### Report on MG11

The Marcel Grossmann Meeting is a huge conference.

A really, really huge one.

Number of plenary talks: 35 (in 6 mornings)

Number of parallel sessions: about 20 in each of 4 afteroons, that is, about 80.

Number of talks per parallel session: about 10 or 12. Total number of talks, then: about 850.

Total number of registered participants: 957.

Number of parallel sessions one was interested in attending in each afteroon: about 4 or 5.

Possibilities of doing this: 0.

Percentage of talks attended that actually included interesting or useful information: about 30%. Percentage of the talks spent sleeping, or trying to decide whether to move to another room or to give the next speaker in this room a chance: 70%.

OK, that was the easy part of the report. Now comes the tough part: an actual day by day report of that 30% of interesting things. (I know nobody forces me to write it, but I feel not doing it would be betraying my Gentle Readers). So:

Monday 24/7

Morning: Opening remarks by Remo Ruffinni et. al. Roy Kerr receives a Marcel Grossmann Award, which is an artistic work inspired in, coincidentally, geodesics on the Kerr metric. More awards given, more opening remarks. Coffee break in which I somehow find myself, with some friends, getting a photo with Kerr himself:

Also, the discovery is made that no food is served with the coffee, which begs the question of where has all the money of registration fees gone to. (Yes, I know that "begs the question" does not really mean that).

Later in the morning: Talks by Thibault Damour and Hermann Nicolai on hidden symmetries in near-singularity (string theory-based) cosmology. Hard stuff. Also: Polyakov gives a nice introduction to basic issues in string theory. The main message of his talk is that the gravitational field is not fundamental, but a mean field generated by completely different (Yang-Mills) objects, according to the string-gauge duality.

Afternoon: Time spent switching between parallel sessions on LQG, on Cosmological Singularities, and on Quantum Fields. Probably I should have sticked to the latter, which is my actual area of research, but there was a second QFT session on Tuesday and I had to go to that one becasue my talk was scheduled for it, so I decided to try some other stuff on Monday. I should say that moving between rooms was not trivial as they where distributed among more than ten buildings, and it could take up to ten minutes to go from one to another.

I started in the LQG session, where Thomas Thiemann opened giving a standard introduction to the theory, stressing his own Master Constraint approach to the Hamiltonian constraint. He said that the Hamiltonian constraint in LQG has nothing to do with "generation of time translations" and that misunderstanding of this creates the artificial "problem of time"; according to him, it is possible to define an operator H which is a Dirac observable and generates physical time translations, but is unrelated to the Hamiltonian constraint. I didn't get the reference he mentioned for this; does anyone know it? He also talked a bit about the results in LQC (loop quantum cosmology) where it is proven that for coherent states peaked on the classical FRW solution the expected value of the curvature operator is finite, even if the operator itself is not bounded. The next talks in this session where very technical and quite over my head, making me regret not having moved to the QFT room earlier. I passed by the Cosmological Singularities room where a friend gave a nice and understandable talk on the validity of different energy conditions for various kinds of cosmological "events" (Big Bang, Crunch, Rip, Bounce, etc.)

After the afternoon coffee break I went to the Quantum Fields session, but I was already very tired and my notes are scarce. The talk I have more notes on was one by Deborah Konkowski on whether quantum mechanics "heals" singularities, in the sense that, for a classical singular background, the evolution of a quantum wave packet is well defined without adding special boundary conditions at the singularity. It seems that for a rather large class of singularities, it is.

Tuesday 25/7

Morning: More talks on string theory and cosmological singularities. To bad I can understand so little of such a hot topic.

Later: Joe Polchinski talks about cosmic superstrings. Explaining that a cusp in them can provoke a burst of gravitational waves like the crack of a whip, he takes out a whip and proceeds to demonstrate his words, Indiana Jones-wise. Laughs in the audience.

Later: Abhay Ashtekar gives yet another standard introduction to LQG, including a list of FAQs at the end which I wrote down completely and include here, because they will sure be of interest to many readers:

Afternoon: My talk was scheduled for 16:00 on the Quantum Fields session, so I headed for there after lunch. Nothing worth mentioning in the talks previous to mine. Two of those speakers didn't appear so my talk was moved 40 minutes earlier, casuing several friends who were coming to the session just for it to miss it. The talk itself went fine and there were no challenging or enlightning questions afterwards.

After coffee break I passed quickly by the Geometric Calculus session; this is a kind of mathematical language for physics made famous by David Hestenes, though it goes back to Clifford, and which I found intriguing since reading this online introduction. There was a talk on that session on application of GC to Rindler observers; the notes I took there will possibly be of use to my work. I went later to the LQG room, where Stephen Zhoren gave an interesting talk on entropy in causal set theory. He has shown that counting the states of a causal set within a given volume gives, in the continuum limit, an entropy proportional to the area (consistent with the holographic bound) in any number of dimensions. The coefficent depends on a numerical parameter (the ratio of the discreteness scale to the Plack length) which is fixed by the requirement S=A/4 and is universal for any volume in a given dimension, but depends on the dimension. However, d going to infinity, the coefficient approaches 1! I have no idea if this has a deep physical meaning, but it sure looked cool.

In the evening we had a popular lecture on "The Fate of the Universe". Lots of cool animations of structure formation. Quote of the day: "In 10^27 years, white dwarfs destroy dark matter". If it doesn't strike you as funny, repeat it to a friend innocent of astrophysics.

Wednesday 26/7

There were talks on the morning on gravitational wave detectors. Unfortunately from now on the lights in the plenary room were turned off during the talks (this was done to reduce the heat) and so I didn't take notes for them. I will start writing directly on the afternoon sessions each day from now on.

Oh, but on Wednesday we had the afternoon free. In the evening we had a conference banquet in the Ritz hotel, which answers at last where all the money that couldn't be spent in biscuits for the coffe breaks went. It was perhaps the poshest dinner I ever attended to, although an English friend said the exact word is not "posh" but "swanky", a term I was ignorant of before.

Thursday 27/7

I started the afternoon in the "Analog Models of and for GR" session, which began with a good talk by Grigory Volovik. This talk first introduced analog (sonic) models for black holes, and then launched a polemical attack against the "cosmological constant problem". According to Volovik, a condensed matter analogy suggests that the natural value for the vacuum energy density of the universe is not the Planxk scale as usually said, but the scale of the matter energy as in fact observed. The reason is that microscopic degrees of freedom (atomic, in the condensed matter case) store energy which cancels the huge value found by cutting off the calculation at the Planck scale (which would be the interatomic distance in the analogy). The argument is developed in this paper. I suspect high-energy physicists would be more convinced if Volovik could provide a concrete microscopical model for vacuum spacetime, instead of just relying on the analogy. Interesting stuff though.

I turned later to the Quantum Gravity Phenomenology session, chaired by Giovanni Amelino-Camelia. The first talk I heard there was by Ralf Lehnert, who sketched a comprehensive strategy for analysing Lorentz and CPT invariance violations in beyond Standard Model physics. There are two kinds of these violations: a kinematical one, with modified transformations between inertial frames alla DSR, and a nontrivial vacuum that picks a preferred frame (dynamical violation; e.g. a fundamental vector field). In this second case passive but not active Lorentz invariance is preserved; changing coordinates between frames does not alter the equations, but rotating an experiment gives different results. Without clear input from fundamental theories on what to expect, the strategy suggested by Ralf was to write the most general Lagrangian which corrects the SM by terms of the form (tensorial background) x (SM field), assume the background is constant at low energies, and study which terms thus generated can be experimentally probed and constrained.

A second interesting talk in the same sesssion was by Jerzy Kowalski-Glikman on DSR. According to Jerzy, the best way to think of DSR is not as "a deformation of SR with two observer-independent scales" (a formulation which makes physical interpretation obscure) but as "an effective theory of particles coupled to gravity, when spacetime is flat and local degrees of freedom of gravity are switched off". Thus, the physical interpretation must be imported from a theory of gravity. In 3D QG coupled to particles it is already proven that when the (topological) gravitational degrees of freedom are integrated out then the net result is that the energy-momentum space for the particles becomes curved: de Sitter, in fact. In the effective theory that results positions are non-commutative. In 4D gravity can be expressed as a topological theory + constraints, the latter enclosing the local degrees of freedom, which vanish in the DSR limit. There is thus reason to hope that some kind of deformed particle kinematics comes out in the flat limit, but it's precise form is yet unknown.

There was a third interesting talk in the same session, by Tomasz Konopka, who works at Perimeter and whom I knew from the Loops 05 conference. He suggested a "5D" approach to 4D DSR: embedd the momentum space of a particle in a flat 5D space where it is represented by the de Sitter hyperboloid with scale parameter k, and add the constraint m^2 = P^2 where P is the momentum four-vector. Thus the value of the "extra" momentum coordinate P4 is constrained to be P4^2 = m^2+k^2, which makes it different for each species of particle. Tomasz discussed physical consequences of this model when applied to QED. The audience seemed a bit skeptical about this, because he did not find a Lagrangian for this theory but just postulate reasonable-looking Feynman rules. Which, moreover, required (for consistency with observation) that the deformation constant k be different for each species of particle, and in particular zero for the photon. Hum...

Friday 28/7

Started in the Black Hole Thermodynamics session, chaired and opened by Don Page. He made a general introduction to the field and then discussed the particular problem of the statistical mechanics of extremal black holes. He talked really fast, so after a while I stop trying to take notes. I can only offer the following quote (about how to distinguish between some two possibilities): "Just wait 10^837 years and see whether the radiation has stopped. Unfortunately, this is slightly longer than the time over which a PhD student generally hopes to complete a thesis".

An interesting talk in the session was by Paul Anderson, who is a specialist in stress-energy of quantum fields in black hole spacetimes. He discussed how for many cases the usual analytical approximations show a divergence in the horizon, which if physical ought to alter significantly the geometry via backreaction -but is it physical? In most cases bumerical calculations don't show it. After the talk, during the coffee break, I discussed with him a bit the results of my first degree thesis project (published here), which showed very different results for vacuum energy of massless scalar field around stars than for black holes, and to my surprise he was aware of the paper and had even discussed it with Bob Wald. It seems that work of him of which I was not aware (almost contemporary with our paper) shows that the vacuum energy around black holes is after all similar to the one we found in stars, not different as previous calculations had shown... but there are intriguing discrepancies for all values of the parameter that couples the field to the curvature other than the minimal one (zero) and the conformal one (1/6). So he is thinking carefully about these issues now. It's nice to see that my old work has been noticed -after two years with zero citations I was feeling a bit discouraged.

And this was about it, insofar as my notes cover -in the rest of Friday I chose badly my sessions and talks and did not hear much of interest. Thus ends my extra-long report on hte conference. How many of you have read it complete?

Oh, and one word of advice for those traveling to Berlin: Don't let tourism guides fool you into making the one hour long queue for enetering the Reichstag. The Schwarzschild geometry-like mirror structure inside the dome (seen below), though nice, is not worth the waiting.

A really, really huge one.

Number of plenary talks: 35 (in 6 mornings)

Number of parallel sessions: about 20 in each of 4 afteroons, that is, about 80.

Number of talks per parallel session: about 10 or 12. Total number of talks, then: about 850.

Total number of registered participants: 957.

Number of parallel sessions one was interested in attending in each afteroon: about 4 or 5.

Possibilities of doing this: 0.

Percentage of talks attended that actually included interesting or useful information: about 30%. Percentage of the talks spent sleeping, or trying to decide whether to move to another room or to give the next speaker in this room a chance: 70%.

OK, that was the easy part of the report. Now comes the tough part: an actual day by day report of that 30% of interesting things. (I know nobody forces me to write it, but I feel not doing it would be betraying my Gentle Readers). So:

Monday 24/7

Morning: Opening remarks by Remo Ruffinni et. al. Roy Kerr receives a Marcel Grossmann Award, which is an artistic work inspired in, coincidentally, geodesics on the Kerr metric. More awards given, more opening remarks. Coffee break in which I somehow find myself, with some friends, getting a photo with Kerr himself:

Also, the discovery is made that no food is served with the coffee, which begs the question of where has all the money of registration fees gone to. (Yes, I know that "begs the question" does not really mean that).

Later in the morning: Talks by Thibault Damour and Hermann Nicolai on hidden symmetries in near-singularity (string theory-based) cosmology. Hard stuff. Also: Polyakov gives a nice introduction to basic issues in string theory. The main message of his talk is that the gravitational field is not fundamental, but a mean field generated by completely different (Yang-Mills) objects, according to the string-gauge duality.

Afternoon: Time spent switching between parallel sessions on LQG, on Cosmological Singularities, and on Quantum Fields. Probably I should have sticked to the latter, which is my actual area of research, but there was a second QFT session on Tuesday and I had to go to that one becasue my talk was scheduled for it, so I decided to try some other stuff on Monday. I should say that moving between rooms was not trivial as they where distributed among more than ten buildings, and it could take up to ten minutes to go from one to another.

I started in the LQG session, where Thomas Thiemann opened giving a standard introduction to the theory, stressing his own Master Constraint approach to the Hamiltonian constraint. He said that the Hamiltonian constraint in LQG has nothing to do with "generation of time translations" and that misunderstanding of this creates the artificial "problem of time"; according to him, it is possible to define an operator H which is a Dirac observable and generates physical time translations, but is unrelated to the Hamiltonian constraint. I didn't get the reference he mentioned for this; does anyone know it? He also talked a bit about the results in LQC (loop quantum cosmology) where it is proven that for coherent states peaked on the classical FRW solution the expected value of the curvature operator is finite, even if the operator itself is not bounded. The next talks in this session where very technical and quite over my head, making me regret not having moved to the QFT room earlier. I passed by the Cosmological Singularities room where a friend gave a nice and understandable talk on the validity of different energy conditions for various kinds of cosmological "events" (Big Bang, Crunch, Rip, Bounce, etc.)

After the afternoon coffee break I went to the Quantum Fields session, but I was already very tired and my notes are scarce. The talk I have more notes on was one by Deborah Konkowski on whether quantum mechanics "heals" singularities, in the sense that, for a classical singular background, the evolution of a quantum wave packet is well defined without adding special boundary conditions at the singularity. It seems that for a rather large class of singularities, it is.

Tuesday 25/7

Morning: More talks on string theory and cosmological singularities. To bad I can understand so little of such a hot topic.

Later: Joe Polchinski talks about cosmic superstrings. Explaining that a cusp in them can provoke a burst of gravitational waves like the crack of a whip, he takes out a whip and proceeds to demonstrate his words, Indiana Jones-wise. Laughs in the audience.

Later: Abhay Ashtekar gives yet another standard introduction to LQG, including a list of FAQs at the end which I wrote down completely and include here, because they will sure be of interest to many readers:

- Q: Shouldn't Einstein equations have quantum corrections? A: Yes, and they do; this is clearly visible in LQC where the Friedmann equation gets corrected in hte semicalssical regime.

- Q: Aren't there quantization ambiguities in LQG, which reflect the ambiguities in the perturbative treatment due to non-renormalizability? A: Yes, there are ambiguities and they are being worked upon; but they just reflect ignorance about the proper method of quantizing in background-independent contexts. They are not the same as those in the perturbative treatment; those arise for inadequacy of the Gaussian fixed point, and there is evidence that a non-Gaussian fixed point exists. (This is a common criticism of LQG I find in Lubos' and Jacques' blogs, and I suspect they would not be satisfied by this answer. Is it wishful thinking or is there solid evidence for it?)

- Q: If the Hamiltonian constraint has not been solved, does this not mean there has been no progress? A: By the same token there has been no progress in reaching a non-perturbative formulation of string theory. (Tu quoque!)

- Q: Isn't LQC too restrictive because of symmetry assumptions? A: Yes, of course. Current work is being done on inhomogeneities; LQC provides useful heuristic intuition.

- Q: In black hole entropy, what is the justification for assuming a Boltzmann statistics for the punctures? A: Misconception! No such assumption is made. The horizon states are counted exactly. Counting punctures with a Boltzmann statistic is an pedagogical presentation device.

Afternoon: My talk was scheduled for 16:00 on the Quantum Fields session, so I headed for there after lunch. Nothing worth mentioning in the talks previous to mine. Two of those speakers didn't appear so my talk was moved 40 minutes earlier, casuing several friends who were coming to the session just for it to miss it. The talk itself went fine and there were no challenging or enlightning questions afterwards.

After coffee break I passed quickly by the Geometric Calculus session; this is a kind of mathematical language for physics made famous by David Hestenes, though it goes back to Clifford, and which I found intriguing since reading this online introduction. There was a talk on that session on application of GC to Rindler observers; the notes I took there will possibly be of use to my work. I went later to the LQG room, where Stephen Zhoren gave an interesting talk on entropy in causal set theory. He has shown that counting the states of a causal set within a given volume gives, in the continuum limit, an entropy proportional to the area (consistent with the holographic bound) in any number of dimensions. The coefficent depends on a numerical parameter (the ratio of the discreteness scale to the Plack length) which is fixed by the requirement S=A/4 and is universal for any volume in a given dimension, but depends on the dimension. However, d going to infinity, the coefficient approaches 1! I have no idea if this has a deep physical meaning, but it sure looked cool.

In the evening we had a popular lecture on "The Fate of the Universe". Lots of cool animations of structure formation. Quote of the day: "In 10^27 years, white dwarfs destroy dark matter". If it doesn't strike you as funny, repeat it to a friend innocent of astrophysics.

Wednesday 26/7

There were talks on the morning on gravitational wave detectors. Unfortunately from now on the lights in the plenary room were turned off during the talks (this was done to reduce the heat) and so I didn't take notes for them. I will start writing directly on the afternoon sessions each day from now on.

Oh, but on Wednesday we had the afternoon free. In the evening we had a conference banquet in the Ritz hotel, which answers at last where all the money that couldn't be spent in biscuits for the coffe breaks went. It was perhaps the poshest dinner I ever attended to, although an English friend said the exact word is not "posh" but "swanky", a term I was ignorant of before.

Thursday 27/7

I started the afternoon in the "Analog Models of and for GR" session, which began with a good talk by Grigory Volovik. This talk first introduced analog (sonic) models for black holes, and then launched a polemical attack against the "cosmological constant problem". According to Volovik, a condensed matter analogy suggests that the natural value for the vacuum energy density of the universe is not the Planxk scale as usually said, but the scale of the matter energy as in fact observed. The reason is that microscopic degrees of freedom (atomic, in the condensed matter case) store energy which cancels the huge value found by cutting off the calculation at the Planck scale (which would be the interatomic distance in the analogy). The argument is developed in this paper. I suspect high-energy physicists would be more convinced if Volovik could provide a concrete microscopical model for vacuum spacetime, instead of just relying on the analogy. Interesting stuff though.

I turned later to the Quantum Gravity Phenomenology session, chaired by Giovanni Amelino-Camelia. The first talk I heard there was by Ralf Lehnert, who sketched a comprehensive strategy for analysing Lorentz and CPT invariance violations in beyond Standard Model physics. There are two kinds of these violations: a kinematical one, with modified transformations between inertial frames alla DSR, and a nontrivial vacuum that picks a preferred frame (dynamical violation; e.g. a fundamental vector field). In this second case passive but not active Lorentz invariance is preserved; changing coordinates between frames does not alter the equations, but rotating an experiment gives different results. Without clear input from fundamental theories on what to expect, the strategy suggested by Ralf was to write the most general Lagrangian which corrects the SM by terms of the form (tensorial background) x (SM field), assume the background is constant at low energies, and study which terms thus generated can be experimentally probed and constrained.

A second interesting talk in the same sesssion was by Jerzy Kowalski-Glikman on DSR. According to Jerzy, the best way to think of DSR is not as "a deformation of SR with two observer-independent scales" (a formulation which makes physical interpretation obscure) but as "an effective theory of particles coupled to gravity, when spacetime is flat and local degrees of freedom of gravity are switched off". Thus, the physical interpretation must be imported from a theory of gravity. In 3D QG coupled to particles it is already proven that when the (topological) gravitational degrees of freedom are integrated out then the net result is that the energy-momentum space for the particles becomes curved: de Sitter, in fact. In the effective theory that results positions are non-commutative. In 4D gravity can be expressed as a topological theory + constraints, the latter enclosing the local degrees of freedom, which vanish in the DSR limit. There is thus reason to hope that some kind of deformed particle kinematics comes out in the flat limit, but it's precise form is yet unknown.

There was a third interesting talk in the same session, by Tomasz Konopka, who works at Perimeter and whom I knew from the Loops 05 conference. He suggested a "5D" approach to 4D DSR: embedd the momentum space of a particle in a flat 5D space where it is represented by the de Sitter hyperboloid with scale parameter k, and add the constraint m^2 = P^2 where P is the momentum four-vector. Thus the value of the "extra" momentum coordinate P4 is constrained to be P4^2 = m^2+k^2, which makes it different for each species of particle. Tomasz discussed physical consequences of this model when applied to QED. The audience seemed a bit skeptical about this, because he did not find a Lagrangian for this theory but just postulate reasonable-looking Feynman rules. Which, moreover, required (for consistency with observation) that the deformation constant k be different for each species of particle, and in particular zero for the photon. Hum...

Friday 28/7

Started in the Black Hole Thermodynamics session, chaired and opened by Don Page. He made a general introduction to the field and then discussed the particular problem of the statistical mechanics of extremal black holes. He talked really fast, so after a while I stop trying to take notes. I can only offer the following quote (about how to distinguish between some two possibilities): "Just wait 10^837 years and see whether the radiation has stopped. Unfortunately, this is slightly longer than the time over which a PhD student generally hopes to complete a thesis".

An interesting talk in the session was by Paul Anderson, who is a specialist in stress-energy of quantum fields in black hole spacetimes. He discussed how for many cases the usual analytical approximations show a divergence in the horizon, which if physical ought to alter significantly the geometry via backreaction -but is it physical? In most cases bumerical calculations don't show it. After the talk, during the coffee break, I discussed with him a bit the results of my first degree thesis project (published here), which showed very different results for vacuum energy of massless scalar field around stars than for black holes, and to my surprise he was aware of the paper and had even discussed it with Bob Wald. It seems that work of him of which I was not aware (almost contemporary with our paper) shows that the vacuum energy around black holes is after all similar to the one we found in stars, not different as previous calculations had shown... but there are intriguing discrepancies for all values of the parameter that couples the field to the curvature other than the minimal one (zero) and the conformal one (1/6). So he is thinking carefully about these issues now. It's nice to see that my old work has been noticed -after two years with zero citations I was feeling a bit discouraged.

And this was about it, insofar as my notes cover -in the rest of Friday I chose badly my sessions and talks and did not hear much of interest. Thus ends my extra-long report on hte conference. How many of you have read it complete?

Oh, and one word of advice for those traveling to Berlin: Don't let tourism guides fool you into making the one hour long queue for enetering the Reichstag. The Schwarzschild geometry-like mirror structure inside the dome (seen below), though nice, is not worth the waiting.

## 5 Comments:

Not completely. *g* The rest after lunch.

As for Thiemanns comment, if you formulate a normal system in a constraint way you get the single constraint C = p_t + H, this does not generate time translations. In fact C vanishes weakly and all states and Observables do not evolve under the flow it induces. The Dirac Observable H does though, in an appropriate sense.

By fh, at 11:44 AM, August 05, 2006

Thanks for going to so much trouble here. I wish I could be there myself!

By Kea, at 6:53 AM, August 06, 2006

Only 2 responses so far, & only one allusion to the HEAT...It was HELLISH ! Florida-in-summer,(30+C)all week. Not the conf.halls, nor hotels, restaurants, niteclubs etc. had fans or AC !!##$@%&*!!

What the hell is wrong with the conf. organizers & Berliners...the usual pathetic response was, "Oh its not usually like this"...Sure.

It utterly negated any enjoyment of the conf. for me and many others I spoke with. The organizers are oblivious to this, and need to be made aware.

Jimbo

By Anonymous, at 1:47 AM, August 13, 2006

Oh, yes, the heat WAS unbearable. But now that I'm in Nottingham and temperature has fallen to a rainy 15 C, I somehow start to miss it... Summer with no heat depresses me.

By Alejandro, at 11:34 AM, August 13, 2006

Next Summer's big relativity conf. will be in Australia (GenRel & Grav, and Amaldi), and the conf. admin has already assurred me ALL facilities will be airconditioned ! But wait: Is'nt summer really WINTER in Aussie land ?

By Anonymous, at 3:06 AM, August 15, 2006

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