Timeline for Does a UV completion of gravity necessarily need to be so drastic as String Theory or LQG?
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| Apr 20, 2012 at 20:58 | comment | added | genneth | @RonMaimon: this is getting long, and perhaps the discussion should be moved, but the technical answer to your question is "by using coherent states". A flat spacetime, like all classical states, must be treated by a coherent state. In a technical way, there are more than one way currently to define such states (though to zero-th order in $\hbar$ they are equivalent). In fact, one can do better, and calculate the scattering of gravitons on such a background. | |
| Apr 20, 2012 at 19:38 | comment | added | Ron Maimon | @genneth: The missing thing in those lectures (which I believe I skimmed when they came out, they look familiar), is the proper spin-network corresponding to a flat space time. The problem is that a generic discrete structure like a spin network doesn't have to "weave" a flat space, but can fork out braches, and the dynamical ideas regarding separation of topology from topological censorship are not incorporated. The spin-foam idea is concrete, it just doesn't look like an SR spacetime in any obvious way. I'll read all the papers, and come back enlightened, but I don't see a revolution yet. | |
| Apr 20, 2012 at 19:19 | comment | added | genneth | @RonMaimon: apologies --- my memory is incorrect. The paper is by Ghosh and Perez: prl.aps.org/abstract/PRL/v107/i24/e241301. The best status update (which is sadly still a little behind the curve) is Rovelli's Zakopane lectures: arxiv.org/abs/1102.3660. I would argue that in those lectures an extremely concrete theory is laid out. Whether it is the correct one with respect to experiment is of course unknown, but one cannot claim that it is not a well-defined theory with calculable observables. | |
| Apr 20, 2012 at 19:15 | comment | added | Ron Maimon | @genneth: I can't find any paper among Ashtekar's with anything new to say regarding the status of LQG. Can you give a title? The only PRL of Ashtekar in 2011 is an analysis of a 2d black hole which has no relation to this stuff. The only other recent stuff was a wordy unilluminating review (which I already read). This stuff is hyped as an alternative to string theory, where unlike string theory, there is no well defined mathematical theory there, just a hunch that there might be a theory (this is just what critics say about string theory, which is wrong of string theory, but correct of LQG) | |
| Apr 20, 2012 at 19:10 | comment | added | genneth | @RonMaimon: it is bizarre that you should say it is hyped --- my impression is that it is exceedingly low key, which makes all the more galling when highly visible figures like Lubos spread misinformation and drown out the small voices that work on the LQG. In any case, on a technical level I don't agree that there has to be convergence to string theory --- there can be more than one quantum theory that has classical GR as a limit -- but that's a discussion best left for somewhere else. | |
| Apr 20, 2012 at 19:03 | comment | added | Ron Maimon | @genneth:I'll look at the recent papers--- you're right, I haven't been keeping up with this. But I think the skepticism is appropriate as a default position, since there is a lot of hype here--- the reason I want higher dimensions is because string theory is fully consistent, so if there is a gravitational representation in terms of spin-networks, it should work wherever there is a quantum gravity, whether it's the world we see or just some hypothetical world we don't see. | |
| Apr 20, 2012 at 18:54 | comment | added | genneth | @RonMaimon: you've just outed yourself as being not up to date. There is no fine-tuning required of the Immirzi parameter (Asheketar et al, PRL 2011). There have been a boatload of papers in the last 12 months showing that the semi-classical spinfoam amplitude is the exponential of the Regge action. You say that the restriction to 4D is a problem --- aesthetically I think that's a bonus --- we only see 4D! It is the highest hubris to claim that because string theory would prefer more that implies it is our empirical observations which are incorrect. | |
| Apr 20, 2012 at 18:26 | comment | added | Ron Maimon | @genneth: You are not right. I have looked at the literature on LQG, and there is no full theory there--- there are only indications of a theory. There are problems of exactly the kind Lubos states, and there is no way you have an embedding of weak field GR today--- this is a major unsolved problem. But I am optimistic that it is possible that there is a theory along these lines, but the fine-tuning of the immirizzi parameter is a problem, as is the restriction to 4 dimensions. You need a principle that turns a spin-network predominantly flat--- and this principle is not present today. | |
| Apr 20, 2012 at 17:15 | comment | added | genneth | @RonMaimon: the comments on LQG are certainly not accurate. There now exists a completely finite theory which has every indication (some might even call it a proof) of having GR as the semi-classical regime. I always find it hard to vote one way or the other on Lubos' answers --- there's so much that is right, tarnished with casual, disingenuous and misleading nuggets. | |
| Apr 20, 2012 at 16:24 | comment | added | Ron Maimon | +1 for accurate technical comments (as far as I can see), as usual, but I don't think you need to be so pessimistic--- perhaps LQG is only missing SUSY to ensure flat space--- it's almost as hard to get flat space is SUSY-less strings (you need a SO(16)xSO(16) projection of SUSY heterotic strings). All near-flat backgrounds have used SUSY to enforce this, and loops lack SUSY. Further, the large N limit in Matrix theory can be reasonably viewed as a "atoms of space" theory, because it reproduces an approximate continuum spacetime from a finite calculation. | |
| Apr 20, 2012 at 16:03 | history | edited | Luboš Motl | CC BY-SA 3.0 |
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| Apr 20, 2012 at 15:58 | history | edited | Luboš Motl | CC BY-SA 3.0 |
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| Apr 20, 2012 at 15:50 | history | edited | Luboš Motl | CC BY-SA 3.0 |
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| Apr 20, 2012 at 15:42 | history | answered | Luboš Motl | CC BY-SA 3.0 |