What is the current state of research in quantum gravity?

Question

I was browsing through this and was wondering what progress in quantum gravity research has taken place since the (preprint) publication.

If anyone can provide some helpful feedback I would be greatly appreciative.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user UGPhysics

Comments

I don't know whether you can call this progress, but the AMPS paper shows that there are significant problems with the "complementarity" idea for resolving the black hole information paradox (which Carlip's paper calls "not very convincing," along with the rest of the ideas for a resolution).

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Peter Shor

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@user2307487: The Ashtekar paper is available on arxiv without a paywall: arxiv.org/abs/grqc/0404018 . It's only about LQG.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Ben Crowell

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On the phenomenological side, it is worthwhile to mention the analysis of an experiment involving bouncing, quantum neutrons in a gravitational field. Even though, the result was not surprising for most people, to my knowledge it is the only experiment that clearly shows that quantum mechanics works with a gravitational potential. See my answer here physics.stackexchange.com/questions/69873/…

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user drake

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@drake Agreed, the neutron bouncer experiments are fascinating and deserve wider recognition. That said they are more properly tests of the weak equivalence principle than quantum gravity. As such they have discovery potential for micron scale modifications of classical gravity (like strongly coupled scalar-tensor chameleon theories) but the Planck scale is still a long way away.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Michael Brown

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@MichaelBrown Agreed, it is a test of non-relativistic quantum matter in a weakly curved background. I hope this is clear in my answer, because I read many people saying totally wrong things about this experiment, such as the experiment showed the quantization of the gravitational field.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user drake

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An update in 2004 <br/> It came up quite easily in this search.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user user2307487

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@UGPhysics, You have to be specific. There are different research groups doing different research in different areas to address same topic- quantum gravity: String Theory, Quantum Loop Gravity being major. They have meetings, briefings on progress and so on. "The State" if research is definitely different from 2001 when focus was mainly on String Theory which has sifted towards LQG. But, I am not an expert, not even a physics major.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user user16682

Answer 1

A few developments since 2001:

There is the new field of loop quantum cosmology, which shows some promising signs of being able to actually calculate things that might conceivably be testable by observation.

The LHC hasn't found any evidence of supersymmetry, which may reduce the appeal of string theory.

There have been some high-precision tests of dispersion of the vacuum: http://arxiv.org/abs/0908.1832 . The results have been negative. This was at one time though to be a potential test of LQG, but now it looks like LQG doesn't yet make any definite prediction.

LQG has been reformulated in the last 10 years, so if you're going to try to learn it, you want to learn it from more recent references. A presentation of the new LQG is given in Rovelli, 2011, "Zakopane lectures on loop gravity," http://arxiv.org/abs/1102.3660 .

In 2004 there was an internet debate between Smolin and Susskind on Smolin's claim that "the Anthropic Principle ... cannot yield any falsifiable predictions, and therefore cannot be a part of science:" http://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Ben Crowell

Comments

what references do you recommend about the recent reformulation?

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user lurscher

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@lurscher: Added a link to a review paper by Rovelli.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Ben Crowell

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Does the recent reformulation of LQG respect Lorentz invariance?

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user anna v

Answer 2

Although; this question's phraseing it's comments and its answers are a bit biased towards LQG (and and friends), the question does not explicitly prohibit string theory.

If you read Lubos Motl's TRF,,, you'd known that Since the advent of M(Atrix) theory and Matrix string theory (c.f. this, this, this, this this, and this), there has been an outburst in a field called String Phenenmenology.

For example, Bobby Acharya, Gordon Kane, and Piyush Kumar predicted that realistic M(Atrix) - theory vacua give rise to a Higgs mass of 125 GeV/c₀² because of relation to the MSSM. in 2011. And it was way before the LHC found the Higgs, so that experimental observation did not have any effect on that (see the comments. It was predicted by the MSSM, which Acharya, Kumar, Kane showed that realistic string vacua predict ). . Instead, it is a GREAT evidence for string theory. Also, note that the Higgs mass did not agree with the Standard Model, only the MSSM, and thus realistic M(atrix) theory vacua.

There have also been outbursts in string cosmology. 

Comments

@luksen: (1) I talked about the 2013 result. (2) If you're talking about the sudden media rage about having found a Higgs and then that died out, then, if I'm not wrong, that's generally dismissed as fluke .

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Dimensio1n0

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The first Kane et al. paper on the Higgs mass was arxiv.org/abs/arXiv:1112.1059, posted on December 5, 2011, slightly before the ATLAS and CMS announcement on December 13. They had the result even earlier-- I was at Michigan on November 16, 2011, and Gordy told me their result then. At that point I hadn't heard the rumor, which doesn't mean none of them had. In any case, it's a straightforward prediction of having heavy scalars in the range they had been advocating for years, although emphasizing the larger tan beta values may have been a choice made in light of the data.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Matt Reece

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Thanks for this important clarification of the issue @MattReece. I hope it will help to do away with some misunderstandings and prejudices some people hold.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Dilaton

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Just to give some perspective on the Kane et al paper: People were saying that the Higgs mass should be less than 125-130 GeV in the MSSM, years before this, and without appealing to string theory. The arguments made by Kane et al are somewhat similar, they just work in a different part of MSSM parameter space - that's the part they motivate using M theory...

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Mitchell Porter

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Some reactions from String Pheno 2012: x-sections.blogspot.com/2012/06/string-phenomenology-day-2.html

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Mitchell Porter

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Have a look at this post in December 2011 Higgs: 125 GeV Higgs rumors were accurate I think it's comical this idea that Kane et al "prdicted" the correct mass beforehand.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user Larry Harson

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last point before this get's too excessive: the remaining window not excluded was already very narrow and first hints that the higgs mass would be below 130 were known already in august 2011 after Lepton Photon 2013, e.g. blog.vixra.org/2011/08/29/… in any case I think it's a stretch to claim the higgs mass was predicted

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user luksen

Answer 3

Approaches to Quantum Gravity: Toward a New Understanding of Space, Time and Matter by Daniele Oriti.

This post imported from StackExchange Physics at 2014-03-07 14:44 (UCT), posted by SE-user UGPhysics