Gravity Renormalizable?

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  • #26
marcus
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... I leave it to others to check if he has since changed his mind.
I think that would be a good idea, Haelfix. Leave it to others who know something about the current work of Percacci, Saueressig and others including of course Reuter.
 
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The authors can redefine conventional physics terms all they want, its irrelevant.
Haelfix---

I may be dense, but I think I agree with you. I simply don't understand how one can make conclusions about quantum gravity based on computer simulations and an effective field theory. Generally, when one finds that higher order terms in the effective action are absent, there is some symmetry which prohibits them---instead of looking for this symmetry (which would be a sign that they don't understand the UV dynamics of their theory), they argue that they have a renormalizable theory of quantum gravity.

Aside from this, I may be stuck in field theorist's land, but calling something ``renormalizable'' (to me) MEANS that you have an effective theory. You still have to cutoff the UV modes somehow. You still have to add arbitrary and infinite constants to the lagrangian to be able to calculate things. At least to me, ``renormalizable'' doesn't mean ``UV finite''---it only means that you can trust your calculations at scales much below the cutoff. I don't know how to separate the ideas of ``renormalizability'' and ``effective field theory'' in me head.

It may be that I am confused by the semantics of the thing, instead of the physics.
 
  • #28
marcus
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Nice to see Jacques Distler getting exercised about Asymptotic Safety and referring to Percacci's review.
See his post of today 30 January
http://golem.ph.utexas.edu/~distler/blog/archives/001585.html

Distler also mentions this four-page paper
http://arxiv.org/abs/0705.1769
Ultraviolet properties of f(R)-Gravity
Alessandro Codello, Roberto Percacci, Christoph Rahmede
4 pages
(Submitted on 12 May 2007)

"We discuss the existence and properties of a nontrivial fixed point in f(R)-gravity, where f is a polynomial of order up to six. Within this seven-parameter class of theories, the fixed point has three ultraviolet-attractive and four ultraviolet-repulsive directions; this brings further support to the hypothesis that gravity is nonperturbatively renormalizabile."

It is nice to see that Distler, when he talks about Asymptotic Safety, does not rely entirely on hearsay or what somebody else TOLD him about it, but instead he seems actually to have read (some of) the papers.

Since I've referred several time in this thread to Percacci's review paper---which extends the definitions of effective theory and fundamental theory to the nonperturbative context of asymptotic safety---and since Distler also refers to it, I may as well give the link to it as well:

http://arxiv.org/abs/0709.3851
Asymptotic Safety
R. Percacci
To appear in "Approaches to Quantum Gravity: Towards a New Understanding of Space, Time and Matter", ed. D. Oriti, Cambridge University Press
(Submitted on 24 Sep 2007)

"Asymptotic safety is a set of conditions, based on the existence of a nontrivial fixed point for the renormalization group flow, which would make a quantum field theory consistent up to arbitrarily high energies. After introducing the basic ideas of this approach, I review the present evidence in favor of an asymptotically safe quantum field theory of gravity."
 
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  • #29
Haelfix
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So I finally read Percacci's review, and well they are doing exactly what we said they were doing (Marcus wth are you talking about, the entire first half of the paper basically spells out *exactly* what we are telling you)

The improvement in recent years from the original paper is essentially throwing in more couplings to test things, but as Jacques and others pointed out, it matters not since even if you think you have found a fixed point eg various vanishings (which actually indicates a gaussian trajectory) you are by no means guarenteed to have that persist order by order upon inclusion, or to be free from nonperturbative renormalizations. Percacci et al know this, even though perhaps others are confused about whats being claimed or not.

In fact, even if they include the first divergence term at order two (see Jacques post), and pass it, it *still* wouldn't suffice as a *proof* b/c the degree and structure of the divergence changes and grows worse (it will go something like x^n factorial, where n is typically 2 or something like that).

All this to say, I don't mind what they are doing, and its an interesting theory to study in its own right, its just some people on this board make it into something that its not and its beginning to get irratating.
 
  • #30
marcus
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So I finally read Percacci's review, and well they are doing exactly what we said they were doing
Glad to hear it! I've never claimed that Reuter or Percacci had a proof. In fact I pointed out a recent paper that (at least for me) raises doubts about the existence of the UV fixed point. But perhaps you didn't notice what I've been saying.

What they have is a growing body of evidence which is suggestive but not conclusive.
And there are definitely unresolved issues, as I pointed out in connection with the recent Saueressig paper.

I am glad you indicate agreement with what Percacci says in his review. I, for one, have not asserted anything more than what he talks about there. So perhaps we should examine what he says in more detail.
 

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