showing that there exist in LQG physical states that, looked at large scales, resemble a nice classical manifold on which GR holds; the semiclassical behaviour of the Master Constraint operator described above implies, by some subtle argument involving projection which I could not follow, that there will be physical states with this property; therefore, the problem is solved. It does not matter at all that most combinations of spin networks, or even most physical states, will not give anything resembling a classical state at large scales. This is the same that happens in ordinary quantum mechanics: coherent states, with nice semiclassical properties, are a very particular kind of state, and most combinations of large number of individual quantum objects do not look at all like anything classical. It is enough that there be nice semiclassical states, not that they be generic.

Of course, this does not mean that one knows how to write a physical state approaching Minkowski (or Schwarzschild or any other classical solution). One doesn’t even know how to write any physical state! But I think that Thiemann’s argument does show, somehow, that this problem is merely technical and that there is no fundamental problem in LQG with respect to the existence of the classical limit. I am interested in hearing my readers’ perspectives on this.

This post is already rather long, and I still have much to say about Martin Reuter’s lectures on Asymptotic Safety, so I think it is a good idea to stop here for the moment. Expect the next post within a couple of days.

Report on the Quantum Gravity School: the pictures

2007-04-05T22:52:00.000+01:00

Being back from Poland only since yesterday, I start here a series of posts reporting on the Quantum Gravity and Quantum Geometry School I attended there. This first one is just for putting up some of the best pictures I have from the two weeks, as many people have been asking me for them. As you can see, Zakopane and its surroundings (the Tatra mountains) are a quite wonderful place.

This group picture of those of us who made it to the peak was taken by Carlo Rovelli.

Thomas Thiemann solving the canonical constraint equations for quantum gravity.

John Barrett and Ileana Naish-Guzman on a long hike we made on the first free day.

Picture taken shortly after the previous one. I urge you to see both on full size by clicking on them; they may easily be the best photos I have taken in my life.

A bunch of happy and slightly inebriated physicists after dining out on one of the last nights

The next post will briefly summarize some of the most important lectures, and the following one will describe at length the discussion session "Where are we in the path to Quantum Gravity?" chaired by Rovelli. Stay tuned!

UPDATE: Oooops, it seems you cannot enlarge the pictures by clicking on them. Well, if you really want to see them just email me and I will send them.

Another hiatus

2007-03-21T11:38:00.000Z

I will be for the next two weeks in Zakopane, Poland, for the First Quantum Geometry and Quantum Gravity School. When I return I am likely to post some things on it; the lectures themselves will probably be too technical, but the general atmosphere and Carlo Rovelli's announced talks should get some discussion. As well, of course, as some photos of the place.

I have not forgotten that I promised to continue the teleporting discussion with arguments for my position, "teleporting is not death". I have all the post actually thought and written in my head, but haven't had time to write it before the trip. Meanwhile you can get more discussions on this subject at Scott's blog. Be sure not to miss his latest Quantum Computation lecture, with a discussion on Penrose's views on consciousness.

Book review: Neal Stephenson, The Baroque Cycle

2007-03-19T22:06:00.000Z

Dileffante has asked me for a review of The Baroque Cycle, and I receive specific requests for posts so infrequently that I feel complied to oblige. So here it goes:

The Baroque Cycle is a huge novel consisting in three volumes, each of them well over 800 pages. It takes place in the late 17th and early 18th centuries, and can be viewed as an attempt to paint a broad fresco of the time, with an emphasis on the emergence of new forms of thought (the Scientific Revolution) and new economic structures (capitalism). It has an immense cast of characters, many of them taken from actual history, and covers as well an immense temporal and geographical scope: from the 1640s to 1715 the former, and from Europe to America to India to Japan the latter. It is also a sort of distant prequel to Cryptonomicon, though I know this only by reading reviews as I haven’t read (yet) Stephenson’s most famous book.

The first book of the cycle, Quicksilver, presents us in its first part the story of Daniel Waterhouse, son of Puritans and natural philosopher, told in flashbacks from the present (1715) in which old Daniel is returning to England from his dwellings in Boston to mediate in the quarrel between Newton and Leibniz. (In one of the first and best of the anachronistic winks present in the novel, Waterhouse is described as having created the “Massachusetts Bay Colony Institute of Technologickal Arts”.) In the flashbacks, we are presented from young Daniel’s viewpoint to the innings of the Scientific Revolution: Daniel is mentored by John Wilkins, founder of the Royal Society, and becomes friends with Newton, Hooke, Pepys, Leibniz, and other key figures of the age. Scientific and philosophical ideas are presented in a witty and engaging way, and the plot mixes them with the political and military intrigues of the time.

The second part of the first book switches social spheres to tell us the story of Jack Shaftoe, who among his many aliases and nicknames takes pride especially in being called the King of Vagabonds. A likeable, colourful, half-crazy hero of picaresque-like tales and swashbuckling adventures (he recounts at one point being a first-hand witness of the death of d’Artagnan), Jack is serving as a mercenary in the battle of Vienna against the Turks in 1683 when he rescues young and beautiful Eliza of Qwghlm from the Sultan’s harem. After they both team up and live some adventures together, Eliza turns up to have an excellent head for business affairs, which makes her raise to a high position in the courts of Europe and end up as a double or triple agent spy, serving at the same time Louis XIV of France and William of Orange. Jack and Eliza eventually drift apart, though the feelings still lasting between them will be key to the action in the other volumes. Leibniz, Daniel Waterhouse and other natural philosophers then get mixed with the intricate and sometimes almost impossible to follow plots that Eliza gets involved in while sailing her way through the treacherous waters of the European international intrigue. Cryptography plays an important role at many points of the story here. The book ends with most plot threads unresolved, leading us directly to the sequel.

The second book, The Confusion, tells two very different stories alternating between them. On one side, Jack is now the leader of a band of galley slaves who become pirates when stealing a cache of Spanish gold. This leads to unexpected amounts of trouble as the gold has unusual properties and many of the shadowy powers pulling the strings of finance and politics are determinate to get hold of it. (One of them is none other than Isaac Newton, who has become the Master of the English Mint). Attempts to get a safe profit from the gold lead Jack and his gang to many exciting adventures all around the world, in Egypt, India, Japan, the Philippines, and Mexico. Meanwhile, Eliza continues to survive and make profits of her own in the complicated high spheres of Europe (though not without some great personal losses either) while continuing her friendship with such figures as Leibniz and Princess Caroline of Ansbach. The two tales cross a number of times and are joined satisfactorily at the end, though this ending is so abrupt that one is left especially eager for the last part of the tale.

In The System of the World, which closes the cycle, we are back in 1715 and following Daniel as he returns to England. This is the book with the most consistent and tight storyline as the three or four plot threads are kept closely interrelated. We have a mystery to unravel, of a series of murder attempts against natural philosophers; the battle of wits and arms between powerful Newton in his character of Royal Minter and vagabond Jack who has now became a coiner bent in undermining the economy; the political tension around the succession of the throne of England (will it go to the Hanoverians or the Jacobites?) and the role Eliza plays in the manoeuvres as friend to Princess Caroline, who will become the Queen if the Hanoverians win; and last by not least the philosophical dispute between suspicious, almost paranoid Newton and courtly philosopher Leibniz.

Though an immense attention is given to accurate historical details, the novel does not feel at all like a realistic account of the Baroque period. This is because Stephenson’s characters often talk and think in a very self-conscious way, making explicit the role they are playing in the grand scheme of history. Witness, for example, this dialogue between a Tory and a Whig in the third volume:

”The question is, shall we be ruled by Money and the Mob –which are one and the same to me, as neither serves any fixed principle- or by one who serves a higher good? That is the point of Royalty, Roger.

Roger paused. “ ‘Tis an attractive prospect,” he said. “And I do understand, Henry. We are at a fork in the road just now. One way takes us to a wholly new way of managing human affairs. It is a system I have helped, in my small way, to develop: the Royal Society, the Bank of England, Recoinage, the Whigs, and the Hanoverian Succession are all elements of it. The other way leads us to Versailles, and the rather different scheme that the King of France has got going there."

It is most unlikely that any real person in 1715 could have thought in this way. It is even more unlikely that Rev. John Wilkins, after Leinbiz discusses with him the binary numeric system and also shows him the mechanical calculator he has built, could have been so prescient as to say: “I believe that binary arithemetickal engines will be of enormous significance”. Or that Leibniz and Waterhouse could have tried to actually build an “binary arithmethickal engine” that follows the Laws of Thought, using punch cards like Babbage would do 150 years later (they call it a Logick Mill). Stephenson has them do this, because he obviously sees the informatic revolution of the 20th century as the natural culmination of the scientific revolution of the 17th, and wants to make it the case that this was already foreshadowed in the beginning. He writes not history as it was, but as it should have been. Which is often not only more entretaining, but also more educating to the reader. I feel I have learnt more from this novel that from many nonfiction books, precisely due to the simplifications and liberties Stephenson takes with historical realism in order to get his larger points across.

(A very different kind of non-realism is provided by the mysterious character of Enoch Root, a shadowy alchemist who appears and disappears from the plot at key moments and may or may not possess the secret of immortality. I mention this because of its potential interest to Cryptonomicon readers, as I understand the same character appears in that book as well.)

The main downside of the novel is of course its length; and also its style, full of long descriptive paragraphs and chaotic complexity in the plot, especially in the first volume. I don’t think as some reviews I’ve read that the novel was “badly needing an editor” because the length and the style are an intrinsic part of the world Stephenson wants to build here; but the task of entering this world for the casual reader can prove daunting. I would say that there are five factors you must consider in order to decide whether to invest money and time (a lot of time) in reading these books:

1) You are a geek.

2) You are deeply interested in the origins of modern science and the philosophical changes that accompained them.

3) You are deeply interested in the origins of modern economic structures such as banking, stock markets, financial enterprises of various kinds, and standarised coinage.

4) You enjoy action-packed swashbuckling adventures, pirates-and-swordfights style.

5) You are very patient and don’t mind reading long books even if there isn’t a clear linear plot to follow.

The first condition is the only sine qua non one. Stephenson is a geek writing for geeks, in the sense of people who love the play of ideas, concepts and abstract structures. (In fact, the novel can be read as making the case that the Royal Society memebers were the geeks of the 17th century.) The remaining four conditions are not all necessary, but if you satisfy less than two of them this novel will be too long and boring for you, no matter how much of a geek you are. In my own case, I am not very patient and I am only mildly interested in economical matters; sometimes Stephenson managed to hold my interest in those parts and those of complicated political intrigues, and sometimes he didn’t and I found myself aching to turn pages without reading them. Overall I enjoyed the novel a lot, especially the last two volumes, and am likely to reread it at some point to get a clearer view of how the plots weave together when viewed in hindsight. If you satisfy more than two of conditions 2)-5), then I recommend you to buy these books, and I guarantee that you will enjoy them hugely.

Poem of the Day

2007-03-14T11:48:00.000Z

Poe, E.
Near a Raven

Midnights so dreary, tired and weary.
Silently pondering volumes extolling all by-now obsolete lore.
During my rather long nap - the weirdest tap!
An ominous vibrating sound disturbing my chamber's antedoor.
"This", I whispered quietly, "I ignore".

Perfectly, the intellect remembers: the ghostly fires, a glittering ember.
Inflamed by lightning's outbursts, windows cast penumbras upon this floor.
Sorrowful, as one mistreated, unhappy thoughts I heeded:
That inimitable lesson in elegance - Lenore -
Is delighting, exciting...nevermore.

...and so on; read the whole poem here. If you can't understand what underlies this variation on Poe's masterpiece and why it was chosen as poem of the day, this link and a moment of thinking should make it clear.

A philosophical poll

2007-03-04T00:55:00.000Z

I’ve been having a long discussion on MSN with my suspicious friend concerning the interpretation of the film The Prestige. The discussion morphed gradually into a purely philosophical one, about abstract questions of self-identity, and at one point in which I voiced an opinion, my friend said: that I was crazy, and that most of our friends would agree with him in that I was crazy in having that opinon. So I decided to make it into a post and ask everybody, friend of ours or not, to leave their opinion and say whether I am crazy or not. Though this post is not about the plot of the movie, there may be a partial spoiler in it because the question discussed is inspired by a major plot point, so you may prefer not to read it if you haven’t seen the film.

The plot point in question is a teleporting machine –and at this point those philosophically savvy amoing you might guess already what the discussion is about, because it is a question popular among many philosophers since Derek Parfit gave it a prominent role in Reasons and Persons. (It is also something of a staple trope in science fiction.) Suppose there existed a “teleporting” machine that worked by scanning your body and creating an exact duplicate of you, atom by atom. Assume that the replica would have the same personality and memories that you have (that is, don’t consider the possibility of your selfhood residing in an inmaterial soul) and that the procedure is instantaneous, so from the point of view of the replica it has just been teleported, having a continuous experience from being at one place to being at another. Then, is there any reason to consider “you” to be the version of you that remains at the same place, instead of the replica? If the machine works by killing the original “you” at the same time that creating the replica, is stepping in the machine a form of suicide or just a convenient way of transportation?

My belief is that there is no reason to say “you” are “the you that remains at the same place”. If both “versions of you” have the same personality, memories, etc (and let’s not forget, also bodily appearance, though I am unsure of the relevance of this) then both have equal call to be called “the future you”. There is simply no answer to the question of which is you: both are to the same extent. My friend disagrees, and says that he is sure he would be the same person that remains at the same place: the other one is just a copy (as he puts it: “the one that is under this skin is me; I want my skin not to die”.)

Who do you agree with? To make things clear, forget about the movie if you saw it, and suppose you are just given the following choice: a) stay where you are, and be subjected to horrible torture (not so horrible that makes you prefer to die, but pretty horrible), or b) step in the machine and have it create a copy and destroy at the same time the original, knowing that the copy (who, in my reading of this scenario, is “you”) will live a blissful life afterwards. I said I would choose b) without a doubt (assuming that the machine is perfectly reliable, etc.) This is the opinion that my friend considers crazy. Who do you agree with?

Three links

2007-03-02T18:03:00.000Z

Philosophia Naturalis #7 is up at The Geek Counterpoint.

My suspicious friend has posted a list of short reviews of every film he saw in 2006. If I counted correctly, they are 50. I have seen only 10 of them.

A new friend enters the blogroll, or will when next updated -which, of course, means probably in several months' time. It it MariaE recounting her Bali experiences, at Blogging A Little Island.

A Riff from Ross to Rorty

2007-02-26T22:58:00.000Z

The topic of Marcus Ross, as Scott Aaronson has noticed, seems ideally suited for being picked up and discussed in blogs; and indeed few among those I read have said nothing about it in the last couple of weeks. (Besides Scott’s own most excellent post, see Janet (also here), PZ, Brandon, Rob Knop and Steve Gimbel for a variety of opinions). In case you have somehow missed all this discussion, Marcus Ross has received recently a PhD in geophysical science at the University of Rhode Island, after presenting a perfectly normal thesis which does research assuming the normally accepted facts about the age of the Earth. However, Ross is at the same time a Young Earth Creationist, who believes that the world was created by God a few thousands years ago, and so –one would presume- does not believe any of the things he wrote in the thesis.

Most of the discussion has centered in whether this is or should have been be grounds for denying him the degree, and whether Ross did anything contrary to the scientific ethos. While acknowledging that in the real world it is a possibility that Ross will attempt to use his scientific credentials to lend credibility to anti-scientific creationist propaganda, and this would certainly be dishonest, I side with those who see nothing wrong in doing research and completing a thesis based on assumptions one does not belief. As many have pointed out, to a lesser degree this is done all the time; it is commonplace for scientists to explore the consequences of a model they do not believe to be true. There is no rule that a scientist must believe what he publishes; only that what he publishes is research done according to the scientific method. Even more, for granting a PhD to someone it is required just that he must display understanding of the topics researched in and of the proper methods to research into them. Understanding, not belief.

Perhaps some think that Ross really believing that the Earth was created as in Genesis is a sign that he does not “really understand” the facts of geophysics he was taught or the scientific method he used in his thesis. But if the examining comitee was satisfied from the thesis that he did really understand all this, then isn’t this a much more reliable criterion? (It is not so easy to write a thesis about something one does not understand; more than “parrotting” is involved!) Others might be concerned that Ross was being untruthful, and violating scientific ethos; to this, I like Greg Kuperberg’s response to Larry Moran at Scott’s comment thread: “He is truthful in his thesis. His thesis has a series of true statements about paleontology. They aren’t statements about his own beliefs, they are statements about the history of the planet. The real issue is that he’s untruthful at other times, when he promotes creationism.”

As these arguments, and counterarguments to them, can be found already all over the place, I will try to riff from the topic of Ross’ thesis to a diferent, philosophical one. Let us ask, what does Ross really believe about the age of the Earth? Of course, I have no way to know the answer. Perhaps he believes there is a fundamental flaw in the methods used by conventional science to date the Earth. Or perhaps he is okey with those methods as far as science goes, but interprets the results in an instrumentalistic way, not as representing reality. Perhaps he is a Gossean, who believes the Earth was created by God with exact signs of old age, so the recent creation is not empirically detectable. But there is another possibility I want to explore. Could he possibly believe both that the Earth is old, and that it is young?

To be clear, I am not considering the interesting and realistic possibility that Ross does not perceive that his set of beliefs is inconsistent; that some of his scientific beliefs have logical consequences incompatible with his creationist ones. I am considering the unrealistic, purely academic and philosophical thought-experiment possibility of a Ross who avowed, loudly, clearly and self-consciously, both the old and the young age of the earth. (From now on, “Ross” will refer to this fictional Ross, not the real one.) Perhaps he avows both propositions at different times, and when pointed out the contradiction he says “I believe one thing in scientific contexts, and another I religious ones”. Moreover, suppose lying detection techinques show that in neither of both occasions Ross is counsciously lying. What would we say then?

Probably, that he is schizophrenical or has some other kind of deep cognitive malfunction. But why? Why would we not be okey with simply accepting his self-description?

The problem cuts deeper than it seems, because on some popular philosophical positions, there would be nothing wrong with the Ross I have described! I will focus on Richard Rorty as a representative of these positions, because he is the only one I have read more or less extensively. Very roughly, Rorty says that beliefs do not aim at representing the world, but are just tools we use to cope with it. Accepting or rejecting a belief is akin to using or discarding a particular tool for dealing with some problem. “Truth” is just a label we use for those beliefs we endorse. It would seem to be a consequence from this account that there is no problem in holding contradictory beliefs, as long as one uses them in strictly separate spheres. The Ross of my thought experiment could believe that the earth is old to “cope” successfully with dating fossils and rocks and publishing scientific papers, and that the Earth is young to cope successfully with reading, interpreting, and teaching the Bible, praying, etc.; and the extent to which this activities did not raise in him any doubts of consistency would not be a mark of schizophrenia but of rational sanity: the enviable ability to cope with full success in two different areas of life using two different tools.

I propose this as a reductio ad absurdum of Rorty’s philosophy. Perhaps he would say that beliefs are tools which only can function when constrained by consistency criteria, but this is a weak reply; it is true in most occasions, but if someone could manage to “compartimentalize” the scientific and the religious aspects of life in such a way that the inconsistent beliefs did not bring about any practical problems, then consistency seems not only unnecessary but positively harmful. I think that ultimately, Rorty should embrace the idea that incompatible beliefs are possible and can even be a feature more than a bug.

The problem with Rorty, of course, is that belief is not just any tool for coping, but one that has as primary purpose the grasping of true facts about the world. Coping successfully is a secondary consequence, achieved via the possesion of true beliefs. Belief, we could say, is intrinsically “truth-aimed”; and thus saying “I believe in X and also in not-X because each of them enables me to cope successfully with some things” is absurd; because one must know that the facts about X cannot agree with both beliefs. However, I don’t think this kind of argument is very good against Rorty, who has a stock-reply for it: “Perhaps with our present concepts of belief, truth, etc. what you say is true and what I want to say is inconsistent; but if we find that these concepts bring upon us too much philosophical trouble to understand what “truth” and “facts” really mean, then we may better switch to a purely pragmatic concept of belief which does not has these problems. Concepts, after all, are just tools as well. So in the future in which we all use the new concepts I propose, there will be no inconsistency.” This reply tends to irritate professional philosophers, who see it as an evasion. (John Holbo whimsically summarises it as “all your bases will have been belong to us”.) But I confess to like it to a certain extent; I do agree that some philosophical matters (unlike “factual” ones like the age of the earth –but don’t ask me to explain the difference!) can be decided pragmatically, and in particular I think this kind of move can be used (with due care) against mind-body dualism or free-will incompatibilism. So I don’t think one can refute Rorty simply by saying that the possibility of incompatible beliefs is "conceptually absurd".

What is possible, however, is to defeat Rorty in his own playing ground, and say that we do not have strong pragmatic reasons to change our concepts of belief; that we want to go on saying that my ultra-compartimentalizer Ross would be crazy instead of rational; because we find the basic concept of beliefs as tools which (unlike others) are intrinsically truth-aiming, a very useful and good one to have. For me at least, the philosophical perplexities which the notions of truth and representation may cause are a small price to pay for using these concepts.

My Erdös Number is 5

2007-02-16T14:26:00.000Z

There are two chains:

1) Alejandro Satz - Jorma Louko - Raymond Laflamme - Peter W. Shor - Nathan Linial - Paul Erdös.

2) Alejandro Satz - Francisco Diego Mazzitelli - Bei Lok Hu - Stephen A. Fulling - Itshak Borosh - Paul Erdös.

You can find your own Erdös number at the AMS Search Page.

Lagrangians, Hamiltonians, and Scientific Realism

2007-02-11T23:28:00.000Z

Up at Mormon Philosophy, Clark has a couple of posts concerning the interpretation of physical theories, with the concrete example of Newtonian mechanics. Probably most of my readers are aware that there are different versions of the theory which are mathematically equivalent, but very different in intuitive terms. Thus the traditional formulation in terms of Newton’s Laws describes the world as elemnts of matter, with the intrinsic property of mass, acting on each other by forces and responding to forces by accelerating; the more “global” Lagrangian formulation says that a system will behave in such as way as to minimize the difference between the potential energy and the kinetic energy across different possible behaviours, and the Hamiltonian formulation gives equations for how position and momentum treated as independent quantities will vary depending on the total energy of the system. In his posts Clark states his belief in scientific realism, defined as the assumption that “the entities we discuss in science correspond in some way to entities in the real world”. Then he ponders:

Now when we consider this statement of realism with the three formulations of mechanics we quickly see the problem. There are three different, yet ultimately incompatible, sets of entities mechanics is framed in terms of. Now I'm sure some will jump up and contest this, arguing for momentum, potential and kinetic energy, mass and so forth. But if we take Sellars to be saying that is the ultimate entities the theory is framed in terms of that we have epistemic justification to believe exist, then we see the problem. We can have multiple mathematically equivalent theories in this sense of theory. This is often called having the theory underdetermined.

An other way of looking at the issue is to talk about empirical adequacy. In this view the acceptance of a theory isn't to necessarily accept the entities postulated by the theory. It is just to accept that the theory accounts for the kinds of phenomena we encounter or would potentially encounter. So, for instance, we might be unable to say if mechanics really is about minimize the difference in kinds of energy or is due to forces between masses. We can just say that whatever's going on underneath, the theory explains the kinds of phenomena we encounter.

This, roughly is the distinction in science between realism and anti-realism.

He thinks that the existence of alternative formulations for mechanics, or other physical theories, is an argument against realism and for anti-realistic instrumentalism; and though he reiterates that he is still a realist, he does not provide any solution for the problem. I will try to suggest one for him.

In a nutshell, I see the problem as a manufactured one due to trying to translate mathematical language into ordinary language. The three formulations of mechanics, being mathematically equivalent, must be saying the same thing “about reality” (if we choose to interpret them realistically). The appearance that they say reality is made of “three different, yet ultimately incompatible, sets of entities” cannot be correct, because equivalent statements cannot be incompatible, and this logical fact is independent of whether we interpret the statements in a realistic or instrumentalistc way. The problem arises only because we are not satisfied with a mathematical equation as a description of reality, but we want an explanation of “what it means”, which amounts to a translation of the statement from mathematical language into non-mathematical language. The translation of Newton’s laws seems to be “objects exert forces and accelerate in response to them” and the translation of a Lagrangian action principle seems to be “systems try to minimize action”, and at first view these descriptions seem hardly compatible, especially if the second is interpreted teleologically. But insofar as there is a problem with the compatibility of the translations, this is only a reason to suppose that the translation has been done carelessly and must be corrected. It is as if we started from two Spanish sentences with equivalent meaning, tried to translate each literally into English and got two English sentences differing in meaning because two Spanish words have a shared possible meaning which is not shared by the most literal English equivalent of each of them.

The three formulations of mechanics, then, say the same thing about reality: that there exist certain quantities (given the names of mass, energy, momentum, etc.) which are related mathematically in ways given by the equations of the theory. It makes little or no sense to ask which of these quantities or equations are “fundamental” and which are merely “derived”. I am skeptical even of the possibility of distinguishing “definitions” from “empirical laws” among these mathematical equations; p = mv (momentum = mass x velocity) is a “definition” in the Newtonian formulation and a “dynamical equation” at equal level with F = ma (force = mass x acceleration) in the Hamiltonian one. The ordinary language translations (“objects exert forces”; “systems try to minimize action”) are just pedagogical or heuristic devices, useful and perhaps indispensable for actual scientific practice but not related to the austere philosophical question “What does the theory say about the world?”

There are theories for which “interpretation” is a serious question: quantum mechanics is the primary example. But the “interpretations” of quantum mechanics are a very different thing from the “interpretations” of classical mechanics we have been discussing, and raise very different issues. The equivalent kind of question would be a contrast between, for example, the Heisenberg and the Schroedinger pictures, asking whether reality consists in “states evolving in time and timeless operators” or “timeless states and operators evolving in time” (both descriptions are related by a simple mathematical equivalence). And these alternative formulations, just like those of classical mechanics, raise no philosophical problems at all.

Fun fact of the day

2007-02-06T23:02:00.000Z

Question: To what food item did Johannes Kepler compare the shape of planetary orbits in the geocentric system?

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Answer: To pretzels.

Who can name the largest number?

2007-02-03T17:59:00.000Z

Via Language Log, a report of an amazingly cool competition:

On Friday, Jan. 26, two philosophers, MIT Associate Professor Agustin Rayo (The Mexican Multiplier) and Princeton Associate Professor Adam N. Elga (Dr. Evil) engaged in the Large Number Duel, in which they attempted to one-up each other by inscribing the largest finite number ever to be written on an ordinary-sized chalkboard. The feat, if successfully accomplished, would be worthy of a note in the Guinness Book of World Records. (...)

The rules of the duel gave free rein to the contestants’ creativity and humor, maintaining only a ban on the use of infinity, and restricting statements about the number proposed to a primitive semantic vocabulary. The battle itself was intense and the room in the Dreyfoos wing of the Stata Center was packed, with people standing on chairs and at least 20 students craning their necks from the doorway.

The contest opened in the style of a boxing match, with competitors presented “in the red corner” and “in the blue corner.” Elga went first, writing the number one. “Ha!” announced Rayo, as he countered with a string of ones across the board. Elga retaliated with a clever trick, erasing a line through the base of half of the ones to turn them into factorials. (...)

Near the end of the duel, Rayo furiously scribbled on the whiteboard: “The smallest number bigger than any number that can be named by an expression in the language of first order set-theory with less than a googol (10100) symbols.”

Although this definition took a bit of tweaking, including what Rayo described as his “second order logic trick,” it soon won him the duel.

As Elga collapsed, slain, the referee closed the ceremony.

Philosophia Naturalis is up

2007-02-02T16:45:00.000Z

... at Science and Reason. Lots of interesting physics blogging collected there. I am usually very bad at keeping up with carnivals and posting links to them, but I will try to keep posting links to further editions.

Happy Birthday to Me

2007-01-31T17:56:00.000Z

Well, not to me, Alejandro Satz. My own birthday was a couple of weeks ago. In the title of the post I am lending my voice to the blog Reality Conditions itself, which was celebrates today its first birthday.

When I started blogging I was much, much, much more enthusiastic about it than now. In my first month I posted 37 times! Afterwards my zest declined, settling now in only one or two posts per week, as you can sumrise from the following chart:

The good news is that I don't think the frequency of posts will decrese significantly in the future. I am satisfied with this rhythm of two or three substantial and longish posts per month, plus as many short posts with links or random comments as I fancy. My readership, after growing fastly in the first four months, has reached a rather stable average as well, of about 1500-1700 visits per month:

(Yes, I am a freaking obsessive neurotic for posting and analyzing all these charts. You don't need to tell me that.) Of course the majority of visitors are from Google and other search engines, but I have typically about 20 people per day that come deliberately to read what I write; as probably most people don't come every day, there may be twice or three times as many people who check this blog regularly and are interested in my writing. To all them, thanks!

Too bad I have never heard of him... er, me.

2007-01-27T23:43:00.000Z

I am:

Gregory Benford

A master literary stylist who is also a working scientist.

Which science fiction writer are you?

I hoped to be Arthur C. Clarke, but (at least according to the test result page) the real Greg Benford once took this quiz, and it told him he was Arthur C. Clarke, so it can't be too far off... Or maybe it can.

Anyway, the test is rather better written than the average Internet quiz, so I recommend you to go and take it.

Book Review: Paul Davies, Cosmic Jackpot

2007-01-20T12:30:00.000Z

It feels strange to be asked by his publishers to review a book by Paul Davies. Many years ago, it was partially the fascination provoked by his popularisation books (especially Superforce) which made me wish to study physics. It is somehow for me like coming back full cycle, if you understand what I mean.

The main theme in Cosmic Jackpot is, as the subtitle aptly puts it, “why our universe is just right for life”. In other words, it is about the apparent “fine tuning” in the basic constants of physics. For many numbers that play a basic role in the laws of Nature, such as the electron’s charge and the magnitude of the strong nuclear force, it happens that if those numbers had been just slightly different, life could not have developed in the universe. The most extreme example is the recently discovered dark energy; it is 120 orders of magnitude smaller than what would be its “natural” value, and if it was just one order of magnitude bigger the universe would have expanded too fast for stars which could support life to have time to form.

Is this just a brute fact of the universe, or something that cries out for an explanation? Davies thinks the latter, though without going through any formal argument to justify the very idea that an explanation is needed. (I’ll argue later that this is a big omission.) He considers several possible explanations, opting at the end for a rather idiosyncratic one, and with little by the way of argument to support it, in my opinion.

Roughly the first half of the book is dedicated to an exposition for the layman of the fundamentals of particle physics and cosmology. Davies is well-habituated to this kind of popularisation, and delivers it with easeful clarity. The expanding universe, the Standard Model of particle physics, the mysteries of dark matter and dark energy, and the basics of string theory are among the topics covered. For those fascinated by the String Wars™, I may mention that the latter exposition is neither hypeful nor negative, giving fair space to the theory’s archievents and to its open problems.

The second half of the book is dedicated to the fine-tuning for life of the constants of phsysics, and its possible explanations. Of course the “anthropic” explanation, and its concrete realization in the string-theoretical landscape and the eternal inflation framework, take a large part of the discussion. For those who have been asleep the past years, the anthropic principle is the speculative idea that if a large or infinite number of parallel universes with different basic constants or laws (a “multiverse”) exists, then the fine-tuning is not surprising, because we couldn’t find ourselves in any universe but in one of the few that support life. Davies is rather critical of the anthropic principle, although not for the reasons usually aduced against it. (Given the wildly speculative metaphysical explorations of the last chapters, it wouldn’t do for Davies to criticize the anthropic principle on non-falsifiability or other methodological grounds!) His main argument is that as universes can almost certainly be simulated computationally at a much “cheaper” cost than creating them in reality, an infinite multiverse will contain many more simulated universes (created by advanced civilizations for amusement, I gather) than real ones, so the anthropic reasoning would imply that we live in a simulated, Matrix-like universe, because that is the most likely situation to find ourselves. But then we have no reason to have a strong belief in the laws of physics that support the multiverse hypothesis in first place, so the whole anthropic argument collapses. I think this reasoning rests on assumptions shaky enough (is it really so easy to simulate an entire universe? do we really have reasons to believe that civilizations will have interest in perfoerming such simulations by large numbers? do we have any reason to suppose that the typical simulated universe will look like ours, any more than the typical real universe will? etc.) that few anthropicists will be troubled by it. (Those who find these whacky discussions fascinating must check Nick Bostrom’s webpage).

I suspect that Davies’ rejection of anthropic explanations does not arise really from the contrived simulation arguments, but from a deeper philosophical conviction: that the ultimate explanation for the universe and its characteristics must be meaningful in some sense. He is not just interested in the limited question “Why are the constants suitable for life?” but in the more ambituous one “What determines what exists and what does not exist?” The existence of a multiverse in eternal inflation described by string/M-theory seems too arbitrary for him to be a suitable final explanation, a good place to find “the turtle over which all others rest”. He also yearns, and says it so explicitly, for an ultimate explanation which involves life and consciousness in an active way, not in an essentially passive way as the anthropic arguments do.

Given these inclinations, some kind of cosmic Intelligent Design seems Davies’ natural resting place. But in fact he also feels uncomfortable with the notion of a God as ultimate explanation. Having the natural sensibilities of a scientist, not a theologian, he can make little sense of the idea of a “necessary being” that can perform the explanatory role of God, and tends to accept the famous Dawkinsian argument that a creative, personal God must be “complex” and stand in need of explanation instead of being a suitable resting point for explanations. (I share those intuitions as well, by the way, while understanding how some may not share them; it is a subject for yet another one of my “science vs. religion” posts.) He ends up saying that he finds a God and a multiverse about equally complex and unsatsifactory as Final Turtles. A unique self-consistent version of M-theory or other Theory of Everything which predicted the values of all constants of physics necessarily would be more simple and satisfactory than both, but he believes it unlikely that such a theory exists, and anyway one could still ask why the theory is realized in actuality. He also considers the view that everything logically possible exists, so there is no special problem in determining what will exist, and has a bit of fun playing with its implications before dismissing it. For philosophers, not the least sign of philosophical amateurness in Davies’ book will be that he attributes this theory to physicist Max Tegmark, with only one endnote mentioning philosopher David Lewis, who developed the idea earlier and more rigorously.

Chapter 10, in which Davies exposes his own views, is the less clear and most jumbled of the book. (As a rule, Davies is much better at explaining ideas of others clearly and critizicing and assessing them than at developing (philosophical) ideas of his own.) His preferred speculation is that life and consciousness might be a creative force in Nature, not by guise of a designing God but by an inmanent, teleological principle that makes their appeareance a necessary feature of the universe. In support of this view he enlists a couple of ideas that loyal readers of this blog will remember I discussed and criticized: Chalmers on the irreducibility of consciousness, and the “backwards causation” interpretation of quantum mechanics. But of course, even if these ideas were true they would fall very short of supporting Davies’ speculation. He also wanders off tangents borrowing from, among others: Deutsch on evolution and quantum information, Tipler on the universe evolving to a “supermind” as final state, Lloyd on the universe as a discrete computer, and Wheeler on self-explanatory causal loops. What I have, honestly, been unable to find in all this “idea-dropping” is any clear and concrete argument for the teleological speculations, which Davies is well aware are far from the mainstream views of scientists. For argument, Davies replaces rethoric; a rethoric that everywhere betrays his deep-seated conviction –and also desire, and even emotional need- that the universe be meaninglful, that it must include us, or generally life and consciousness, among its fundamental principles and not only as a casual and fortitious outcome as modern cosmology seems to imply. (He calls this mainstream view “The Absurd Universe”, and seems to believe it renders somehow the scientific enterprise unjustified and pointless.)

As readers must have gathered, I have little sympathy for Davies’ philosophical speculation. The “fine tuning for life” problem seems to me rather arbitrary: as Carl Sagan pointed out once, about the same constants and laws necessary for a universe containing observers are necessary for a universe containing rocks, so why not discuss the “fine tuning for rocks” problem? To frame the discussion the way it is done presupposes that life and consciousness are important or significative features –and despite all Davies’ posturing, we have really no reason to believe they are in an objective sense, apart from our obvious (self-) interest in them. Multiverse-anthropic arguments for adressing these questions have in my opinion a very slight, but not zero, chance of becoming some day testable and scientific; while Intelligent Design arguments have an effective chance of zero, and so do teleological or causal loop principles in absence of a much stronger motivation and articulation for them. So for the time being count me by default as a citizen of the Absurd Universe.

However, despite all this the book does provide a good and clear explanation of many basic areas of modern physics and cosmology, and provides also reasonably clear and fair discussions of the anthropic and the intelligent design controversies. It is only when Davies lets his own views take the forefront that I start to dislike it. Another virtue of the book is that, for all its philosophical amateurness (in fact, because of it) it can be a refreshing reminder of those “big problems” like why does something exist instead of nothing, that not only physicists but even also philosophers lose sometimes in the pressure of professional specialization. A breezy and engaging exploration of the mysteries of “life, the universe and everything”, no matter its flaws, can be welcome if only for that reason.

You can also read Chad Orzel's review, which is a bit more dismissive of the whole subject-matter of the book than I am; although I certainly agree that this subject matter can only be clasified as "philosophy" and not as "science", if there is any kind of distinction between them. I will add links to other reviews as they appear.

I suspect Good Old-Fashioned Hibernation may be cheaper

2007-01-17T14:45:00.000Z

Andrew J. Janca, So you want to stop time

A model of a stasis chamber, slowing the passage of time in its interior down to arbitrarily small rates relative to the outside world, is considered within classical general relativity. Real and apparent (gravitational) forces as perceived by an interior observer are altered, but in opposite ways. Comparison with special-relativistic time dilation shows the use of such a static chamber to be economical only when the most drastic slowing of time relative to the outside world is desired (d(tau)/dt < 10^{-20}) or when one wants to avoid spending the time needed to accelerate to relativistic speeds.

Dinosaur Comics

2007-01-16T16:45:00.000Z

Note to self: next time I update my blogroll (one of those often talked of but never actually happening events) I must include a link to Dinosaur Comics. I have already many times found links to some of its wonderful strips, including the ones on philosophical zombies and zombie epidemics, but it was today's awesome light cannon (discovered by Chad Orzel) that clinched the decision.

This, by the way, is a post done just to keep the blog alive while I finish writing my second comissioned book review, which will hopefully be posted in two or three days' time.

Dr. Atheism

2007-01-08T16:31:00.000Z

I'm sure I am not the only one tired of the byzantine discussions about Richard Dawkins. In the last two weeks we had two similar examples, in which recorded opinions of him were analyzed to the highest degree of semantic precision to determine whether he had or not overstepped the limits of reasonableness into some dark form of scientistic totalitarianism. First it was the topic of "should religious indoctrination be prohibited", next "should Saddam have been used for scientific research on psycopathy instead of executed". Why do people from all sides of the ideological disputes rush so quickly to these discussions to do their best to convict or acquit Dawkins of this or that nefarious opinion?

It is, of course, because he has become the Public Face of Atheism. He's the first one the media will try to get when they need the opinion of an atheist about something, and he's the one that will represent atheism in the imagination of the general public. Hence the effort by atheists of his persuasion in making him look good, and of the enemies of atheism or of his particular kind of atheism to make him look bad. But it all gets a bit ridiculous at the end, and everyone on every side should remember that Dawkins is only one man, that he does not speak for anybody but himself, and that both his good and his mistaken opinions may or may not be shared by all other atheists. He's not the Pope of our atheist church, not even its spokesman. And I say this not to distantiate myself personally from him (though some of my disagreements have been recorded already) but just to put some perspective into all this.

This transformation of biologist Richard Dawkins in "Dr. Atheism" brings to mind a game I thought of long ago, which consists in coming up with the "Dr. X" figure that represents discipline or intellectual position X for the general public. It usually is a top figure in his or her field, or a very successful popularizer, or both. Let's see:

Dr. Physics: Used to be Richard Feynman. Since his death, I think Stephen Hawking has taken the post. We can also mention here the existence of a Dr. String Theory (Brian Greene) and a
Dr. Loop Quantum Gravity (Lee Smolin).

Dr. Astronomy: Used to be Carl Sagan, be a large margin. I'm not sure who it is now.

Dr. Biology: Again, I am sure that before his death Stephen Jay Gould had this role. (My argument: any intellectual that appeared in The Simpsons is the almost certainly a Dr.) But for now I'm not sure; maybe Dawkins can claim this one as well as the Dr. Atheism one?

Dr. Medicine (or Dr. Dr.): Zero ideas here. Suggestions?

Dr. Economics: Another case of recent deaths (J.K. Galbraith and Milton Friedman) making the decision difficult. Perhaps for Americans it is Paul Krugman?

Dr. Linguistics: By right it ought to be Noam Chomsky, but perhaps he is so absorbed by his role of "Dr. Anti-Imperialism" that he has resigned the position. In that case, I don't know who replaced him.

Dr. Semiotics: Umberto Eco, of course. The proof is that he is the only living one I can mention.

Dr. Philosophy: I don't think there is any worldwide one, but perhaps several of local recognition. In Britain, perhaps A.C. Grayling?

I leave the rest of the game for you. Who are the "Drs." for disciplines I left out like psycology and political science? Who the hell is Dr. Mathematics? And for other intellectual positions than atheism, such as vegetarianism, feminism, libertarianism or Marxism? Have fun!

End-of-the-Year Miscellany

2006-12-30T19:24:00.000Z

Physics blog post of the week: Scott Aaronson's Mercenary in the String Wars.

Physics blog comment of the week: Sean's admission at a follow-up to Scott's post, where after some litigation he forfeited to me all the money he made by using the phrase "String Wars", which belongs officially to me now.

Books bought this week at the cheap prices for secondhands in Buenos Aires: Kim Stanley Robinson's The Years of Rice and Salt, Vladimir Bartol's Alamut, and Louis Menard's The Metaphysical Club; all in perfect condition and at a total price of 34 pesos, or about 6 pounds. I'm aiming at finishing the first one before I fly back to Nottingham; it is an alternative history based on the premise of the total extermination of the European population by the Black Death and the gradual rise of Islam and China as major world powers. Engrossing at some parts, boring at others.

Dream of the week: a doze I had today, in which I figured out the final theory of quantum gravity. The only thing I remembered in awaking is that the key concept was the notion of "more or less".

Friend of the week: my cynical friend, aka Bad Santa, aka The Ghost of Christmas; he knows why.

Greeting of the week for all my readers: Have a happy 2007!

I've been Tagged!

2006-12-21T22:23:00.000Z

Bee has tagged me with the following meme:

Grab the book closest to you.
Open to page 123, go down to the fifth sentence.
Post the text of the next 3 sentences on your blog.
Name the book and the author.
Tag three people.

She expects me to have some interesting book close to me. Had I been in my room in Nottingham, that would probably be the case. At least, I would have one of "my" books near, and probably a recently read one. Here I am typing in a room at my family home in Buenos Aires, a room used for the computer and the books of all the family, not my own. Moreover, the book-shelf closest to me has some Atlases and map books, and some humour books; I am not sure which book is mathematically closest to me, there are no sentences on a map, and the comic strips I find in pages 123 of nearby humour books have 6 or 7 sentences in the first strip, so the 3 following the fifth one would belong to two separate strips (the punchline of the first one and the beginning of the next one). I prefer to tweak a little the rules of the meme to get a better result.

So I go to page 123 of one of the closest humour books, which is Diez años con Mafalda, and I copy the full first strip instead of the three senteces after the fifth one. It is a dialogue between Mafalda and her friend Susanita. (For the uninitiated, descriptions of the comic and the main characters here; some examples of strips with English translation here; English translation of Volume 1 of the comic here; strongly recommended.)

(first panel) Mafalda: Tener hijitos está muy bien Susanita, pero los tiempos cambian.

(second panel) Mafalda: Además de ser madre, hoy la mujer debe contribuir al progreso, hacer cosas importantes! Susanita: ¡Tenés razón!

(third panel) Susanita: ¡Desde mañana mismo, aprenderé a jugar al bridge!

(fourth panel) Susanita: ¿Qué pasa?...¿Acaso no juegan al bridge las señoras importantes? Mafalda (leaving, with a weary expression, thinks): ¡Dios mío!

My translation of the dialogue:

M: "To have children is all very well, Susanita, but times change. Besides being a mother, today woman must make a contribution to progress, do important things!"
S: You are right! Starting tomorrow, I will learn how to play bridge!
S: What?...Do not all important women play bridge? M (leaving, with a weary expression, thinks): Oh my god.

My suspicious friend, claiming to be the voice of my conscience, told me to play by the rules and copy the exact sentences I was commanded to. If he enjoys strict adherence to meaningless arbitrary rules, I tag him with the meme. And, let's see... My cynical friend and my nonintelligent friend as well. The first one to pre-empt the trademark silly sarcastic comments he is bound to make to this post, and the second one because I am curious to see what is he reading now that he got his degree.

What I believe

2006-12-12T14:59:00.000Z

This is the religion test many ScienceBloggers are taking. You can take it here.

1. Unitarian Universalism (100%)
2. Secular Humanism (98%)
3. Mainline to Liberal Christian Protestants (93%)
4. Liberal Quakers (90%)
5. Nontheist (76%)
6. Theravada Buddhism (70%)
7. Bahá'í Faith (65%)
8. Neo-Pagan (64%)
9. Church of Jesus Christ of Latter-Day Saints (Mormons) (62%)
10. Christian Science (Church of Christ, Scientist) (57%)
11. Taoism (52%)
12. New Age (46%)
13. Reform Judaism (45%)
14. Mainline to Conservative Christian/Protestant (44%)
15. Jehovah's Witness (42%)
16. Mahayana Buddhism (39%)
17. New Thought (39%)
18. Orthodox Quaker (38%)
19. Sikhism (32%)
20. Scientology (29%)
21. Jainism (23%)
22. Seventh Day Adventist (20%)
23. Eastern Orthodox (16%)
24. Islam (16%)
25. Orthodox Judaism (16%)
26. Roman Catholic (16%)
27. Hinduism (16%)

A couple of comments. The definition of Unitarian Universalism is so wide that anyone who is not a fundamentalist will get a very high score on this one. The most surprising result for me was how low I scored on Reform Judaism, which would probably be my religion if I had one (or maybe, probably is insofar I have one). Of course this is so for reasons of familiar and cultural identification which the test does not track well, as it is focused on abstract beliefs; but I was surprised all the same. (I'm almost as much a Jehovah Witness as a Jew? WTF?) Reading the descriptions the test links to, I do not feel more far away from the Reform Judaism one than to the ones for much higher scored faiths such as Liberal Protestantism, so my guess is that for some reason the test allows much more divergence from the mainline in these other faiths than in Judaism (that is, a belief -or in my case probably a lack of belief- that some Protestants and some Jews hold but is not mainstream for either will be given a higher Protestant score than a Jewish one).

On holiday

2006-12-11T17:55:00.000Z

I am at Buenos Aires for my holidays until January 2nd. As leisure seems to leave me less free time for blogging than work does (someone should analyze this apparent paradox) posting is liekly to be severly reduced until my return to Nottingham.

Book Review: Lee Smolin, The Trouble with Physics

2006-12-05T13:25:00.000Z

I know I am arriving a bit late to this game; I only got hand of the book after returning from from Canada, and I have been very busy these weks since then. Frankly, on normal circumstances I wouldn’t bother writing a review –why do it, when there are so many excellent reviews around already? (See the complete list here; I recommend especially Bee's and Sean's) But the publishers were kind enough to send me a copy on the understandment that I would write a review, and so I am forced to try to find something at least half-original to say.

The first point is that the book is not an attack on string theory, at least in the strong and uncompromised sense Peter Woit’s Not Even Wrong probably is. (I haven’t read that one, but I’d be surprised if it was half as sympathetic to string theory as Smolin’s is). Smolin makes clear that he does not think that string theory is a failure or that research in it should be abandoned. The provocative title and cover of the book (as Smolin has said) and the promotional material from the publishers (as Bee remarks) are misleading in this respect, being clear attempts to boost sales by fueling controversy. I am not going to speak ill against the nice publishers who gave me a free book, but just say that readers put-off by what seems an aggressive, unconstructive attack should give the book a chance.

(I’m just having an absurdist vision of Woit and Smolin playing a bad cop-good cop routine against a handcuffed string theorist! Probably inspired by this old Cosmic Variance comment, by far the funniest take on the String Wars I have ever seen.)

Smolin opens the book with a provocative question: Given that the past twenty-five years have been the only quarter of a century in the past two hundred years in which fundamental theoretcal physics has not made a significant progress in our understanding of the universe, what are the causes of this situation and what can we do to revert it and bring back the glorious times of the Einsteins, the Heisenbergs, the Feynmans or the Weinbergs? What does physics need to get moving again?

He next describes the five “Great Problems” which need to be solved: finding a theory of quantum gravity; solving the foundational problems of quantum mechanics; finding (if there is) a theory that unifies all the forces and particles found in nature; explaining the values taken by all the parameters of the Standard Model; and explaining dark matter and dark energy. Then comes a fast tour through the history of physics with an emphasis on the idea of unification, that takes us through several chapters from Galileo’s of principle of inertia “unifying motion and rest” to the formulation of the Standard Model which describes all forces as manifestations of gauge symmetries. The explanations of different topics in these chapters are gentle, clear if not too comprehensive, and directed always to the lay reader. The choice of “unification” as a theme has some odd consequences as for example more space in the story is dedicated to the Kaluza-Klein theory than to the discovery of quantum mechanics and quantum field theory (which, one could even argue, provided a fundamental unification not mentioned by Smolin, between discrete particles and continuous field waves). The next chapters build up to the origins of string theory by chronicling the first (failed) attempts to unify forces beyond the Standard Model and to construct theories of quantum gravity. There is an interesting personal memory of the days in the mid ‘70s in which supergravity seemed the wave of the future; Smolin had a chance to start working on it with a top group but declined, and many years later he came to the conclusion that the reason was:

Having learned physics by studying Einstein in the original, I had obtained a sense of the kind of thinking that went into a revolutionary new unification of physics. What I expected is that a new unification should start from a deep principle, like the principle of inertia or the equivalence principle. You would gain from this a deep and surprising insight that two things you had once seen as unrelated were actually at root the same thing…

Supergravity was not doing this. Although it was indeed a proposal for a new unification, it was one that could be expressed, and checked, only in the context of mind-crushingly boring calculations. I could do the math, but this was not the way I had been taught to do science by my readings of Einstein and the other masters

This foreshadows the conclusions of the book as to what is the trouble with physics.

The second part of the book is a very readable history of string theory, explaining the two “revolutions” in the mid-80s and mid-90s, and how they convinced many people that string theory was the ultimate theory of nature. Smolin does not hide the many successes and insights provided by string theory, but does take a more cautelous approach to them that most string theorists would. He expresses frustration, for example, that key points such as perturbative finiteness at all orders, the strong version of the Maldacena conjecture, and the Bekenstein-Hawking entropy of realistic black holes, have not been proved but are commonly accepted as true just because more limited versions of them have been proved. He constantly says that “the pessimist” could take the partial results as nothing more than a coincidence, and that string theorists should not rest in the assumption that these facts are true. These cautionary remarkes may be provide a useful balance against some of the most triumphalist rethoric from the string community, but they are not ultimately very convincing –string theorists who see partial successes as a reason for optimism seem to me perfectly justified.

As everybody knows, Smolin also makes much of the issue of background independence. I certainly agree that the complete theory of quantum gravity ought to be background independent, but my understanding is that string theorists agree as well, and just say that trying to construct from the beginning a manifestly background independent theory is not the most promising route to it, but we ought instead to gain step by step in understanding from the perturbative versions of string theory to the full non-perturbative theory. Now from the LQG camp some will say that pursuing manifest background independence is the most promising approach, and a comparing and debating the archievements and potential of both approaches is indeed legitimate, but the insistence on “background independence” as such a key issue strikes me as a red herring.

Smolin is more convincing when talking about the landscape business, and on his criticism of the anthopic principle. On the latter he concludes, after explaining the situation and providing a list of quotes from famous “anthropicists” including Polchinski and Linde:

There is not a person quoted here whom I do not deeply admire. Nevertheless, it seems to me that any fair-minded person not irratioanlly commited to a belief in string theory would see the situation clearly. A theory has failed to make any predictions by which it can be tested, and some of its proponents, rather than admitting it, are seeking leave to change the rules so that their theory will not need to pass the usual tests we impose on scientific ideas.

Strong words, but I agree that, for the moment at least, anthropic arguments stand outside the boundaries of science. Perhaps if someone came up with a plausible mechanism to bestow a probability distribution on the landscape, together with a plausible definition of the sector of the landscape that allows intelligent life, some testable prediction could be somehow extracted from the approach. But I am extremely skeptical that any of those things will happen in our lifetime.

Some excerpts from the ending of Part 2, after a critical evaluation of how string theory fares at solving each of the Five Great Problems:

String theory succeeds at enough things so that it is reasonable to hope that parts of it, or perhaps something like it, might comprise some future theory. But there is also compelling evidence that something has gone wrong…

So string theory is certainly among the directions that deserve more investigation. But should it continue to be regarded as the dominant paradigm of theoretical physics? Should most of the resources aimed at the solution of the key problems of theoretical physics continue to support string theory? Should other approaches continue to be starved to support string theory? Should only string theorists be eligible for the most prestigious jos and research fellowships, as is now the case? I think the answer to all these questions must be no. String theory has not been successful enough on any level to justify putting nearly all the eggs in its basket.

Part 3 is concerned with those “other approaches” that in Smolin’s opinion deserve as much support as string theory. Its first chapter is about puzzling observational data that don’t fit current frameworks: those supporting the MOND alternative to dark matter, the Pioneer anomaly, the large angular scale anomaly in the CMB data, and others. Next comes a discussion of speculative post-Einsteinian ideas which may be capable of experimental testing soon, the most prominent of which is Doubly Special Relativity. Here I tend to agree with Smolin: the ideas of DSR may prove wrong, but I find them fascinating and deserving of wider interest; especially given that (unlike string theory) there may be hard data coming soon to test them. The third and last chapter of this part explains the “alternative approaches” to quantum gravity: LQG, CDTs, non-commutative geometry, etc. I think Smolin tends to hype the promise of these approaches as much as string theorists do with their own, but (with that caveat in mind) the lay reader will profit a lot from this chapter and the preceeding one, as few popularizations are available on these topics. (Penrose talks about them in The Road to Reality, but how many lay readers can claim to understand half of it?)

Finally, in the fourth part, we get to the practical diagnose and proposed solution to “the trouble with physics”. Smolin finds the root of the problem in the approach to research prevalent in the “elementary particle community”, which continues to be that in string theory, and which he describes as a “brash, aggressive and competitive atmosphere, in which theorists vie to respond quickly to new developments and are distrustful of philosophical issues”. This contrasts with the “relativity community”, whose core values are “respect for individual ideas and research programs, suspicion of fashion, a reliance on mathematically clear arguments, and a conviction that the key problems were closely related to foundational issues about the nature of space, time and the quantum.” Smolin argues that the elementary particle style of research was adequate for the times in which new experimental data were found every other day, but that it is inadecuate to tackle the questions of today, when we have no incoming flux of data to build theories of quantum gravity. In this situation, this style of research leads to fashion-following fads and overcompetiveness with little fundamental progress. After giving many examples in which the string community seems to be falling in “groupthink”, drawing a distinction between “seers” and “craftpeople” (t’Hooft, Penrose and Julian Barbour are paradigmatical “seers”, while it would seem that most string theorists fall on the other side), and going over an interesting discussion on philosophy of science (Smolin is a bit of a fan of Feyerabend) we come to the advice:

We must recognize and fight the symptoms of groupthink, and we must open the door to a wide range of independent thinkers, being sure to make room for the peculiar characters needed to make a revolution. A great deal rests on how we treat the next generation. To keep science healthy, young scientists should be hired and promoted based only on their ability, creativity, and independence, without regard to whether they contribute to string theory or any other established research program. Peopler who invent and develop their own research programs should even be given priority, so that they can have the intellectual freedom to work on the approach they judge the msot promising. (…)

A research program should not be allowed to become institutionally dominant before it has gathered convincing scientific proof. Until it does, alternative approaches should be encouraged, so that the progress of science is not stalled by overinvestment in a wrong direction.

Smolin, in summary, does not so much take issue with the soundness of string theory as physics, but mostly with the way research in it is pursued and the way it is overinvested in in comparison with other approaches. Despite Smolin’s own admiration for Feyerabend, for me it seems natural to put his point in Kuhnian terms: Smolin wishes the area of quantum gravity and fundamental theoretical physics to be treated as a “pre-normal science”, the situation of a discipline before any paradigm becomes dominant and frames the research, as the field of dynamics was before Newton for example. The opposite view that we already have surpassed the pre-normal science stage and have now a paradigm guiding research in quantum gravity, namely string theory, is clearly expressed in Lubos Motl’s comment on a review by Aaron Pierce:

Aaron has an interesting idea that the author of the blue book is really complaining that it is no longer possible for the authors of seemingly fringe theories to get a lot of attention. The book is a lamentation for a bygone era, not an introduction to the field.

(I am not including any new links to Lubos' blog, as a modest and ineffective means to express my outrage at his praise of Augusto Pinochet a couple of days ago.)

Leaving aside Lubos’ usual lack of respect for those who disagree with him, the quote captures quite well the basic disagreement: should we consider that the successes of string theory are strong enough to merit its paradigm status (which automatically makes any other approach a “fringe theory”)? In this key issue I agree with Smolin. Regardless of how impressive we evaluate the successes and potential of string theory to be, it is dangerously unscientific to elevate to this status a theory that seems so far away from any possible experimental confirmation.

However, this does not mean that I necessarily agree with his diagnose and advice to make better progress. I would like to, because I am deeply interested in conceptual/foundational issues and because I have the instinctive dislike that many others share for the fads and fashions prevalent in the string community. (I know a very intelligent young theorist who started studying string theory but abandoned it disgusted by these mores.) But is funding dozens of individual researchers, each with his or her vision of the path to the Holy Grial, a better prospect? It seems to me that lacking experimental input, a focus in conceptual clarity cannot lead to truth anymore than frenetic calculations can. The only example we have in history of a fundamental theory discovered by something like “pure thought” is General Relativity –and it may have been a unique and unrepeatable thing. Moreover, a community of researchers on a subject, even if it is only of five or ten, seems to me necessary to have some standards of intersubjective validity.

The truth is that, insofar as new and unexpected data do not appear, I suspect the situation Smolin decries as a “crisis” will not be overcome. The reason why we are not making progress faster is that we can't build acceperators to probe Plack scale physics and that quantum gravity is very, very hard to develop without experimental clues; the sociological quirks of the string community, if regrettable, play a much smaller role in the explanation. In the meanwhile, we ought by all means to try to develop alternative research programs (preferrably ones that have prospect of connection with experiment (say I, as if that was easy!)). And not put too much energy into semi-scientific anthropic calculations, please.

This book is extremely readable and I would recommended for anyone interested in the present situation in theoretical physics. It is an extremely personal book, much more than most popularizations, so those who are new to the subject should probably balance it with some Brian Greene or Lisa Randall to get a more even view. But the questions it raises about the way science is and should be practised put it beyond the normal popularization and transform into an argument to which members of the field should pay attention, no matter if they ultimately disagree.

Mathematicians and computers

2006-11-29T22:54:00.000Z

[UPDATE: I apologize to all those who couldn't see the images and were asked for a password yesterday. I have fixed the mistake I made when uploading the images.]

There is an old joke about mathematicians that comes in many variations, but the basic idea is something like this:

A mathematician is asked how he would prepare tea given a mug, an empty kettle, a heater, and a jar with cold water. After thinking a minute, he answers he would fill the kettle with cold water, put it on the heater, and when it is hot pour it into the mug and make tea. Then he is asked what would he do if the kettle was already full with hot water. He now immediately answers that he would empty the kettle, thus reducing the problem to a previously solved one.

I was strongly reminded of it when I saw two amusing chess positions from computer games, both at Tim Krabbe's wonderful chess site. It seems that modern chess programs come with a table of theoretically won and drawn endings and the complete algorithm to win and draw them; they are also programmed to strive to reach one of the won endings at any cost. This makes sense because reaching it is exactly equivalent to reaching checkmate from the computer's point of view, but it gives rise to curious situations like the following:

Instead of playing the natural 1.Qxf4+, or the even stronger 1.Kb2 which leads to mate in 6, the program Hiarcs 7.32 plays 1.Qf7+!! The reason is, of course, that the resulting ending with two pawns against one is in the tablebase of won positions. Krabbe comments: "He'd rather look something up than think - how human." (You can find the position and the commentary at the bottom of this page).

An even more amusing case:

From a game between programs Deep Junior and Deep Fritz. White is a rook and a pawn up and things should be easy for it. Junior decides that it would be even easier if it had only a pawn up, because then the position is "theoretically won". So it played 1.Rc6+! But Fritz is no fool -it has the same tablebase and knows better than to fall into the trap, so it played 1...Kb5! The game continued in a display of masterful chess: 2.Rc5+! Kb4! 3.Rb5+! Kc4! 4.Rd4+! Kc3! 5. Rc5+! and now Black inexplicably gave up and played 5... Kxd4?, which after 6.Rf5 reaches a tablebase and White wins.

Krabbe's comment on this (item 114 of this page):

It's like buying a can of beer, then taking the plane to Zimbabwe because you have a friend there who knows how to open them.

What people search for here

2006-11-27T19:50:00.000Z

Life imitates Mathematics far more than Mathematics imitates Life

An interesting variation on Wilde's dictum, which brings to mind the old joke: engineers think that their equations approximate reality, physicists think that reality approximates their equations, and mathematicians don't think about reality.

bayes in pictures

If you wanted pictures of Thomas Bayes, here is one. Google Images is more useful for this than ordinary Google, you know.

what is time but the correspondance of your mind with reality

Ummm...

Scientists don't agree on whether zombies exist

Philosophers, and also some philosophically minded scientists, don't agree on whether zombies could exist. But I am not aware of any scientist thinking that they do in fact exist. In fact, I would be very surprised if any one did.

forma de la tierra

Manolito: "Un esferoide"
Maestra: "Muy bien! Con un ligero achatamiento en..."
Manolito: ..................... "En el animo?"

cuanto es un centimetro

Ehhh... la centesima parte de un metro? O preguntabas en pulgadas o en codos egipcios?

What are the laws of nature and physics in Tolkien's world

An interesting question! I don't think we have enough information from canon to write them up. Probably there are fundamental laws, but they are different enough in character from those of our world (involving, for example, irreducible ethical and aesthetical concepts) that to call them laws of physics is misleading.

time for earth to go round sun

One year. (You wouldn't believe how often I get variations of this question.)

Platonic solids, 1500 years before Plato

2006-11-20T17:49:00.000Z

My next post was scheduled to be my long review of The Trouble with Physics, almost finished by now, but I saw this in John Baez's last This Week's Finds column and thought it was too cool not to post it. The Neolithic inhabitants of what now is Scotland were familiar with the five Platonic solids already by the year 2000 BC or so, as evidenced by these stone carvings dating from that period:

Baez points to this paper for more on regular polyhedra in different areas of science, and to this great talk by himself on the dodecahedron.

More Shameless Self-Promotion

2006-11-14T10:48:00.000Z

If you wondered wht I had not posted during the past week, the answer is that I was busy finishing this:

Then again, how often does the Unruh-DeWitt detector click if we switch it carefully?

The transition probability in first-order perturbation theory for an Unruh-DeWitt detector coupled to a massless scalar field in Minkowski space is calculated. It has been shown recently that the conventional $i\epsilon$ regularisation prescription for the correlation function leads to non-Lorentz invariant results for the transition rate, and a different regularisation, involving spatial smearing of the field, has been advocated to replace it. We show that the non-Lorentz invariance arises solely from the assumption of sudden switch-on and switch-off of the detector, and that when the model includes a smooth switching function the results from the conventional regularisation are both finite and Lorentz invariant. The sharp switching limit of the model is also discussed, as well as the falloff properties of the spectrum for large frequencies.

Links

2006-11-06T17:43:00.000Z

- What has contributed more to our basic comprehension of the universe, computer science or cosmology and particle physics? Scott Aaronson says the former; Sean Carroll (and surely most other people with him) the latter.

- While we are at it, the lecture notes on quantum computation Scott Aaronson is putting on the Web are an unmissable read. The title says it all: Quantum Computing since Democritus.

- If you just can't get enough of the Dawkins wars, go to Evolution Blog where Jason is reviewing the reviews of The God Delusion, and mostly defending Dawkins against them. I skimmed a bit through the book at the university bookshop a few days ago, and it seemed more substantial that I had been lead to expect. I may even buy it when it comes in paperback.

- A discussion at The Volokh Conspiracy on wether voting is rational according to rational choice theory. Ilya Somin says yes (also here and here); Jim Lindgren says no (also here), or at least no according to Ilya's arguments (which are inspired by philosopher Derek Parfit). Of course, if the answer turned out to be "no" we could say "so much the worse for rational choice theory" instead of deciding not to vote.

- A philosophy blog that will enter my blogroll soon (or in the next update, which may not be so soon): The Splintered Mind. Recent highlights are a thumbnail summary of positions in philosophy of mind and why all of them are "weird"; a discussion on what is metaphysics, and an intriguing puzzle about vision: when we have our eyes closed, do we see the interior of our eyelids?

On not taking a stance

2006-11-01T23:29:00.000Z

After a typically excellent explanation of the hierarchy of energy scales that is crucial to allow life as we know it to exist in the universe, Sean Carroll says:

Because we don’t yet fully understand the origin of these fantastic hierarchies, we can conclude that God exists. Okay, no we can’t. Really we can conclude that we live in a multiverse in which all of the constants of nature take on different values in different places. Okay, we can’t actually conclude that either. What we can do is keep thinking about it, not jumping to too many conclusions while we try to fill one of those pesky “gaps” in our understanding that people like to insist must be evidence for their personal favorite story of reality.

This is exactly the kind of thing I was going to say in a post scheduled to write one of these days, but not on the fine-tuning problem but on the interpretation of quantum mechanics.

Just as in the fine-tuning problem we have theologians pressing on one side that the most rational solution is to accept the existence of God, and people like Leonard Susskind pressing on the other side that the only rational solution is to accept the real existence of the multiverse, on the interpretation of quantum mechanics we are also pressed on all sides by people who want to convince us of accepting "their personal favorite story of reality". Many physicists insist that we should give up the goal of a physical theory that describes reality "as it really is" and accept that theories are only tools to predict experimental outcomes. Some others urge that the most rational interpretation of quantum mechanics is one that allows a complete and deterministic description like in classical physics, even if this requires nonlocal hidden variables or backwards causation. Still others think that the only reasonable thing is to expect orthodox quantum mechanics to break down at a certain level, replacing unitary evolution of the state vector by a real, physical "collapse". And many others vehemently insist that the only interpretation that makes justice to quantum theory as it is, without adding any extra element nor relinquising realism, is the Everettian one that implies that millions of copies of us "branch out" into other universes at every minute.

And I... find every one of these alternatives unpalatable. And I don't think I am rationally required to take a stance and accept one of them, not even provisionally, any more here than in the fine-tuning case. The day we have a final theory of nature (meaning one that predicts correctly and explains all actual and possible experiments we can think of, including quantum gravity and everything else under the sun), and if it still includes quantum mechanics in its present form, I will have to decide which interpretation to suscribe to. But meanwhile, I think it is perfectly acceptable to go on, using quantum mechanics as every other physicist does, but without accepting any interpretation of it; not even the pragmatical one that rejects all interpretations as a matter of principle.

There is still much we don't know, and not only just "facts" but also whole realsm of nature we do not know how to think coherently about; not the lesser one, spacetime itself at a quantum level. It seems to me a reasonable expectation that a correct theory of quantum gravity could involve changes in the very structure of quatum theory, that would reveal the theory as we know it to be a weak-energy limit of some other kind of physics, with a conceptual structure we cannot even guess at the moment, but which would not be as "paradoxical" and "un-interpretable" as quantum mechanics as we know it, while not involving in any sense a return to classical (pre-quantum) notions.

Someone might say that this (and the similar position on the fine-tuning issue) is just a cop-out. If all the alternative ways of understanding something are on the table, and I can't think of any other, am I not supposed to decide which of the alternatives seems most likely to be true, and accept it until something else comes up? Am I? But why? There is a similar and very familiar fallacy commited by Intelligent Design defenders: if they point to some biological system which we can't explain at present how it evolved, then the only available explanation for the moment is God and we ought to accept it until we can give an evolutionary explanation. The fallacy is much more obvious here, because in the biological case we have already a good idea of what kind of explanation is needed, and we have excellent reasons to believe that with more research we will find it. While in the physical problem I discussed we have no idea at present of how to develope deeper theories that may solve them, which makes some people insist that quantum mechanics as we know it (and particle physics, string theory and cosmology as we know them) are final; that we have already reason to believe that we will be no further revolutionary overturning of our knowledge on those areas. This sounds hubristic to me. If the history of science shows something, it is that Nature is cleverer than we are.

I have a particular historical analogy on my side. When Newton proposed his theory of gravity, it was considered a strong criticism against it that it involved action at distance. The prevailing philosophy of nature, Cartesianism, implied that all physical action occured by direct contact, and the idea of a body exerting a gravitational force directly on another one, without any intermediate, seemed philosophically absurd. When it became clear the Newton's theory predicted accurately the planetary orbits, people had to come to grips with the fact of action at distance; and sure enough, just as with quantum mechanics there were many that said: "You are not supposed to understand how it works -that is a metaphysical question! It predicts the movements we observe, what else do you want?" Still others try to develope theories that mimic Newton's Law starting fom Cartesian models, like Le Sage's theory, and these efforts (which ultimately failed) can be compared to the "hidden variables" or the "physical collapse" models for quantum mechanics, which try to limit the weirdness going back to something more familiar. And there were certainly no lack of people who thought that the model of particles affecting each other at distance was there to stay forever; I think some even said that it could be known a priori that Nature behaved in that way, which shows how much familiarity with an initially weird idea can make for you. (I have seen also a priori deductions of quantum mechanics.) What finally happened, of course, was that Einstein came up with General Relativity, which explained how the gravitational force is transmitted through spacetime using mathematical tools, concepts, and a whole way of seeing nature which were completely unthinkable in the 18th century. The puzzle was finally solved when a much deeper and comprehensive theory was found. That, exactly, is what I am hoping for the puzzle of quantum mechanics

[Note: Does the expression "to take a stance" strike native speakers of English as an eggcorn? I hesitated between it and "take a stand"; checking Dictionary.com only the latter phrase appeared, but Google finds results for "take a stance" in a ratio of 1/10 to it. Which seems high enough to be linguistically acceptable, and it sounded much better to my ears. What do you think?]

Atheism, Religion, and Rationality; or, do you think that all those who believe in God are stupid?

2006-10-30T23:48:00.000Z

When I was 17 years old I went with some high school friends on a holiday to a sort of seaside resort, where one of us had somehow won a week staying as prize for something. We passed most of the days playing paddle (a sport popular in Argentina and unknown everywhere else; the closest thing I found on Wikipedia is plataform tennis) and most of the nights discussing philosophical matters, in the wonderfully passionate and earnest way you can discuss at that age. Our discussions covered the nature of the self, the soul, infinity, determinism, ethics, and God. I was quite vehement in those days about my then rather recent atheism, while one of my friends was equally convinced of God's existence and lectured to us on the Hinduistic conceptions of God and reincarnation; the discussions between us both were Titanic, unless they have been magnified by memory, which is certainly possible. What I am sure of is that one day, while we were in the paddle court and he was about to serve, he stopped, looked at me and asked suddenly: "Do you think that all those who believe in God are stupid?"

I think I, taken by surprise, told him that was not the moment, and to continue playing that we would discuss this later. And on the evening the question was posed again, and I can't remember what I said but we drifted into another long and unresolved debate on the existence of God, and many other things. But the question has stayed in my mind, sometimes making me feel a bit ashamed of how arrogant and certain of things I talked like in those times. And the memory has come back reading all the discussions all around the blogosphere on Richard Dawkins's new book, The God Delusion.

I have not read it yet, and I don't know if I will, but (as even someone who hadn't read anything by Dawkins before should guess from the title) it seems to be a rather aggresive attack on all forms of religious belief, which for Dawkins are both irrational and harmful. The reason it is so discussed is that Dawkins is not only a well-known biologist but a major public intellectual; he seems to fullfill more or less the kind of role Richard Feynman had twenty years ago, that of being the public Voice of Science. And the question under discussion, echoing my friend's one, is: is it true that all religious people are being stupid, or at least irrational?

I have no patience at the moment to trace and link to all the posts I have read discussing the book and its reviews; it seems that almost all the blogs I read have had something to say about it. To a rough approximation, they can be divided in three groups. First, those who think that there is no God and Dawkins is great (PZ Myers is the prime example). Second, those who think that there is no God but Dawkins is a jerk (Chad Orzel and John Wilkins are two good examples, though there are much more; perhaps more than in the first group.) And last, those who think there is a God and Dawkins is a jerk. (Brandon is one example among those I read regularly.) Unsurprisingly, I have not yet found any blogger who believes that God exists and Dawkins is great.

So what do I think? I tend to fall more into the second group; but there are at least two senses in which I could think "Dawkins is a jerk". [Perhaps needless to say, I do not think really that Dawkins is a jerk (nor do Chad or John or Brandon, I would think!). I admire him greatly and have been much inspired by his books on evolution. I use "Dawkins is a jerk" as a substitute for "I don't agree with/admire him for/respect him for his attacks on religion."] The first sense is pragmatical: even agreeing fully with him on philosophical grounds, and believing that religion is wholly irrational, it could be that scathing and disrespectful attacks on it are likely to backfire, to give a bad reputation of arrogance to atheists, to fuel the evolution controversy instead of defusing it, etc. The second is philosophical: I could think that the question of God's existence is not nearly so simple as Dawkins (and PZ and others) make it. In this post I will discuss only the second sense. There is much to be said for the pragmatic question, both for (perhaps strong atheistic voices are something our culture needs) and against. But at least if examining the philosophy we find that Dawkins is right on it there is a prima facie case for saying it loudly and clearly, and viceversa if we find he is wrong there is an even stronger case for not doing so, so examining the abstract matters first seems sensible.

So, is belief in God necessarily irrational? In one sense I agree with Dawkins that it is. (Don't I sound like a real philosopher, making one new pedantic distinction on every line? Be patient.) I have never seen any convincing argument for the existence of God; I believe all the evidence we have should compel us to reject it. I even accept what may be called the Master Argument for Scientistic Atheism, which implicitly or explicitly is used by Dawkins, PZ, and so many others, and which goes more or less:

1) Science can explain many things about the world without assuming the existence of God, and on those which it cannot explain, there is no reason to believe that it cannot explain them eventually; the hypothesis of God's existence is useless as a scientific one. When it makes specific predictions (like in creationism or studies on the efficacy of prayer) they are invariably disproved, and when it is stated in a metaphysical way that doesn't make predictions it becomes untestable, unnecessary and eliminable by Occam's Razor. In summary, following the rules of scientific evidence we ought not to believe in God.

2) But science is the only reliable source of knowledge we have about the universe, so we can only accept rationally those beliefs about the universe that are endorsed by the scientific method.

3) Therefore, belief in God is irrational.

I think most "reasonable" theists (whatever that means until we have reached a conclusion in our argument!) would accept some version of 1) and take issue with 2). And here comes the second and crucial sense in which I disagree with Dawkins et al. While I accept 2), and therefore am commited to say that belief in God is irrational, I don't think that 2) is so obvious that there cannot be "reasonable disagreement" about it.

What 2) is doing is to propose a standard of rationality, a standard by which to judge beliefs on factual mattes: to only accept those that science can endorse. If someone accepts this standard and then goes on to believe on, say, UFOs or ESP on presumed scientific grounds, we can say that the person is irrational (or misinformed about the evidence) and point out why. But if someone rejects wholly the standard 2), the situation varies. There are at least two ways in which the theist could reject it: saying that one can "in a way impervious to rational criticism" accept beliefs for which there is no evidential support when they are of great existential importance (Fideism), or that we can form rational beliefs upon factual matters which science cannot touch, using metaphysical reasoning. When any of these is embedded in a whole consistent philosophical system, be it William James's or Aquinas's, it is much more difficult to prove irrationality.

It would be tempting to say that it is impossible to do so, and that 2) is self-refuting in practice. The theist could say: "Your grounds for claiming 2) cannot be scientific, because your acceptance of 2) must be prior to your acceptance of scientific beliefs to justify them; so by your own standards you are being irrational". (Haven't you seen many times a theist argue on a discusison forum or blog comment that science is ultimately based on faith?) A similar argument is usually credited with killing logical positivism, the doctrine that statements which cannot be scientifically verified are meaningless; beacause this doctrine does not seem to be scientifically verifiable. But this would be going too fast. The scientistic atheist could answer "I propose 2) based on what science has shown us about the universe and ourselves, that we do not posses faculties to grasp untestable metaphysical facts, and that arbitrary beliefs will tend to be false no matter how existentially consoling they are. The whole of science and 2) support each other in a consistent way, forming an harmonious belief web which needs no external standard. [Insert references to Quine's naturalized epystemology, Neurath's boat, etc.] The theist's alternative standards of rationality will inevitably conflict at some point with the rest of his ordinary, scientifically endorsed beliefs and practices." (Perhaps logical positivism is also rescuable in a similar way, but this seems less likely.)

But the theist would now point out that sophisticated forms of fideism or of metaphysics are not shown to be inconsistent so simply; that a lot of philosophical work is needed to dispose of them. And this is what Dawkins, PZ Myers and the rest do not seem to see. It may be the case that naturalism (which Dennett defines as "the idea that philosophical investigations are not superior to, or prior to, investigations in the natural sciences, but in partnership with those truth-seeking enterprises" is the correct philosophy, and I in fact accept that it is and that there are compelling reasons for accepting it. But those reasons are philosophical; being a naturalist implies that "philosophical" does not mean for me "superior to or prior to" science, but it does mean that the arguments operate at a rather high level of abstraction, and that charging a Thomistic theologian or a Kirkegaardian fideist with simple irrationality is much more difficult than charging a believer in UFOs or in Creationism. The "sophisticated" believer has embedded the belief in God in a philosophical web of concepts and reasons that legitimitizes it while not conflicting overtly with undisputable scientific facts or practical-life rationality.

To many people trained in the sciences, who tend to be philosophical naturalists by default, these conceptual structures called theology are so weird, alien and uncomprehensible that they look sometimes rather like the elaborate "knowledge" exhibited in fandom (Star Wars, Harry Potter, etc.), based on shared, intersubjective, but purely fictional premises. There seems to be (I speak from the first person) just no motivation for thinking of the world in these terms. And so we fall into the objectifications that so annoy theists: "They just believe it because they were brainwashed into it. " "It is a substitute for the Father." "It is the opium of the people." It is perfectly possible, indeed likely, that more sophisticated scientific theories on this same "objective" line will provide some day a complete understanding of why people are religious (this seems to be Dennett's program in Breaking the Spell, another book I haven't read) and allow us naturalists to account for religion "leaving no residue". But a large percentage of makind will in all likeness continue to believe in religions. And given that on issues so basic an central to peoples different "webs of belief" there is little possibility, as I said, of proving that the other is being irrational by the standards the other can accept, I think we ought to treat beliefs which people regard as central to their lifes with a modicum of respect, no matter how weird they seem from our perspective, as long as those beliefs do not become a clear danger to others as in militant fundamentalism.

These issues of rationality and meta-rationality are, as you see, tricky. And meanwhile, we must all concede that the "rationality" discussed here is just theoretical rationality, the standard by which we judge beliefs; but there is a commonplace practical meaning of rationality or at least of "reasonableness" which is up to a point independent of these philosophical standards. It consists simply in being open to arguments, criticism, and discussion, admiting the possibility of error, being fair to one's opponents positions while discussing them, and so on. And this is much more important for assesing a person in real life than his or her conformity to this or that standard of theoretical rationality. And by this token, there are many, many theists who are very rational indeed. And certainly not stupid.

Back!

2006-10-24T20:59:00.000+01:00

I have been back in Nottingham for several days already, but with little time to post. Several posts are half-composed, but all only inside my head for the moment. Two of them are on old and eternally recurring topics in this blog: the interpretation of quantum mechanics (stimulated by an interesting talk I heard on Everettian interpreatations the day before leaving Vancouver) and the relation of science and rationality to religion (triggered by the discussions found all around the Web on Dawkins' new book The God Delusion). Another one is my long-promised review of Lee Smolin's The Trouble with Physics, a book which I have finally got hold of and started to read now.

I'm having a bit of trouble to upload pictures to Blogger, so the Californian photos views Bee) requested will have to wait. They were all excessively touristy anyway.

Chinatown, San Francisco:

Hiatus Announcement

2006-10-08T05:04:00.000+01:00

For some unknown reason, Internet connection is no longer available at the room where I am staying in the University of British Columbia campus. It is not known when the problem will be solved, but anyway I will be leaving Vancouver next Thursday, going to California for one week before returning to Nottingham. So posting is likely to be severly reduced, if not cut off altogether, for the following couple of weeks. I hope my Loyal Readers remain, well, Loyal, in spite of this interruption.

Irony Meter Explodes

2006-10-03T23:44:00.000+01:00

If you are oversensitive to irony don't even read the following link, let alone click on it:

Offended parent wants to remove Farenheit 451 from school curriculum.

Pictures of the week, links of the week

2006-10-02T07:41:00.000+01:00

Not much time or inclination to write a proper post, so here are some pictures of Vancouver sightseeing, and interesting links for you to check:

View of Vancouver; Capilano Suspension Bridge; Victoria (Vancouver Island); Whale Watching near Vancouver Island.

Now the links:

-Cosmic Variance discussion on the best popular science books on quantum mechanics. I'll have to be unoriginal and go for Feynman's The Character of Physical Law and QED, the merits of whic I don't need to promote here. Two more idiosyncratic preferences of mine are Alberto Clemente de la Torre's Física cuántica para filo-sofos and Bernard D'Espagnat's In Search of Reality. The first one is an excellent introductory explanation of quantum mechanics and its philosophical problems, probably untranslated into English. The second one is not really "popular science" but a rather heavy (if very interesting) philosophical discussion, but contains a superb explanation of the Bell inequalities.

-Jason Rosenhouse explains how the World Chess Championship is jeopardized by Vladimir Krammik's frequent excursions into his private bathroom.

-Brandon has an excellent post explaining why "Bayesian arguments" can't be used to prove anything, let alone God's existence, even if Bayesian epistemology is sound. The only thing one could prove with these arguments, he says, is that one is being rational in one's beliefs. To my mind this is still a large problem in Bayesianism (at least in its "pop" version), because it seems that almost any beliefs could be justified as rational by it.

-Doesn't this remind anybody of this?

Helen: You've got to lead our protest against this abomination!
[shows newspaper article]
Marge: Mm, but that's Michelangelo's David. It's a masterpiece.
Helen: [gasp] It's filth! It graphically portrays parts of the human body,
which, practical as they may be, are evil.

-On the Friendly Blogs Circle: my nonintelligent friend re-creates the Sargent Pepper's cover; my sinful friend imagines how The Da Vinci Code would have been directed by other filmmakers; my cynical friend lists ten hateful things; my rock star friend doesn't like to be called "sir"; my suspicious friend reviews United 93; and my tsumani-rowing friend seems to have quitted after only three posts.

-Now I've seen it all: Agatha Christie anime.

Units, fundamental constants, dimensional analysis, and all that stuff

2006-09-24T04:57:00.000+01:00

That is what is being discussed in this wonderful thread at The n-Category Café. It is one of those tricky subjects where everybody thinks it is all so simple, the true answer to all questions is obvious... and then everybody gives different answers. How many fundamental constants are there? Must fundamental constants be dimensionless? What are units? Do coordinates in GR have units, or are they in the metric tensor, or where? I once spent a couple of hours discussing these questions after lecture hours with my particle physics teacher, and no conclusion was reached. Great food for thought. (You can also check this paper for more.)

Eddington's Turing Test & Eddington's Path Integral

2006-09-21T22:55:00.000+01:00

In the (brief, I swear it!) breaks I take from work here at the University of British Columbia, I have been skimming through an old copy of Arthur Stanley Eddington's Space, Time and Gravitation I found in the office I was assigned. It is a fascinating book, written in 1920 and explaining -better than many more recent efforts- the central concepts of the then brand-new Theory of General Relativity in a fairly non-technical way, sprinkled with interesting conceptual and philosophical dicussion. I will quote two paragraphs, one worth reading for philosophers and the other one for physicists.

The first one: Eddington stresses constantly the formal nature of scientific laws and the way they capture structure, not content. E.G., in a discussion of Weyl's theory of electromagnetism (which failed in its original form but introduced into physics the inmensely fruitful concept of gauge symmetry) he says:

The geometrical potentials (k) obey the recognised laws of electromagnetic potentials, and each entity in the physical theory -charge, electric force, magnetic element, light, etc.- has its exact analogue in the geometrical theory; but is this found correspondence a sufficent ground for identification? The doubt which arises in our minds is due to a failure to recognise the formalism of all physical knowledge. The suggestion "This is not the thing I am speaking of, though it behaves exactly like it in all respects" carries no physical meaning. Anything which behaves exactly like electricity must manifest itself as electricity. Distinction of form is the only distinction that physics can recognise; and distinction of individuality, if it has any meaning at all, has no bearing on physical manifestations. [Emphasis added]

I have added the emphasis to remark what seems to me a bold conjecture, which Eddington ventures here but does not pursue in other places. returning to a conventional distinction between "form" and "content", which of course entails together with the formality of physics that the content or intrinsic structure of the physical world is unknowable. The temptation of believing that in our conscious thoughts we capture definite content, in turn, leads then naturally to panpsychism, or proto-panpsychism, or perhaps phenomenalism. But if we resist it and hold, despite initial implausability, that content has no meaning independent from a particular highly complex form (in Douglas Hofstadter's words, content is fancy form) then the obstacles for a naturalistic metaphysics disappear, the Turing Test is embraced as trivially valid, and zombies become an absurdity.

The second one: after remarking that in Weyl's theory curvature is no longer an absolute, gauge independent quantity, but action is, he says that action can be expressed as a pure number which, however, can take fractional values (though he mentions Planckian quantization of action he does not give it a fundamental importance. I don't understand very well why, but I forgive him; after all, it was 1920 and full quantum mechanics was still a thing of the future). He speculates then:

I can only think of one interpretation of a fractional number which can have absolute significance, though doubtless there are others. The number may represent the probability of something, or some fuinction of a probability. The precise function is easily found. We combine probabilities by multiplying, but we combine the actions of two regions by adding; hence the logarithm of a probabiliy is indicated. Further, since the logarithm of a probability is necessarily negative, we may identify action provisionally with minus the logarithm of the statistical probability of the state of the world which exists.

The suggestion is particulary attractive because the Principle of Least Action becomes the Principle of Greatest Probability. The law of nature is that the actual state of the world is that which is statistically most probable.

Isn't this a remarkable anticipation of Feynman's path integral? Okay, Eddington uses a real exponential intead of an imaginary one, and doesn't hit upon the quantum principle of summing over all paths and making the least action path the one with greatest weight only in the classical approximation. But still, I find the connection between action and probability, made on intuitive grounds years before quantum mechanics was formulated, to be a great insight bearing the mark of genius. Don't you agree?

UPDATE: Upon further reading, Eddington would definitely not endorse the Turing Test. He says in the last chapter: "The matter of the brain in its physical aspects is merely the form; but the reality of the brain includes the content". What a pity.

Revenge of the Pants

2006-09-21T08:13:00.000+01:00

Chances are any Star Wars geek worth his salt among my readership has already seen this page I discovered today, but just in case, don't miss The Star Wars Pants Page, based on the premise that any line from Star Wars can be made hilarious by taking a word from it and replacing it by "pants". You doubt it? See if you can read these selected examples without Laughing Out Loud:

"This is my apprentice Darth Maul. He will find your lost pants." --Darth Sidious

"This is where Chancellor Valorum's pants will disappear." --Senator Palpatine

"I find your lack of pants disturbing." --Darth Vader

"These pants are now the ultimate power in the Universe. I suggest we use them." --Motti

"A tremor in the pants. The last time I felt this was in the presence of my old master." --Vader

"I cannot teach him. The boy has no pants." --Yoda

"And I thought pants smelled bad, on the outside." --Han

"Yeah, well short pants are better than no pants at all, Chewie." --Han

"You have controlled your fear, now release your pants. Only your pants can destroy me!" --Darth Vader

"Search your feelings, Father, let go of your pants!" --Luke

P.P Cook on Penrose on the Arrow of Time

2006-09-19T07:35:00.000+01:00

At the Tangent Space you can read the report of a talk with Penrose's latest thoughts on the cosmological arrow of time. As book blurbs say: if you liked my posts on Price, you're gonna love this one!

Superhero Physics

2006-09-19T07:01:00.000+01:00

Don't miss the videos at Pharyngula.

On Price (and Penrose) on Time Asymmetry in Quantum Mechanics

2006-09-18T01:01:00.000+01:00

The latter chapters of Huw Price's book Time's Arrow and Archimedes' Point contain much of interest for the philosopher, including an account of the notion of causation which distinguishes clearly and convincingly its subjective, "anthropocentric" aspects from its objective ones. For the physicist, however, the most interesting thing is bound to be Price's original attempt to provide an interpretation of quantum mechanics which is realistic in Einstein's sense, involves local hidden variables, and gets around Bell's theorem by accepting a form of "advanced causation" in which the future affects the past. According to Price physicists ought to consider seriously the possibility of such advanced causation, because the ordinary interpretation of quantum mechanics is time-asymmetric, and his interpretation restores time-symmetry and makes QM fall in the line with the rest of physics in this respect.

Such strong claims need a strong defense, of course. Price starts by considering the proposition: "Two systems have typically their properties uncorrelated before interacting but often have them correlated after interacting". This time-asymmetric proposition is uncontroversially true for macroscopic systems (think of meeting a guy and talking with him; after the talk you will both remember things about the other so there will be a correlation between your brains which did not exist previous to the meeting) but this asymmetry is just a consequence of the lower entropy if the initial state, and can be explained statistically in the same way as the Second Law of Thermodynamics. (The temporally reversed process of you two knowing of each other before the meeting and not afterwards is physically possible, just reversing the trajectories of each particle, but is extremely unlikely in the same way that a gas going spontaneously all to the corner of its container is.) Price claims, however, that we accept implicitly the same asymmetry for microsystems (in this case he calls it "micro-innocence"), and that the statistical explanation is not available for them.

His thought experiment is a photon that passes succesively through two polarizers aligned in different directions. We expect the state of the photon between the polarizers to be correlated with the state (i.e. the direction) of the first polarizer but not with that of the second. In other words, the state of a photon after going though a polarizer is correlated with it, but the state of a photon before going through a polarizer is not. Price views this asymmetry as "suspicious" because according to him it is a) not related to the statistical asymmetry of macroscopical systems, and b) not based on any asymmetry in the dynamical laws of physics, which are (with an unimportant tiny exception) time-symmetric. He therefore suggests that we should consider abandoning the assumption of micro-innocence and allowing correlations between microscopic systems before they interact. And, lo and behold, he finds that this assumption is just what we need to get a "local hidden variables" interpretation of quantum mechanics work, despite Bell's theorem; because if the probability distribution of hidden variables can depend on the future measurements we will make on them, then the correlations in the Aspect experiments and their like are easily explained without any superluminal influences.

The argument has therefore two steps, which look like a sort of bait-and-switch: first comes the claim that micro- innocence is an independent principle used currently in physics without good justification, and then the proposal to reject it and allow "incoming correlations". But the first step can and should be contested, in my opinion. Micro-innocence does not seem to me to exist as a principle independent of "macro-innocence", which has a harmless statistical explanation.

It is telling that Price's example involves a photon interacting with polarizers, which are macroscopical objects. If we consider instead a completely microscopical process, like two electrons repelling by exchange of a photon, we don't expect the outcoming properties to be any more correlated than the incoming ones -conservation of momentum allows to calculate any of the four momenta from knowledge of the other three, without regard to whether they are in- or outcoming; the process is completely time-symmetric. So the suspicion arises that an apparent asymmetry appears only when we consider the interaction of a microsystem with a macrosystem, and in particular, when we view the interaction as a measurement.

It is in fact true that in the naive "measurements collapse the wave function" interpretation of QM measurements are irreduciblely time-asymmetrical. Knowing that there has been a measurement at time t allows us to infer that at time t' close to t in the future the state is close to an eigenstate of the measured quantity, but does not allow a similar inference for times t' close to t in the past. Roger Penrose has made much of this asymmetry with a purpose diametrically opposite to Price's; while Price tries to restore lost time symmetry by allowing incoming correlations in a hidden-variables theory, Penrose tries to posit the collapse of the wave function as an objective, robustly time-asymmetric process with an ultimate explanation in a speculative quantum gravity theory. Both seem to be making too much of the time asymmetry of measurements, in my opinion. It is a more plausible and conservative suggestion that measurements (involving as they do macroscopic systems) are time-asymmetrical for the familiar statistical reasons of increase of entropy. As John Baez pointed out in person to Penrose, if the measured quantum system and the measuring macroscopic apparatus were in thermal equilibrium measurements wouldn't have any asymmetry. Penrose responds to this objection in his more recent book, The Road to Reality, but his answer seems unconvincing. (If I remember correctly not having the book with me, he says that making inferences to the future we apply quantum mechanics confidently and it is only when we try to use it to the past we need to specify whether there is thermal equilibrium or not, etc. It seems to me that this is just because we normally take the macroscopic time-asymmetry by granted.) The decoherence approach to the measurement problem gives my contention that any asymmetry has familiar statistical roots a strong support, it seems to me.

However, this only undercuts partially Price's argument. He may not be able to make the "bait-and-switch" argument for his interpretation of QM if ordinary QM has no fundamental asymmetry in need of purging; but his proposal that local hidden variables theories with incoming correlations may account for the Bell inequalities survives intact. He skillfully and in my opinion successfully defends the idea against many intuitive but philosophically dubious objections (such as the one that these correlations would render free will impossible as the coming particle would "know" already which measurement we decide to perform on it) and makes it worth for any physicist working in the area to consider it seriously. And this, for a philosophical outsider to the physics communtiy, is no mean feat at all.

New Physics Carnival

2006-09-14T17:44:00.000+01:00

For all of those who wondered, I have arrived fine to Vancouver and am settling in. I have two substantive posts "under construction", the second part of the Price review and one on science and religion, but I don't know when will I have time for completing them. In the meantime enjoy the first edition of the new Blog Carnival for the Physical Sciences: Philosophia Naturalis.

To Canada I go

2006-09-08T21:48:00.000+01:00

Next Sunday I will be leaving Nottingham. For the next month and slightly more, I will be in Vancouver, working at the University of British Columbia by the grace of a Universitas 21 scholarship. The research group there is ideal for furthering my work, as it includes William Unruh, discoverer of the eponymous effect my research is centered on, and Kristin Schleich, who has done work with my supervisor in related areas. It is bound to be a unique and engaging experience, and I am looking forward very much to it.

Posting will probably take a short rest until I have settled down there. I apologize for the delay in my promised second posting on Huw Price and the arrow of time, but I have been extremely busy this week. Anyway, I am sure you are not taking seriously my promises any more! I have returned Price's book to the university library instead of taking it with me in my trip, but I may still write my post on it if I can remember enough and find the time.

The recently bought books I am taking with me, and on which you might get some comments later, are:

The Baroque Cycle by Neal Stephenson. A huge and by the looks of it vastly entretaining trilogy set in the times of Newton and making a swashbuckling epic out of the Scientific Revolution. When I read in my teens The Sleepwalkers, Arthur Koestler’s exhilarating (if not too reliable, I discovered later) history of the Scientific Revolution, I dreamt of writing one day a novel covering the 17th century like a tapestry. This seems to be the book I wished to write.

Truth, by Simon Blackburn. Looks like a clear and careful discussion of those philosophical issues that fascinate me most: the nature of truth, the correspondance (or lack thereof) of mind and language with reality, and the contest between scientific realism and various forms of relativism or pragmatism. In other words, another book I would have liked to write, or at least blog.

In the meanwhile, while you are (I hope) waiting eagerly for me to resume posting, you can entretain yourself by reading the tales of nerdom and geekdom the Science Blogs crowd have to tell. Just go to Tim Lambert's post and follow the individual links on the table. I am quite low on that scale, having scored only 64 at the nerd test on which most ScienceBloggers score over 90. I do have some pretty impressive tales of nerdiness to tell: dedicating breaks at elementary school to draw (out of memory) a map of the world with all the national flags I knew (more than I know now); inventing chess problems at age 12 and retrograde analysis chess problems at age 15, and in a party at age 14 while my friends were starting to flirt with the oppsite sex going instead to a quiet room with a copy of The Memories of Sherlock Holmes I had found. But I must confess (oh, the shame!) that I never saw a full episode of Star Trek. I guess this makes all my extensive knowledge of Tolkien, Star Wars and Asimov worthless and disqualifies me automatically from any nerdish competition.

Uncle Albert in the sky!

2006-09-03T22:40:00.000+01:00

Religious people usually get all the fun of spoting their veneration figure of choice, be it Jesus, the Virgin or whatever, in unlikely places like a piece of chocolate or the womb. Rationalists are more short on their list of sacred pareidolia, probably because our list of figures of veneration is also shorter, and because we don't call the press when we see one in an unlikely place. There are exceptions though: the Holy Flying Spaghetti Monster has been seen in the sky, and now our greatest secular saint, Albert Einstein, has appeared there as well:

(Seen at The Cloud Appreciation Society, which was linked to at Leiter Reports.)

On Price on the Arrow of Time

2006-08-31T16:45:00.000+01:00

Apologies for the lack of posting recently. I had made my mind to write my long-delayed review of Huw Price's book Time's Arrow and Achimedes' Point, but rereading it made me think so much about the issues it raises that I decided insteadt to write two or more separate posts discussing them. My evaluation of the book is simply that it is a must read if you are intereseted in the philosophy of time and/or conceptual problems at the foundations of physics.

Price assumes more or less the "atemporal" or "block universe" perspective in the philosophy of time, and discusses the asymmetries to be found within time, their physical and conceptual relation to each other and the possible ultimate explanations for them. It is a commonplace that all the asymmetries we see in ordinary life (broken glasses that do not reassemble spontaneously, etc.) are ultimately traceable to the second law of thermodynamics and the growth of entropy. (The fundamental dynamical laws are time-symmetrical, with the exception of some very specific particles subject to weak interactions and that so far as we know play no role in ordinary physics.) Price explains very clearly that the growth of entropy means only the tendency to macrostates of greater probability, and as such requires no particular explanation; what does require an explanation is the surprising fact that entropy is so low to begin with, when the "natural" state of the universe is a thermal equilibrium with maximum entropy. Boltzmann suggested once that the original state of the universe was really a high entropy one and that our low-entropy universe was just product of a fluctuation. This proposal has unacceptable quasi-solipsistic consequences, explained by Sean Carroll some weeks ago: by its reasoning it is far more likely that I have sprung directly into existence by a small fluctuation that created my present brain, not by a large one that created a whole universe in which my brain could eventually appear (the "Boltzmann's Brain" problem).

Price also does an excellent job of noting the "double standard" many authors commit in discussing time asymmetries: applying different criteria of plausability in different temporal directions. For example, a common assumption is that "incoming influences are independent": sistems interacting and with no common cause have their properties uncorrelated before the interaction but correlated after it. This seems entirely plausible and commonsensical, but if dynamical laws are time-symmetrical there is no good justification for it. A priori we should find the correlation of systems prior to interacting and uncorrelation afterwards to be just as possible. For macroscopic systems the principle can be justified from the second law and shown to arise from the special low-entropy state at the beginning of the universe (instead of positing it to explain the second law, as some physicists used to hope), whereas for microscopical systems the situation is more obscure (more on this later). Price is extraordinarily good at thinking in "atemporal" terms and uncovering hidden assumptions and double standards in the way we usually think about these questions.

The third chapter of the book contains a discussion of the so-called Arrow of Radiation (we see "retarded" waves going out from emmiting systems and not "advanced" wave focusing spontaneously in absorbing systems, a situation which is the time-mirror image of the first and just as possible on dynamical grounds). Price explains the well-known Wheeler-Feynman theory for explaining this asymmetry and gives a thought-provoking reinterpretation and reassessment of it. Alas, he does not discuss the how his version of the theory fares when quantum electrodynamics replaces classical electrodynamics. Can anybody point to a good discussion of a quantum version of the Wheeler-Feynman theory, if such thing exists?

The fourth chapter discusses cosmology, adressing the fundamental question: "What could be a possible explanation for the unnatural, extraordinarily low-entropy state of the early universe?" After considering and subjecting to criticism the opinions of many well-known physicists (the names of Davies, Penrose and Hawking come up repeatedly) Price concludes that there are four possible alternatives:

1) The anthropic approach: there is a multiverse with vast, vast, number of other universes, with all possible initial conditions, and ours is just a lucky one that got a sufficently-low entropy initial condition to allow for life to develope. Two objections to this view are that it postulates a huge number of unobservable entities, and that it is not clear whether a universe like ours is really the "least costly" way of creating life; if it isn't, we may face a Boltzmann's Brain problem here.

2) The asymmetric law proposal. Roger Penrose is the main contemporary defender of this idea, which essentially postulates a fundamental asymmetric physical law that constrains initial singularities (like the Big Bang) to have low entropy but puts no such constraint on final singularities (like stars collapsing into black holes, or a possible eventual Big Crunch). The main problem with this idea is how arbitrary it seems, especially if the dynamical laws are time-symmetric. (It may become more palatable if a future Unified Theory shows the tiny time asymmetry in weak interactions to derive from some deep time asymmetry at a fundamental level, but this is, I think, pure speculation at the moment. Do string theory folks out there have any idea about the possible status of T-symmetry in M theory?

3) The Gold universe (named for cosmologist Thomas Gold, the first one to propose it). There is a fundamental constraint that makes singularities have a low entropy, but it applies symmetrically. If the universe recollapses to a Big Crunch entropy would revert and decrease in the collapsing phase (at least from the point of view of our present standpoint; from the point of view of observers in the collapsing phase it may seem that the universe in expanding and increasing entropy, if as it seems likely our psychological arrow of time depends on the thermodynamical one). Also black holes should are constrained to have low entropy. Price favours this kind of view for its attractive stmmetry, but most physicists reject it. I will discuss below who has the upper ground in this discussion.

4) The "corkscrew" view. Like a factory that produces an equal number of left-handed and right-handed corkscrews, it might be that the fundamental theory of the universe is time-symmetric but only allows strongly time-asymmetric realizations. Price understands Stephen Hawking's present views (after retracting from a Gold-like view in the 1980s) as an example of this kind of view. Hawking claims that by using his "no boundary proposal" for Euclidean path-integral quantum gravity it can be proven that one extreme of the universe must have low entropy. He further agues that the connection between the thermodynamical and psychological arrows of time warrant calling the low-entropy extreme the "initial" one instead of the "final" one; and that once we have the universe securely expanding to a higher-entropy future we can use ordinary statistical arguments that say that a reversal of entropy is overwhelmly improbable, so the final state will be high-entropy instead of low-entropy.

Setting aside the anthopic view for the reasons outlined, Price favours the symmetrical Gold view (3) over Penrose's and Hawking's asymmetrical views, or at least argues that the Gold universe deserves a more serious consideration that most physicists are willing to give it. The standard argument against the Gold view is that the reversal of entropy requires events of huge statistical imporbability (broken glasses spontaneosly mending themselves, etc.) in the contracting phase. Price argues that this rebuttal invoves a Double Standard: we know already that the argument does not apply in one time direction, so why apply it confidently in the other? From an atemporal perspective, a glass breaking is a trajectory in configuration space precisely as improbable as a glass "unbreaking". We assume that an initial boundary condition of low entropy, whether imposed by fiat as in Penrose or dynamically as in Hawking, can override statistical considerations in one direction; so why not in the other? If we have a thoery saying that a "natural" initial boundary condition is a low entropy one, then the theory will have more symmetry, therefore be more plausible, if it also applies to final boundary conditions. Price devotes a lot of space to showing that there are (and are likely to be) no empirical contradictions to the Gold universe, at least until and if the universe actually starts recollapsing.

I think love of symmetry takes Price to far away in this. His reasoning neglects a very simple fact: we have lots of empirical evidence supporting a low-entropy past, none for a low-entropy future. Thus an explanation for the low entropy past is the only thing we need -and according to Ockham's Razor, we better take the simplest explanation that we can find. If we take Penrose's "Weyl Curvature Hypothesis" or Hawking's dynamically generated low-entropy singularity to be satisfactory explanations, then the question is: does the additional "simplicity" of temporal symmetry gained by extending them to the future as per Gold's universe compensate the additional "non-simplicity" of this making the universe's history far statistically unlikelier than it is without this extension? Statistical considerations are usually very powerful; they enable us, after all, to make reliable predictions from the Second Law in all ordinary situations. It is my opinion that they should not be overriden by a mere desire for symmetry. Absent any empirical evidence that the future is low-entropy (as we have regarding the past) the statistical unlikeliness of such a future takes precedence for me over symmetry, making me reject Gold's universe.

There is an additional consideration: it seems pretty certain that the universe will not in fact recollapse to a Big Crunch, but continue its expansion indefinitely at an ever greater rate, driven by dark energy. Morevorer, it seems to me as plausible as not that this is not an "accident", but a necessary feature of universe. (To have a Big Crunch, the dark energy content of the universe would have to be much lower and the matter energy content much larger; but the closeness of the actual values of both quantities, which are of the same order of magnitude, suggests that there may be a dynamical reason for it, which would make recollapse impossible). If this speculation is solid, then either Penrose's or Hawking's theories would support the "corkscrew" result of a naturally asymmetric universe. Price mentions the possibility of the universe expanding forever, but does not give it the importance he should in my opinion. (The book is written before the discovery of dark energy.) Price argues that even if the unverse does not recollapse there are local collapses to black holes; but I am not sure if these local singularities must be subjected to the same conditions than the global one. (For example, does anybody know if Hawking's "no boundary condition" can be applied to a black/white hole as well as to the universe? If yes, there is indeed a question of why does it not imply that collapsed black holes have low entropy; but I suspect the answer must be no, or Hawking would have realized this!)

There is a fifth possiblity which Price does not discuss because it is more recent than his book. It is suggested by Sean Carroll and Jennifer Chen in this article, which Sean is fond of citing in his Cosmic Variance posts. It claims to give a model for the emergence of the universe as a statistical fluctuation in a high-energy background, but avoiding not only the usual problems of the Anthopic Principle but also, and crucially, the Boltzmann Brain problem as well. I do not really understand how it manages to do so; it has appearently something to do with a distinction between entropy and entropy density, but I am not sure how this does the required job. I have asked Sean about it yesterday in this comment, but the question seems to have been lost among the discussion of sexism in science that highjacked the thread.

My next post on Price's book will discuss how he frames the question of "independence of incoming influences" with respect to microsystems, and the radical interpretation of quantum mechanics and Bell's theorem this leads him to. Stay tuned.

UPDATE: In case you are interested, this paper contains most of the the ideas of Price on cosmology I have discussed here. Check his list of publications for more material on this and other philosophical problems. I find Price to be one of the most interesting contemporary philosophers.

Pluto no longer a planet

2006-08-24T16:14:00.000+01:00

So says the definitive resolution of the International Astronomical Union. The provisional decision of last week, which kept Pluto's status as a planet and added three more planets to the Solar System by definitional fiat (see the discussions linked in this post) has been overturned by pressure of the "hard-liners". The main argument for the decision was, of course, that Pluto is known now to be just one more in a large class of trans-Neptunian objects, with no particular distinction among them justifying a special status for it. This is not clear, however, from the text of the press report:

Much-maligned Pluto doesn't make the grade under the new rules for a planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit."

Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's.

Instead, it will be reclassified in a new category of "dwarf planets," similar to what long have been termed "minor planets." The definition also lays out a third class of lesser objects that orbit the sun -- "small solar system bodies," a term that will apply to numerous asteroids, comets and other natural satellites.

The new definition makes little sense to me. What does "clear the neighborhood around its orbit" mean exactly? How does the overlap between Pluto's orbit and Neptune's disqualify "automatically" Pluto but not Neptune? I suspect there is some sloppy journalism going around here, and that the official resolution must be clearer.

Language Log's comment is:

The International Astronomical Union has spoken, and Pluto is no longer to be called a "planet." It is, however, still considered a "dwarf planet." Don't be fooled by any preconceptions you might have about English hyponymy: a dwarf planet is not, in fact, a planet.