The new spinfoam and strings as only game in town

[ensabah6]

For 30 years string theorists have said string theory is the only game in town. It is the only known consistent theory of quantum gravity. LQG has not been shown to have GR in the low energy regime so it is not been shown to be even related to gravity. The only reply I've seen is that string theory is not background independent whereas LQG is, so string theory is not a complete candidate for QG.

Recently, http://www.icra.it/MG/mg12/talks_plenary/Freidel.pdf

has shown that the newer Rovelli spinfoam models, which incorporate an immirizi parameter and reproduce canonical LQG kinematics apparently have a good semiclassical limit, it reproduces GR in the low-energy large volume regime, where GR has been shown to be experimentally verified, along w/quantum corrections at high energy.

If these claims are verified, then will this new SF models be considered "quantum gravity" or is there still more work to be done before the statement "string theory only game in town" is falsified?

 

[humanino]

One must not think too far to claim "only game in town". This is absurd even before you end the sentence. Please stop repeating partisan, biased, claims. For instance, even though some perturbative formulation of string theory are obviously background dependent, do you see gauge invariance in Feynman rules ?

 

[atyy]

Recently, http://www.icra.it/MG/mg12/talks_plenary/Freidel.pdf

has shown that the newer Rovelli spinfoam models, which incorporate an immirizi parameter and reproduce canonical LQG kinematics apparently have a good semiclassical limit, it reproduces GR in the low-energy large volume regime, where GR has been shown to be experimentally verified, along w/quantum corrections at high energy.

Is there a new claim that it reproduces GR in the low energy limit, or is it still the more limited claim of http://arxiv.org/abs/0809.2280 "These results provide considerable evidence in favor of the proposed spin foam amplitude as a valid amplitude for quantum gravity, in the sense that it reproduces expected semiclassical behavior. There is, however, more work to be done to fully confirm this picture. ... That is, we have demonstrated the proper semiclassicality for certain histories that one should sum over in computing amplitudes. What we are ultimately interested in is the semiclassical property of the sum over amplitudes. Given a boundary spin network, we would like to sum over all spins in the interior compatible with the boundary spin network and show that the resulting amplitude gives an object that can be interpreted as the exponential of the Hamilton–Jacobi functional of a gravity action. Our result is a necessary condition for this to happen, but we have not shown that this is sufficient."

 

[marcus]

Atyy, in order to get some idea about the answer to your question you need to read the PDF file of Freidel's 14 July talk carefully.

The source you offer for comparison is 10 months earlier--September 2008. By the same author. It is the same voice, with the same degree of openness, objectivity and clarity, giving a report on the program at two different times.

It helps to have watched Freidel over the course of several years (and lately the two Freidel and Conrady). Also listen to him talking in a 3-way seminar with Ashtekar and Rovelli on the ILQGS. He doesn't ever seem to rest. His work, and the field as a whole at present, is developing rapidly.

We don't know the actual state of things. In the past, for example, they've often proved stuff for the Euclidean signature first because it's simpler, and then when they see how the proofs go they extend the results to Lorentz signature. Personally I think the semiclassical limit is a done deal but they may still be finishing details. It's clear to me that a lot of progress has been made since the September 2008 paper that you cited. But you have to judge for yourself by reading carefully.

Strings as the only game in town is mere hype, as I think you realize. I, for one, starting as early as 2003, have been hearing a whole succession of invalid reasons why the Loop-etc. QG program could not ever possibly succeed. As soon as one reason is discredited, another is invented to take its place, and naive people take to repeating it.
Loop is a family of approaches which morph--they are to some extent fluid. Thinking up reasons why the program can never succeed seems increasingly futile to me---a waste of time motivated by a need to cling to the fantasy of an "only game in town" status. This clinging to an illusion of privileged status is not constructive. But that's how it is, we get a a certain amount of that here.

 

[MTd2]

Loop is a family of approaches which morph--they are to some extent fluid.

Aren't they converging?

 

[marcus]

Aren't they converging?

Everybody judges for themselves and we can't know the future---we can't tell if the converging tendency we saw in the past couple of years will continue, or which particular approaches will hit the wall and die and which will keep on evolving. You and I know this and take it for granted, I think, as a basic assumption or rule of the game.

So yes! I think there have been signs of convergence. Loop/foam now looks to me much more simplical than it did 2 years ago, much more like Regge.
Ashtekar and Rovelli both use the word "lattice" a lot more than I recall them doing before.
Modesto seems to have gotten that (4 -> 2) dimensional reduction with zooming into small scale to carry over from Loll's triangulations.
The whole renormalization group flow and AsymSafe thing.
So far Loll CDT does not have a cosmological bounce (so is unlike Loop cosmo) but it might get one. Loop people are talking about renormalization group stuff which they didnt do earlier.

And it is to some extent you MTd2 who have been finding papers where this convergence thing happens. Oh yes, the Horava business. There is a lot of convergence.

but fundamentally I am agnostic. I don't know how it will go. I only have an impression of the immediate past.

Another thing is Group Field Theory (Daniele Oriti is a leader in GFT, but I think it was Freidel or Freidel Krasnov who started it perhaps even 10 years ago, I forget). GFT has a potential to unify a number of approaches. In a sense different approaches are simply different ways to calculate and combine measurements---it is all looking at the same nature, using whatever method of calculating is appropriate as long as they seem consistent with each other.

So yes I think so, but no I don't know for sure. As usual

 

[ensabah6]

what exactly is group field theory?

 

[marcus]

This post has some useful GFT links
https://www.physicsforums.com/showthread.php?p=2300906#post2300906
Because of its overlap with several QG approaches:
Loop/Spinfoam
Causal Dynamical Triangulations
Quantum Regge Calculus
it can be seen as potentially a unifying framework.
(this is the content of slide #18 of Oriti's Azores talk, 10 July 2009. This is probably the same as the invited plenary talk he is giving in Beijing today 7 August.)

Here is the Azores talk:
http://www.fqxi.org/data/documents/Oriti Azores Talk.pdf

It gives the basics of GFT at a very introductory level in slides #10 thru #15.

A more advanced introduction was given in Oriti's 10 June talk in Wroclaw. That is available both as slides and as a 40 minute video.
The link to the Wroclaw talk video is in that PF post I just gave the link to.

========================

Besides potentially serving as a unifying framework, GFT is already used by Spinfoam QG researchers as a technique to aid calculation. When e.g. Rovelli's associates calculate the graviton propagator in Spinfoam QG, I recall at some stage they convert the problem to a GFT problem, to make it easier to calculate the answer. Though it is not something that a researcher, for example Rovelli, would use all the time or in every situation, it is nevertheless already a good option to have in the toolkit.

The title of Oriti's Azores talk was:
"The Group Field Theory Approach to Quantum Gravity: a QFT for the Microscopic Structure of Spacetime"