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Arguments against LQG

by jdstokes
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MTd2
#19
Jan19-09, 03:53 AM
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Quote Quote by humanino View Post
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,
There were some thoughts about 2005 in which LQG was a kind of limit of M-Theory just like superstrings. Sergei Gukou and Ashok Sen were guys that wondered about that. Some simple things made that conjecture invalide... But I guess some recent articles made that possible, I think.

I will post a thread about that later.
wolram
#20
Jan19-09, 07:20 AM
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I do not know if this is the best paper but it is the best i can find, nor do i pretend to even understand all of it, but it doe's seem to give a good overview.

http://arxiv.org/abs/gr-qc/0501053

G. Amelino-Camelia, C. Lämmerzahl, A. Macias, H. Müller
(Submitted on 17 Jan 2005)
Abstract: We give an overview of ongoing searches for effects motivated by the study of the quantum-gravity problem. We describe in greater detail approaches which have not been covered in recent ``Quantum Gravity Phenomenology'' reviews. In particular, we outline a new framework for describing Lorentz invariance violation in the Maxwell sector. We also discuss the general strategy on the experimental side as well as on the theoretical side for a search for quantum gravity effects. The role of test theories, kinematical and dymamical, in this general context is emphasized. The present status of controlled laboratory experiments is described, and we also summarize some key results obtained on the basis of astrophysical observations.
MTd2
#21
Jan19-09, 07:32 AM
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I found this one, among the citations for the above article:

Quantum Gravity Phenomenology
Authors: Giovanni Amelino-Camelia
(Submitted on 2 Jun 2008)

http://arxiv.org/abs/0806.0339

Abstract: I review the present status of the development of Quantum Gravity Phenomenology. Among the accomplishments of this young research area I stress in particular the significance of studies which established that some appropriate data analyses provide sensitivity to effects introduced genuinely at the Planck scale. The objective of testing/falsifying theories that provide comprehensive solutions to the quantum-gravity problem appears to be still rather far, but we might soon be in a position to investigate some "falsifiable quantum-gravity theories of not everything".
humanino
#22
Jan19-09, 11:52 AM
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Quote Quote by gendou2 View Post
For what it's worth, I suspect that Lee Smolin would argue they can't be different paths to the same solution because LQG is "background independent" where as ST is "background dependent".
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.
Newton's theory is not BI however it is known to be a low energy limit of GR. Incidentally, Lee's last paper was on the landscape of ST. It might suggest, considering he was paving the way to search a link between ST and LQG, that he still has interest in ST.

The weak anthropic principle and the landscape of string theory
The case for background independence
marcus
#23
Jan19-09, 03:18 PM
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Quote Quote by humanino View Post
... Lee's last paper was on the landscape of ST. It might suggest, considering he was paving the way to search a link between ST and LQG,
The weak anthropic principle and the landscape of string theory
...
http://arxiv.org/abs/0901.2414
"It is interesting to note that while the understanding of the string landscape is evolving, present evidence on the nature of the landscape allows such an argument to be made, leading to the conclusion that the WAP favors a negative value for the cosmological constant, Lambda, in contradiction to the result of astronomical observations. The viability of applying the WAP to string theory then requires that either there are found an infinite discretum of anthropically allowed vacua for Lambda >0, or the recently found infinite discretum of solutions for Lambda <0 be reduced to a finite value."

Smolin's co-author on this one is George Ellis, a well-known cosmologist who earlier co-authored The Large Scale Structure of Space Time with Stephen Hawking.

Basically they argue that applying the Weak Anthropic Principle to the String Landscape tends to discredit string/M as a whole.
The argument goes as follows. Suppose one assumes that our universe was chosen at random from the habitable sector of the String Landscape as presently understood. Then it is infinitely more likely to have Lambda < 0----at least as things stand presently. (Only a finite discretuum with Lambda >0 and an infinite one with negative cosmo constant.) Therefore String/M predicts negative Lambda.

But at least so far, observations point to positive Lambda. This would seem to discredit String/M at least with it's present Landscape of possible vacuua, if one follows Anthropic reasoning.

My impression is that this paper does not "pave the way" towards a reconciliation.
Nor is it really a serious attack on Stringy thinking. At least I do not consider it anti-string.
What I do see is that Ellis and Smolin disapprove of resorting to Anthropic Landscape arguments---and in that they agree with a considerable part of the string research community itself!
I think the main thing their paper is intended to show is what a liability the Anthropic Landscape is to string/M.
humanino
#24
Jan19-09, 03:48 PM
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Quote Quote by marcus View Post
My impression is that this paper does not "pave the way" towards a reconciliation.
When I said that, I was talking about the other paper.
Quote Quote by marcus View Post
I think the main thing their paper is intended to show is what a liability the Anthropic Landscape is to string/M.
I personnally did not take too seriously the "result" that string theory has to say about the cosmological constant sign. They are very clear themselves about the fact that our current understanding is limited.

Not even to mention the fact that, a situation which I would find wonderful, if one could prove that the landscape has a unique solution compatible with any known experimental fact, that would not by itself disprove string theory because the infinite (?) remaining landscape is wrong. That would mean that we have found a sufficient constraint for string theory to become fully predictive !
marcus
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Jan19-09, 04:11 PM
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Exactly! In that case Anthropic reasoning would be superseded and laid to rest. I was just about to add that the other paper (Case for B.I.) does make a constructive suggestion to string theorists---working on a nonperbative background independent version---and in that sense does pave the way to more collaboration. I didn't realize that is the recent paper you meant.
But I see you already made that point.

A propos of prospects for linkage, Smolin is one of a dozen or so Loop community people (Loop and allied approaches) that will be taking part in an interesting weeklong symposium in early July (if all goes as planned) bringing together roughly the same number of String/M and Noncommutative QFT people. This is the 25th Max Born symposium at Warsaw.

What you say in general terms (not about the Smolin/Ellis paper specifically) I think is very true---the time is ripe for an exchange of ideas and methods. A lot of people in several areas seem eager for this and ready to take the initiative. I will get the link to the XXV Max Born preliminary list of speakers
Warsaw 29 June-3 July
http://www.ift.uni.wroc.pl/~planckscale/
http://www.ift.uni.wroc.pl/~plancksc...?page=speakers
http://www.fuw.edu.pl/~meissner/home.html


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