Reformulation of Loop gravity in progress, comment?

atyy

Yes, critical configurations is their term. To me at looks like a "saddle point approximation" - ie. configurations which extremize the action? In AdS/CFT, the gravity is classical when the saddle point approximation becomes very good, which is the large N condition, which is why I expect there should be a similar condition in spin foams - just saying that the saddle point configuration is correct is necessary but not sufficient, I think.

atyy

The next two papers following http://arxiv.org/abs/1304.5626 start to follow up on this.
http://arxiv.org/abs/1304.5627
http://arxiv.org/abs/1304.5628
"The semiclassical analysis is carried out by taking into account the sum over spins in the regime where all the spins are uniformly large. Such an analysis is a natural continuation of the previous studies of large spin asymptotics [6–9], which don’t take into account the sum over spins."

References [6-9] include the first in Muxin Han's new series http://arxiv.org/abs/1304.5626, and the most important papers on the semiclassical limit before this.
http://arxiv.org/abs/0809.2280
http://arxiv.org/abs/0907.2440
http://arxiv.org/abs/0902.1170

Good job! With almost 5 years since the first of those, I thought they'd abandoned ship for relative locality or a reformulation. I guess the Muxin Han and Mingyi Zhang papers were preparation for this. Now what do they find ...

tom.stoer

One should not overrate the semiclassical analysis. It is an important consistency check and a calculational tool for quantum corrections, but not more. The quantization ambiguities we still face in LQG need not be visible in these approximations. The deep QG regime is beyond this semiclassical analysis.

So this is an important research program, but not the one that will tell us the ultimate truth about LQG.

atyy

Well, if this fails, that would tell us the ultimate truth about LQG;)

But yes, I agree if this works we still need to know whether the infinite sums implied in Eq 30 of http://arxiv.org/abs/1303.4636 work out.

marcus

Although the Han Krajewski paper (and other recent ones by Han) are very interesting and in my view contribute to a sense that LQG may be on the right track, this post is about something else. I continue to be surprised by the ecumenical breadth of the upcoming Loops conference. Not only are several allied (also in a sense rival) background independent QG approaches are represented but also continuing observational efforts to constrain energy-dependence of speed of light. For instance among the invited plenary speakers I see Henrique Gomes, Fay Dowker, Dafne Guetta.

Henrique Gomes has done research in spinfoam asymptotics and more recently on shape dynamics.
http://inspirehep.net/author/H.Gomes.1/
Fay Dowker, as we know, is one of the main researchers in Causal Sets
Dafne Guetta http://inspirehep.net/author/D.Guetta.1/ is an expert on Gammaray Bursts (GRB) with 80 citable papers of which the two most recent are
http://inspirehep.net/record/1223049?ln=en
http://inspirehep.net/record/1222810?ln=en
She has been a frequent collaborator with Tsvi Piran.

Vincent Rivasseau and Razvan Gurau, two of those most active in tensor model QG, are also among the plenary speakers. Also Steve Carlip and Bill Unruh. It's a speakers list drawn from a wide range of research interests. I wonder if this will establish a pattern to be followed in subsequent Loops conferences.

I see also David Skinner, whose most recent papers have been about gravity in twistor space and about N=8 supergravity:
http://inspirehep.net/search?p=autho...ion%3Aciteable

Frank Hellmann is also one of the plenary speakers.

marcus

It's interesting to see how the Loops 2013 organizers are allocating the plenary talks. 19 invited speakers are listed so far. The conference is scheduled for 5 full days and unless they break with tradition they will have to save most afternoons for parallel sessions of contributed talks. So my rough guess is there's time for somewhere around 25 plenary speakers---just a really rough guess.

A lot of the 19 announced so far are younger--rising generation people. Some of the names are not all that familiar to me. Some that are familiar (such as Frank Hellmann) have been working on new variants of LQG. Maybe I shouldn't say "reformulation"---the new versions might turn out to be largely equivalent: the same theory couched in a different mathematical language. Or might not. I'll continue to call these efforts reformulation. And there are close relatives that aren't LQG but connect with it, like shape dynamics and tensor models.

Here are some of the younger speakers and some (including senior folk) whose talks seem to indicate a thematic branching out. I've indicated my non-expert guesses as to topics their talks might cover.

Ivan Agullo, DAMPT Cambridge (pre-inflationary, bounce) cosmology
Aurelien Barrau, Universite Joseph Fourier observational tests of loop cosmology
Eugenio Bianchi, Perimeter Institute (several including) loop black holes and thermodynamics
Fay Dowker, Imperial College, London causal sets
Henrique Gomes, University of California, Davis shape dynamics
Dafne Guetta, Braude College constraints from GRB and neutrino astronomy
Razvan Gurau, Université Paris-Sud tensor models
Frank Hellmann, Max Planck Institute for Gravitational Physics holonomy spinfoams
Etera Livine, Ens de Lyon (several possibilities including) spinorial LQG
Alejandro Perez, Centre de Physique Theorique (several including) loop BH and thermodynamics
Vincent Rivasseau, Universite Paris-Sud XI Orsay tensor models
David Skinner, DAMPT Cambridge, IAS N=8 supergravity?
Bill Unruh, University of British Columbia analog models of QG?
Madhavan Varadarajan, Raman Research Institute completing the LQG Hamiltonian approach

Bill Unruh is certainly no youngster, but I've included his name in this list because he might be talking about research outside of what has normally been covered at Loop conferences. Likewise Rivasseau. I find myself unable to predict with any assurance what some of these people will be talking about.

Madhavan Varadarajan is an interesting speaker because his recent papers (solo, with Casey Tomlin, or with Alok Laddha) show progress towards completing the original LQG program involving a satisfactory Hamiltonian constraint operator: e.g. http://arxiv.org/abs/1210.6877
Casey Tomlin gave a [video] lecture on some of this work today:
http://pirsa.org/13040104/

For clarification about recently developed spinorial LQG among the work by Etera Livine see e.g. http://arxiv.org/abs/1302.7142

marcus

Someone who is not on this list is Yasha Neiman (complex part of GR action, and entropy) but I want to remind us of his papers and also mention a video talk that was given just last week:
http://pirsa.org/13040106
The imaginary part of the gravitational action and black hole entropy
Speaker(s): Yasha Neiman
Abstract: I present a candidate for a new derivation of black hole entropy. The key observation is that the action of General Relativity in bounded regions has an imaginary part, arising from the boundary term. The formula for this imaginary part is closely related to the Bekenstein-Hawking entropy formula, and coincides with it for certain classes of regions. This remains true in the presence of matter, and generalizes appropriately to Lovelock gravity. The imaginary part of the action is a versatile notion, requiring neither stationarity nor any knowledge about asymptotic infinity. Thus, it may provide a handle on quantum gravity in finite and dynamical regions. I derive the above results, make connections with standard approaches to black hole entropy, and speculate on the meaning of it all. Implications for loop quantum gravity are also discussed.
Date: 18/04/2013

The papers I mentioned earlier. Some are with Norbert Bodendorfer. It is conceivable that either Norbert or Yasha could be talking about this at Loops 2013.

Here are the three most recent papers by Yasha, who recently joined the Penn State group as a postdoc.
http://arxiv.org/abs/1304.3025
The Wald entropy formula and loop quantum gravity
Norbert Bodendorfer, Yasha Neiman
16 pages

http://arxiv.org/abs/1303.4752
Imaginary action, spinfoam asymptotics and the 'transplanckian' regime of loop quantum gravity
Norbert Bodendorfer, Yasha Neiman
22 pages, 5 figures

http://arxiv.org/abs/1301.7041
The imaginary part of the gravity action and black hole entropy
Yasha Neiman
37 pages, 8 figures

You can see that the January solo paper by Yasha has almost the same title and the PIRSA video talk that he gave last week.
Also it is noteworthy that the ILQGS schedule was recently revised to give him the 7 May timeslot.
His ILQGS online talk will be:
The imaginary part of the GR action and the large-spin 4-simplex amplitude
I'm not sure but this 7 May talk may turn out to be related to one he gave at Perimeter in 2011:
http://pirsa.org/11110111/
Parity and the Immirzi Parameter in Lorentzian Spinfoams
Yasha Neiman
The parity invariance of spinfoam gravity is an open question. Naively, parity breaking should reside in the sign of the Immirzi parameter. I show that the new Lorentzian vertex formula is in fact independent of this sign, suggesting that the dynamics is parity-invariant. The situation with boundary states and operators is more complicated. I discuss parity-related pieces of the transition amplitude and graviton propagator in the large-spin 4-simplex limit. Numerical results indicate patterns similar to those in the Euclidean case. In particular, parity-related components of the graviton propagator differ by a phase. I discuss possible resolutions of this issue.
02/11/2011

marcus

I focused earlier on only 14 of the 19 invited speakers listed so far by the Loops organizers and so didn't properly consider what the talks by the following major people might be about.
Abhay Ashtekar, Pennsylvania State University
Steve Carlip, University of California, Davis
Viqar Husain, University of New Brunswick
Kirill Krasnov, University of Nottingham
Carlo Rovelli, Le Centre de Physique Théorique
That will have to wait until more information is available.

Meanwhile here's a short list of the themes identified in the previous post#193. The project of completing LQG Hamiltonian dynamics, pursued by Varadarajan and by Tomlin among others could also be called "closing the quantum constraint algebra" off shell, I suspect. The quantum constraint algebra corresponds classically to the hypersurface deformation algebra, which closes in GR. The snag which the Hamiltonian approach hit in the late 1990s seems essentially to have been that the quantum operator version of HD algebra did not close off shell. Correct me, anyone, if this is isn't clear. I will omit a couple of themes I'm not at all sure about (mere guesses in connection with talks by Unruh and Skinner) and highlight the last four, because less familiar.

cosmology/observational tests
black holes and thermodynamics
causal sets
shape dynamics
tensor models
holonomy spinfoams
spinorial LQG
closing constraint algebra

It should be remembered (I should remind myself frequently) that the main themes of a conference do not necessarily all have to be reflected in the list of plenary talks by invited speakers. Presumably there will be contributed talks in parallel sessions and some of these will arouse significant interest. I'm guessing that something mentioned in post#194, "the imaginary part of the gravitational action" will figure in what the conferees take away. In line with that it could be recommended to watch last week's PIRSA video ( http://pirsa.org/13040106 ) and listen to the 7 May ILQGS talk ( http://relativity.phys.lsu.edu/ilqgs/ )