Is Loop Quntuam gravity plausible formulation of quantum gravity

[kodama]

There simply are no other theories that have achieved a plausible formulation of quantum gravity in some universe. That universe is not necessarily ours, but recall that Einstein applied his tools first to another consistent relativistic theory of gravity that was wrong, as a step towards general relativity. If you stop researching string theory, you may be taking away things that would in fact help other theories.

Secondly, at best evidence is against low-energy supersymmetry. String theory is about high energy supersymmetry.

what about loop quantum gravity? plausible formulation of quantum gravity in some universe. not necessarily ours.

 

[atyy]

what about loop quantum gravity? plausible formulation of quantum gravity in some universe. not necessarily ours.

No, LQG is in a much weaker position regarding the emergence of general relativity, and a non-perturbative formulation.

 

[wabbit]

This is as far as I know a debatable position. LQG is a plausible approach just as much as ST and neither is a fully developped theory nor has made a successful prediction yet. There are other approaches too and the proof is in the pudding.

 

[marcus]

what about loop quantum gravity? plausible formulation of quantum gravity ...

A number of researchers do see it as a plausible formulation of QG, in this universe.

 

[kodama]

No, LQG is in a much weaker position regarding the emergence of general relativity, and a non-perturbative formulation.

i didn't mention spinfoam but now i will. lqg is non-perturbative formulation

 

[atyy]

i didn't mention spinfoam but now i will. lqg is non-perturbative formulation

Can you show a single paper where LQG is non-perturbatively formulated and gives general relativity?

 

[kodama]

A number of researchers do see it as a plausible formulation of QG, in this universe.

currently what is the status of "the emergence of general relativity" from LQG?

 

[atyy]

currently what is the status of "the emergence of general relativity" from LQG?

LQG does not even have an agreed upon non-perturabtive formulation, and there is evidence against GR emerging from EPRL which is the most "current" proposal for LQG.

http://arxiv.org/abs/1111.2865
http://arxiv.org/abs/1502.04640

 

[kodama]

LQG does not even have an agreed upon non-perturabtive formulation, and there is evidence against GR emerging from EPRL which is the most "current" proposal for LQG.

http://arxiv.org/abs/1111.2865
http://arxiv.org/abs/1502.04640

If Marcus can produce a paper(s) that (1) has a non-perturabtive formulation, and (2) shows GR emerging, would this satisfy the criteria you outline for string theory, that it is a theory of gravity in some universes not necessarily ours?

 

[atyy]

If Marcus can produce a paper(s) that (1) has a non-perturabtive formulation, and (2) shows GR emerging, would this satisfy the criteria you outline for string theory, that it is a theory of gravity in some universes not necessarily ours?

Yes. But I'm pretty sure he cannot :) The reason I'm pretty sure is that the bulk of recent work has been on EPRL, and there certainly are papers indicating that it is a non-perturbative formulation, and that GR emerges from it. However, the two papers by Engle linked above indicate that other positive indications notwithstanding, there are problems with EPRL regarding the emergence of GR.

I believe the non-perturbative definition of EPRL is also problematic, because the existence of the continuum limit is unknown. For example, http://arxiv.org/abs/1205.2019 states "In the general case, the definition of the refinement limit has recently been studied and formalized in [327]. There is no evidence that the amplitudes of any of the present models for 4D converge in such a limit."

 

[martinbn]

No, LQG is in a much weaker position regarding the emergence of general relativity, and a non-perturbative formulation.

What's weaker than none? Or do you mean to say that string theory has these to features?

Can you show a single paper where LQG is non-perturbatively formulated and gives general relativity?

Can you do that for string theory?

 

[atyy]

What's weaker than none? Or do you mean to say that string theory has these to features?

Can you do that for string theory?

Yes: AdS/CFT.

 

[wabbit]

Please. Neither theory is complete and neither has a single successful prediction to its credit. Can we wait till they do intead of speculating on which one might or might not eventually do so or comparing their looks?

 

[martinbn]

Yes: AdS/CFT.

Can you clarify? In what sense is the AdS/CFT conjecture a formulation of string theory that has GR as a classical limit? Can you show the papers?

 

[atyy]

Please. Neither theory is complete and neither has a single successful prediction to its credit. Can we wait till they do before speculating on which one might or might not eventually do so?

You have to remember my original point was made in the context of an argument for stopping all research on string theory, because it was diverting focus from other good research on quantum gravity. It is undeniable that string theory has the best foothold at the moment, and not LQG. I don't think putting down string theory is a good way to advance the interests of LQG research, which I do support.

Here is the thread in which I originally made my point: https://www.physicsforums.com/threads/evidence-against-string-theory-paper.808802/#post-5077563.

 

[atyy]

Can you clarify? In what sense is the AdS/CFT conjecture a formulation of string theory that has GR as a classical limit? Can you show the papers?

Lots of them, just google.

 

[martinbn]

Lots of them, just google.

In that case I have no more comments!

 

[kodama]

YYes: AdS/CFT.

does that work in 4 AdS dimensions and 3 dimensions CFT?

 

[atyy]

does that work in 4 AdS dimensions and 3 dimensions CFT?

The best studied example of AdS/CFT is in 3+1 CFT dimensions and 5 AdS dimensions.

http://arxiv.org/abs/gr-qc/0602037
http://arxiv.org/abs/1010.6134
http://arxiv.org/abs/1106.6073
http://arxiv.org/abs/1412.5704

 

[kodama]

The best studied example of AdS/CFT is in 3+1 CFT dimensions and 5 AdS dimensions.

http://arxiv.org/abs/gr-qc/0602037
http://arxiv.org/abs/1010.6134
http://arxiv.org/abs/1106.6073
http://arxiv.org/abs/1412.5704

5 AdS dimensions is probably unphysical

 

[atyy]

5 AdS dimensions is probably unphysical

There's some work on 4 also. But the main problem is that AdS itself is probably unphysical.

 

[kodama]

There's some work on 4 also. But the main problem is that AdS itself is probably unphysical.

what about LQG and the Kodama wave function? might be unphysical, but it is a theory of QG in some universes.

 

[atyy]

what about LQG and the Kodama wave function? might be unphysical, but it is a theory of QG in some universes.

Hmm, I don't know what the status of that is in LQG. Off the top of my head, I think it was proposed for canonical LQG. But canonical LQG in those days was not a theory, and the spin foam approach was meant to help complete canonical LQG. The most promising proposals of the last few years have been the FK and EPRL spin foam formulations. I don't know the status of FK, but as the papers above show EPRL seems to have problems.

 

[kodama]

Hmm, I don't know what the status of that is in LQG. Off the top of my head, I think it was proposed for canonical LQG. But canonical LQG in those days was not a theory, and the spin foam approach was meant to help complete canonical LQG. The most promising proposals of the last few years have been the FK and EPRL spin foam formulations. I don't know the status of FK, but as the papers above show EPRL seems to have problems.

what about CDT, GFT, NCG, twistor theory ?

 

[atyy]

what about CDT, GFT, NCG, twistor theory ?

I think they are not even close to LQG. Actually, GFT is closely related to LQG, but it has the potential to develop independently also.

 

[wabbit]

I find NCG intriguing - from the outside it seems mathematically strong, well developped, and includes a version of the standard model. I'd be happy to find a non-technical overview of its current state as a theory of QG if such a thing exists.

 

[atyy]

While we're running randomly through all QG approaches, I'd like to remind people of this bizarre link between LQG and string theory found by Freidel, Krasnov and Livine in http://arxiv.org/abs/0905.3627: "Interestingly, the integration kernel that appears in the decomposition formula turns out to be the n-point function of bulk/boundary dualities of string theory."

 

[Dale]

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