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Steven Weinberg offers a way to explain inflation 
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#109
Nov2409, 06:22 PM

Astronomy
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PF Gold
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And this range E < E_{Planck} is exactly where Bonanno's assertion applies. It is also where Roy Maartens and Martin Bojowald found, in 2005, that black holes could not form (given the Loop context). We may in fact not have a problem. The sheer existence of black holes of less than Planck mass is questionable. There is no evidence that they exist, and there are analytical results to the contrary. 


#110
Nov2409, 07:12 PM

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OK, looks like we all agree on the physics heuristics but maybe not the names of various hypotheses.



#111
Nov2409, 07:47 PM

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AS is basically not very rigourous (Rivasseau complained about this in a footnote in his GFT renormalization paper) and kinda hopeful, but my impression is that it's often that way in condensed matter. For example in Kardar's exposition at some point he says (I'm doing very free paraphrase) well, how do we know there's not nonperturbative fixed points  we don't, but luckily we can do experiments and they even more luckily match our perturbative calculations! He also says there are several different coarse graining schemes which actually no one has proven are mathematically equivalent, but they all seem to match experiment, so we live in blissful ignorance! In condensed matter the predictions are "universal", so for example the critical temperature is different for all sorts of materials and the theory cannot predict the temperature  what it gets right is the critical exponent which seems to be independent of material and dependent only on symmetries and dimensionality. So I guess Weinberg and co are hoping for some such generic predictions. 


#112
Nov2409, 07:53 PM

P: 343

Just a note on possible confusion. When one says "high energy" in gravity it can be confused for "low energy" and vice versa. The reason is the following: Newton's constant is dimensionful. It has mass dimension [G]=2 such that when I write GM this is a length or an inverse mass [GM]=[G]+[M] =2+1=1.
One consequence of this is the strange property of black holes that when I increase there mass their temperature drops T=1/(8 pi G M) i.e. they have a negative specific heat. Other consequences of [G]=2 are that the entropy of a black hole goes as the S=area/(4G) since G is the Planck area and the infamous power counting nonrenormalizability of general relativity. 


#113
Nov2409, 08:09 PM

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Does AS really need a fixed point? Could it live with, say, a limit cycle?



#114
Nov2409, 08:09 PM

P: 343

The relevant paper is http://arxiv.org/pdf/0907.2617 Also checkout Frank Saueressig's talk at perimeter. 


#115
Nov2509, 02:02 AM

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#116
Nov2509, 01:03 PM

Astronomy
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PF Gold
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The paradigm of colliding two particles at higher and higher energy, and equating that with physics, has become less interesting. It's a mental rut (almost an obsession) left over from the accelerator era. For example Weinberg was talking about inflation, which is a different business. Different concepts, and different sources of data, come into play. You could say that the range 10^{9} TeV up to 10^{16} TeV is the range from just over "cosmic ray" energy up to "early universe" energy. A billion TeV is kind of approximate upper bound on cosmic ray energies. It's quite rare to detect cosmic rays above that level. And 10^{16} TeV is the Planck energy. I would say this is a new erogenous zone for theoretical physics. The putative "GUT" scale, of a trillionplus TeV, comes in there. But it impressed me that in Nicolai's new model there is no new physics at GUT scale. What Nicolai and Meissner have done is project a model which *is falsifiable by LHC (once it gets going) and *is conceptually economical, even minimalisticbased on existing standard model concepts, *pushes the breakdown/blowup points out past Planck scale, so it *delays the need for fundamentally new physics until Planck scale is reached. Whether Nicolai and Meissner's model is correct is not the issue here. What this example suggests is that this kind of conservative unflamboyant goal, this kind of unBaroque proposed solution, will IMO likely become fashionable among theorists. You could think of it as a reaction to past excesses, or a corrective swing of the pendulum. This same economical or conservative spirit is the essence of what Weinberg is doing. The new paper of his that we are discussing simply carries through on what he was talking about in his 6 July CERN lecture, where he said he didn't want to discourage anyone from continuing string research, but string theory might not be needed, might not be how the world is. How the world is, he said, might be described by (asymptotic safe) gravity and "good old" quantum field theory. I assume that means describing the world pragmatically out to Planck scale (10^{16} TeV) so you cover the early universe. An important part of the world! And not worrying about whatever new physics might then kick in, if any does. It's a modest and practical agenda, just getting that far, compared with worrying about putative seamonsters and dragons out beyond planck energy. But of course that's fun and all to the good as well. ================================ In case anyone new is reading this thread, here is a link to video of Weinberg's 6 July CERN talk: http://cdsweb.cern.ch/record/1188567/ It gives an intelligent overview of what this paper is about, where it fits into the big picture, and what motivates the Asymptotic Safe QG program (which he describes in the last 12 minutes of the video). As a leading example of extending known and testable physics out to Planck scale, here is Nicolai's June 2009 talk: http://www.ift.uni.wroc.pl/~rdurka/p...=1.3%20Nicolai Here's the index to all the videos from the Planck Scale conference http://www.ift.uni.wroc.pl/~rdurka/p...ndexvideo.php 


#117
Nov2509, 04:02 PM

P: 343

"Just a note on possible confusion. When one says "high energy" in gravity it can be confused for "low energy" and vice versa. The reason is the following: Newton's constant is dimensionful. It has mass dimension [G]=2 such that when I write GM this is a length or an inverse mass [GM]=[G]+[M] =2+1=1. " So for the argument about the nonrenormalizability of gravity based on its scaling in the UV to be valid the "Asymptotic" in Asymptotic darkness and needs to be the same as the Asymptotic in Asymptotic safety. The reason it is false is because they are not for exactly the reason above. If I have a large mass black hole M>>Mpl then r=2GM is large r>>lpl. This is what the "Asymptotic" in AD refers to and as you say you get closer and closer to classical GR. But the "Asymptotic" in AS refers to exactly the opposite limit that is when k>>Mpl where k=1/r this is where we are very far from classical GR and hence where we need a full theory of QG to answer any questions appropriately. This is exactly the point David Tong is making ""Firstly, there is a key difference between Fermi’s theory of the weak interaction and gravity. Fermi’s theory was unable to provide predictions for any scattering process at energies above sqrt(1/GF). In contrast, if we scatter two objects at extremely high energies in gravity — say, at energies E ≫ Mpl — then we know exactly what will happen: we form a big black hole. We don’t need quantum gravity to tell us this. Classical general relativity is sufficient. If we restrict attention to scattering, the crisis of nonrenormalizability is not problematic at ultrahigh energies. It’s troublesome only within a window of energies around the Planck scale."" So you see its not the AD scenario that I'm arguing about. Its that AD(an IR property of classical gravity) has any baring on AS/renormalizablity(which is a UV problem of quantum gravity). 


#118
Nov2509, 04:27 PM

Astronomy
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PF Gold
P: 23,125

Could it be that some people want to deny or dismiss the significance of AS suddenly coming to the forefront? It seems to me when something like this happensgreatly increased research, first ever AS conference, possible alliance with CDT and even Horava, connection with cosmology revealedthat the appropriate thing to do is to pay attention, and focus on it, not try to dismiss (especially not by handwaving about transplanckian black holes ) Haelfix, could you have been misled by someone with a vested interest that felt threatened by Weinberg's CERN talk, or recent paper, and is grasping at straws? or just blowing smoke? Be careful, maybe a bit more skeptical? 


#119
Nov2509, 07:18 PM

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One thing I don't understand is that Weinberg's paper (the one being discussed in this thread) starts with the most general generally covariant Lagrangian (http://arxiv.org/abs/0911.3165)  but Krasnov has recently proposed an even more general generally covariant Lagrangian (http://arxiv.org/abs/0910.4028 )  so presumably Weinberg's is less general  is that because Weinberg admits only local terms, while Krasnov's contains nonlocal terms? Usually renormalization flows don't generate nonlocal terms, I think, and naively I would expect the same for AS, but is that true? Edit: Krasnov says his terms are all local  so what is the difference between his stuff and AS? Litim's http://arxiv.org/abs/0810.3675 says "A Wilsonian effective action for gravity should contain ... possibly, nonlocal operators in the metric field." So I guess nonlocal terms can come about through coarsegraining, which is not intuitive to me  can someone explain? Also what are these terms, and did Weinberg include these? Edit: As far as I can tell, Weinberg, as well as Codello et al, only included local (or quasilocal) terms. So what are these nonlocal terms Litim is talking about, and why would they arise? 


#120
Nov2509, 07:43 PM

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P: 8,395

http://relativity.livingreviews.org/...es/lrr20065/
"a canonical formulation is anyhow disfavored by the asymptotic safety scenario" What!? 


#121
Nov2609, 08:55 AM

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P: 1,677

"So you see its not the AD scenario that I'm arguing about. Its that AD(an IR property of classical gravity) has any baring on AS/renormalizablity(which is a UV problem of quantum gravity). "
AD is a UV property of QUANTUM gravity by definition ... You are summing up ladder diagrams and things like that after all. THe peculiarity here is that it effectively looks semiclassical again. The quantum effects which may have been important at the Planck scale as well as the nonperturbative physics, at transplanckian energies, must drop out. 


#122
Nov2609, 05:54 PM

P: 343

What your saying here is not the case and I assure you you have been mislead. Please can you cite a paper where ladder diagrams in QG are being computed and the result is that gravity becomes semiclassical again? 


#123
Nov2709, 11:52 AM

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"you can't go beyond 2 loops in perturbation theory one has to go to effective field theory and work at energies below the Planck scale. "
You can sum up however many orders of perturbation theory that you want in gravity, the thing is you may or may not get an underspecified answer (for instance, depending on constants arising from the counterterms of the next order) or alternatively a divergent answer (for E > infinity). But for E finite, you will get some number. Incidentally thats what AS presuposes. Namely that as you sum up the perturbation theory, there are cancellations that take place within the divergence structure of the theory (so bad '2' loop terms like GS and the Rs coupling will presumably cancel out) But anyway, here we are talking about a theory of 2 body scattering. The approximation under consideration is where you take the first exchange term with a graviton, and then 'exponentiate' it by summing up all the associated ladder diagrams. For large impact parameters, this approximation is valid and exact (this is the Eikonal regime). 


#124
Nov2709, 04:00 PM

P: 343

I'm not sure that AS presupposes anything. AS is a possible scenario in which taking the cutoff to infinity will give you a finite theory. I still insist that whatever approximation you are on about is certainly not valid at the UV fixed point of gravity. For sure it neglects nonperturbative effects if your just exponentiating the tree level graviton exchange. My point has always been that all these arguments based on perturbation theory and the Einstein Hilbert action have nothing to say about AS. The Eikonal regime is subplackian GE<lpl. It says nothing about graviton loops. Look at fig 1. in http://arxiv.org/pdf/0908.0004v1 Its in the semi circle at the bottom that we need to know QG and can make comments on nonperturbative renormalisation. If AS is realised in nature this regime is controlled by a UV fixed point and we don't expect black holes to be formed. AD is valid in the strong gravity regime where arguments can be made that we must see black holes here but these arguments have no bearing on the physics of a full nonpertubative theory of QG. 


#125
Nov2709, 05:05 PM

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P: 1,677

"The Eikonal regime is subplackian GE<lpl"
For the 10 th time.. Its transplanckian : E (CM) >>> Mpl!!! The papers I have already listed explain this in great detail, or see Veneziano's papers in the 80s (which are cited in Srednicki's paper) Every point in that semicircle in figure 1 are at transplanckian energies!!! 


#126
Nov2709, 06:30 PM

P: 343

I agree E>>Mpl but this means GE>>lpl because GE is a length not a mass. So we're in the IR physics of gravity 1/GE<<Mpl. Sorry I meant GE>lpl in my last post. I know its confusing that theres this UV/IR thing with gravity. But you need to think of the physics here. If I collide two tennis balls together then the energy E>>Mpl but I don't need QG to describe the physics. If I further take the mass of the tennis balls and compact them down such that when they collide there within a radius r<2EG then a black hole must form but the curvature at the horizon will be subplackian therefore I can still describe the physics without QG I only need semiclassical physics. Its only when I take a small amount of energy E~Mpl and confine it to a very very tiny space r=2GE~lpl that the curvature becomes Plackian and we need QG. In the Fig. 1 in Giddings paper this is the semi circle with the ? at the bottom left were both E and b are small i.e. a small energy confined to a small radius, its here and only here that the curvature is Plackian and we're in the UV. 


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