Reality Conditions

Sunday, July 29, 2007

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

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]


Wednesday, July 25, 2007

Loops 07: Conference report (part 2)

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.


Wednesday, July 18, 2007

My next three days

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!)

Tuesday, July 17, 2007

Loops 07: Conference report (part 1)

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!


Thursday, July 05, 2007

Loops 07: pics and links

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.