
#1
Jan1709, 06:33 AM

P: 527

I've read a lot of arguments in support of LQG over string theory, mainly focusing on ST's lack of background independence.
I'm also told that there are some pretty good arguments against LQG. What are some of these arguments? Have they been summarized somewhere? 



#2
Jan1709, 06:43 AM

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#4
Jan1709, 12:28 PM

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Arguments against LQG 



#5
Jan1709, 01:59 PM

P: 448





#6
Jan1709, 02:14 PM

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It is about time you string lqg guys came out, reveal what you have, i guess it will not get to many hot under the collar, unless they have a good imagination.




#7
Jan1709, 08:38 PM

P: 478

Another thing we notice about our universe is that all of the matter lives in small representations of gauge groupsfermions are ALWAYS in the fundamental representation of the gauge group. In string theory, this is the typical state of affairsthat is, we don't have to work too hard to see everything we need to describe nature just fall out of the theory "for free". For example, suppose you want to build a theory based on open strings. You go through the quantization procedure and have a perfectly happy theory, with no gauge symmetries. But now, you talk to Joe Polchinsky about your theory, and tells you "Ah, there are things in your theory you MISSED, called d branes, that MUST be there." Once you include dbranes, you find that closed string MUST begin and end on them. Now you get creative, and you imagine taking stacks of dbranes (which are already in your theory) and intersecting various stacks at different angles. Suppose you take N dbranes and intersect them with M dbranesif you study the different types of open strings that you can have, you find out that you have EXACTLY and SU(N)xSU(M) gauge theory, with matter in the fundamental representations of the gauge group. And all of this just by playing around with states that are already in your theory. This, to me, is the most remarkable thing about string theoryit tells you how to get quarks and leptons, and nonAbelian gauge groups. String theory is a more ambitious program than the other approaches to QGthey aren't trying to explain the standard model, they're ONLY trying to quantize gravity. In fact, all of the attempts to explain where the gauge groups actually COME from have failed. In string theory, this was done as early as the mid to late eighties. {irrelevant comment removed  Zz} 



#8
Jan1709, 09:01 PM

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BenTheMan: that looks like an argument for ST, not an argument against LQG.




#9
Jan1709, 10:01 PM

P: 527

Hi Ben,
Thanks for your comments. I have a couple of questions for you. I didn't understand the relevance of the statement I'm not sure about the gauge group problem, but I read about an idea in Lee Smolin's book about treating particles as braids in the spinnetwork. Does this idea hold much water? 



#10
Jan1709, 10:37 PM

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Bianca Dittrich, Thomas Thiemann http://arxiv.org/abs/0708.1721 Infinite Degeneracy of States in Quantum Gravity Jonathan Hackett, Yidun Wan http://arxiv.org/abs/0811.2161 



#11
Jan1709, 11:08 PM

P: 478

I perhaps should have made a different statement, that all of the things we have observed so far live is representations smaller than the adjoint, but you would have pointed out to me the large higgs representations that are needed in GUTs in general. The statement is correct in the context of the standard model, but is not true in SU(5), where the fermions come in a 5* + 10, or SO(10) where the fermions live in the spinor rep. Of course, very few people believe in SU(5) anymore (the minimal SUSY models are firmly dead by dim 5 and dim 6 proton decay, see Pierce and Murayama, http://arxiv.org/abs/hepph/0108104), and evading the constraints with SO(10) gets a bit tight (see "SUSY GUTs under siege" by Dermisek, Mafi, and Raby). So what DOES this say about strings? First of all, when you get your hands dirty with string models, you find that it is difficult to get representations larger than the adjoint. In fact, I only know of one way to get large representations (i.e. bigger than adjoints) out of string theory. This is in a paper by Kieth Dienes here: http://arxiv.org/abs/hepth/9604112. Conversely, I know of TONS of ways to get small representations out of string theory. "Small" here means "smaller than adjoint". So, the question again comes: what does this say for the SO(10) models that require higgses in the 45 + 120 + 210 + ... representations. I would say that the question isn't "what does this say about strings", but "what does this say about SUSY GUTs"? I don't know what the right answer is, but I have never seen a string model that gets something like the complicated SO(10) models that people build. (The Dienes paper only shows that it is possible, and I know one of his students is working on that.) My feeling is that these models don't have a good stringy embedding. (Please, don't take my word for itI'd LOVE to see some realistic SO(10) models come out of string theory :) ) Of course, you can even construct models that don't NEED SO(10) or E6we certainly have never seen SO(10) or E6. You can break E8 (or SO(32))directly to the standard model at the string scale and have a perfectly happy model. Or, you can view the unification of forces as an accident, and imagine that we live on intersecting stacks of 6 branes wrapped around various cycles of various Calabi Yau shapes. Or (...) Getting particle physics from string theory is a robust field, with many approaches. But from what I know, you are more or less limited to the smallest (adjoint or smaller) representations of whatever gauge group you have. There was a Connes' noncommutative geometry standard model a few years ago, but that was firmly ruled out earlier this year by CDF, when they killed a higgs at 160 GeV. (Of course, as a good model builder, Connes found a way to fix his model, but this is the same things that people deride string theorists for...) And, of course, there is the Lisi model which has nice cartoons, but probably can't describe Nature. If getting standard model looking things out of other QG (notice absence of L!!!) approaches were easy (as is the case in string theory), then someone would have done it. This means that it is probably hard, or not possible. "Hard" doesn't mean that it should be eliminated from the spectrum of possibilities, it just makes it more difficult to be optimistic about it being true. But either way, in my mind this is the main argument for string theorynot only is it a finite theory of quantum gravity, it contains nonAbelian gauge symmetries and chiral matter, and it even contains the right KIND of chiral matter (small reps). 



#12
Jan1809, 12:49 AM

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P: 1,664

The arguments against LQG are more technical than anything else. The weird nonseperable hilbert space really grates me more than anything else about the formalism. I don't mind lorentz breaking so long as you can convince me that its smaller than observation bounds, eg tiny etc etc.
Still you have to realize even the best models on the market are extremely naive, the inclusion of matter (braids or whatever) is poorly developed and not taken seriously by phenomenologists. I hate to say it, but the immense majority of scientists are highly critical of the subject, despite all the popularizing on the internet and by a few silly books. What might be interesting about the program is if they are successful or not in the whole canonical quantization of gravity endeavour. For years that was a dead end, and the certain success they've had might be an indication that what they've worked on is at least valid in some limit or as a toy model. So interesting in its own right. Still the entire field is very, very far from being fleshed out. Its a bit like string theory or Regge Calculus was in the 70s, or nuclear physics in the 60s. Many calculations can still go wrong in principle and they don't really have a smoking gun yet to draw the best people in. Of course this can be said about most QG programs. For instance the much talked about noboundary proposal and euclidean path integral advocated by Hawkins et al is also greeted with extreme skepticism b/c of how naive it is. 



#13
Jan1809, 10:54 PM

P: 1,928

As for LQG, expecting that something gravitational weird can be measurable, that cannot be aproximated by a more classical theory, is hopeless, pretty much like string theory. But unlike string theory, it does not even try to tackle the oter forces. Anywaym, that is a sad state of affairs for both fields. 



#14
Jan1809, 11:15 PM

P: 2,828

As a side note to the motivation of this discussion, I'd like to ask if anybody would have strong arguments to claim that string theory, loop quantum gravity, and Connes' noncommutative geometry must be incompatible with each other. It might as well be, to me, that they are different approaches and ingredient to an unique solution.




#15
Jan1909, 12:18 AM

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P: 1,664

Thats a little hard to say exactly without comparing specific LQG theories, with a particular NC model and string theory with a given vacua. I'd guess lorentz breaking/nonbreaking would be a big indicator.
String theory doesn't really make sense without the inclusion of matter, theres really no wiggle room possible for decoupling the matter degrees of freedom from the gravitational ones so hard to make contact with the better understood LQG models. In a related fashion, many LQG versions seem to have EinsteinHilbert to all order, which cannot at face value be compatible with ST unless theres a duality somewhere (but then the physical predictions don't necessarily match either) The NC models seem to have low energy matter relationships thats close to something like a nonsupersymmetric GUT and I guess that could be made close to st. but the high energy predictions looks completely different from what I've read. 



#16
Jan1909, 12:27 AM

P: 235

This conclusion seems based on the argument that a fixed background model can't be made to agree in general with a model lacking such a fixed background. I don't have the tools to judge wether this is correct, but it sounds convincing while reading his book. 



#17
Jan1909, 01:22 AM

P: 478





#18
Jan1909, 02:14 AM

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There is a specific testable prediction of a LQG model relating to the finescale structure of the vacuum. Fotini Markopoulou predicted years ago that we would see a frequencydependent delay in the arrivaltimes of gamma rays from GRBs. Her idea was that the more energetic gamma rays would interact more frequently with the space through which they propagate, and would therefor be slowed more than EM of lower frequencies. Though she hung her observational hopes on GLAST, such an effect may already have been recorded by the MAGIC project. If such a frequencydependent delay can be observed and confirmed multiple times, it would help to rule out sourcebased systematics. If the magnitude of the delays could also be shown to be proportional to the redshift of the source, things are going to get exciting.



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