Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Important AsymSafe QG development

  1. Jul 13, 2009 #1

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Everybody should listen to the the last 10-12 minutes of Steven Weinberg's 7 July Cern talk, drag the button so it starts at minute 58 and go to minute 70, where he stops and takes questions.
    http://cdsweb.cern.ch/record/1188567/

    He describes his own work in progress in AsymSafe Cosmology, where he gets inflation and a graceful exit from inflation as a natural consequence of A.S. QG.

    There already was a paper by Martin Reuter and Alfio Bonanno about this. If you start out at the UV fixed point (at the highest energy/smallest scale/highest wavenumber) then you naturally get inflation because of the values of the running constants. And then as scale increases the constants run towards more normal levels and inflation naturally stops.

    The point is that Weinberg has now done his own paper about the same business. In inflation scenarios there always was a problem of "graceful exit". Once you have set up things with assumptions constructed so that inflation happens, how do you get inflation to stop? the RBW (reuter bonanno weinberg) scheme takes care of that.

    Also inflation scenarios always used to involve fantasizing an exotic "inflaton" field different from anything in known physics. The RBW scheme using AsymSafe QG avoids that bizarre conjecturing. It does not need an "inflaton" field to drive the inflation.

    The biggest point Weinberg made in his Cern talk was to highlight the paper of Percacci Codello Rahmede. He gave a 2008 date for it but I recall seeing it in 2007 preprint, and discussing it here at PF. Percacci is the senior author. It is where they truncated so as to allow a 7D UV critical surface and they still only got 3D. The first really convincing evidence that the UV fixed point (on whose existence the whole AsymSafe QG program depends) has a finite dimensional UV critical surface.
    There was a followup in 2008 by Saueressig, Benedetti, Machado which got similar results with a different truncation, still the critical surface was only 3D which is very good news. Weinberg reminds us that both sets of authors, Percacci et al and Saueresssig et al, were able to include matter and still have it work.

    Keep in mind the Percacci Codello Rahmede paper. It was a turning point in Weinberg's story. Seeing that result was the point where Weinberg came back into the game after 30 years.

    So this is a major QG research development and it needs a conference or workshop. And it should involve Percacci, Reuter, Bonanno, Weinberg, maybe also Saueressig, Benedetti...
    A conference is how you gauge the strength of a research thrust, is it taking on new people? How mainstream is it? Who are the major players?

    Well it turns out that Roberto Percacci and Dario Benedetti are organizing the conference. It is 3-8 November 2009 at Perimeter and here are the confirmed invited speakers so far:

    http://www.perimeterinstitute.ca/en/Events/Asymptotic_Safety/Invited_Speakers/ [Broken]

    ==quote==
    Confirmed Invited Speakers To Date

    Jan Ambjorn, Utrecht University
    Alfio Bonanno, INAF, Catania
    Daniel Litim, University of Sussex
    Max Niedermaier, University of Sussex
    Martin Reuter, Mainz University
    Frank Saueressig, CEA, Saclay
    Lee Smolin, Perimeter Institute
    B.F.L. Ward, Baylor University
    Steven Weinberg, University of Texas, Austin
    Jean Zinn-Justin, CEA, Saclay
    ==endquote==

    Smolin has a 1982 paper on this AsymSafe-type stuff that he did at IAS Princeton, not long after Weinberg proposed AsymSafe (1979). Weinberg cited Smolin's IAS paper (on gravity at small scale) in his recent Cern talk. Jan Ambjorn is a clear choice for invited speaker because of the close connection between AsymSafe and Causal Dynamical Triangulations (Ambjorn Loll...). They get the same running dimensionality result. Weinberg cited Ambjorn and Loll's work in his Cern talk.
    Daniel Litim organized a conference last fall (September 2008) in the UK that brought together these and allied approaches, with many of the same people. He is the main UK guy involved in this. Niedermayer is normally based in France, don't know why he is listed from Sussex, maybe on sabbatical there.

    It strikes me that the confirmed invited speakers at the Perimeter November conference (plus the conference organizers Percacci and Benedetti) are essentially the same people that Weinberg cited in the last 12 minutes of his Cern talk.

    Now we need to put the new AsymSafe QG research thrust into broader perspective. I will try to do that as time permits later today.

    We've been actively following AsymSafe here at PF ever since 2005 when Martin Reuter and Renate Loll were plenary speakers at the Loops 2005 conference. And especially after Reuter's talk at Loops 2007 where he reported the Percacci Codello Rahmede result. There are a lot of papers and links I could give, if anyone wants. The best review is Percacci's chapter written for Oriti's book Approaches to Quantum Gravity. The book is 600 pages and expensive but Percacci's chapter is free online at arxiv.
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 13, 2009 #2

    MTd2

    User Avatar
    Gold Member

    Marcus, unfortunantely, I cannot listen to his talk. This stream is above my fiancee's bandwidth conditions, and I am not coming back to my home until next week... And I am not even sure if I will be able to watch there. Can you describe a little bit more of what he says?
     
  4. Jul 13, 2009 #3

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    First of all, congratulations to you and your fiancee! I am sure there are more important matters for you to attend to than Weinberg's talk and bandwidth considerations. I shall do as you ask and give some context and some detail about his talk, but I will not expect you to pay much attention, or to respond, until you have more time (which might only be next week.)

    Before reviewing what Weinberg said, I will just copy the overview of the November workshop organizers:
    ==quote==
    Asymptotic Safety - 30 Years Later
    November 5 - 8, 2009
    Perimeter Institute

    One of the most salient challenges in modern theoretical physics is the construction of a quantum theory of gravity. This challenge has been addressed over time from many original, and sometimes exotic, points of view. A very conservative solution, going under the name of "asymptotic safety", exists somewhat surprisingly within the realm of usual quantum field theory and the renormalization group. Proposed 30 years ago, it has recently received strong support thanks to new techniques in non-perturbative quantum field theory.

    This workshop will bring together researchers who are currently working on asymptotic safety (or worked on it in the past) and researchers who are working on alternative ideas which can bring a new light on the subject. The aim of this meeting is to combine these alternative points of view and reach a better understanding of the results obtained so far, of their interpretation and relevance for quantum gravity.


    Organizing Committee:
    Dario Benedetti, Perimeter Institute
    Razvan Gurau, Perimeter Institute
    Roberto Percacci, SISSA
    ==endquote==

    http://www.perimeterinstitute.ca/en/Events/Asymptotic_Safety/Asymptotic_Safety_-_30_Years_Later/ [Broken]

    Abstracts received to date:

    Alfio Bonanno, INAF, Catania
    The mass-inflation phenomenon in the asymptotic safety scenario

    Max Niedermaier, University of Sussex
    Gravitational fixed points and asymptotic safety from perturbation theory

    Lee Smolin, Perimeter Institute
    Asymptotic safety in the light of our modern understanding of quantum geometry
     
    Last edited by a moderator: May 4, 2017
  5. Jul 13, 2009 #4

    MTd2

    User Avatar
    Gold Member

    No, I am just resting here for my vacations... I am waiting her finish college so that we can marry! :redface:
     
  6. Jul 13, 2009 #5

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Hi MTd2, I will give some impressions from Weinberg's talk. One of the things that impressed me was how often he said things like:
    "I don't want to discourage anyone from working on string theory, but..."

    "I'm not against string theory, but..."

    He warned of the possibility that if "good old QFT" was all there is, string theory "might not be needed", and "might not be the way the world is".
    That the way the world is might be just plain old QFT.

    He presented what he was working on as a modest alternative that should be pursued, a conservative alternative. He stressed that he did not mean to undermine confidence in string (of which he has the highest opinion etc etc). I gather there were a lot of string theorizers in the audience (the Globe auditorium was packed and there were people outside who couldn't get in according to Luis Alvarez-Gaume). So the message was that string is great but not the only game in town, no longer using the conventional "our one best hope" protocol.

    The way the talk was constructed, he gave history for the first 58 minutes. Put up a "QFT stock market" graph of rising waves. Showing successive steep rises in QFT "stock price" (credibility researcher-interest progress) starting back in 1920, where each sharp rise was followed by a slow stagnation. There would be a kind of plateau/slow decline while people tried other things (like S-matrix, or string/M) because of some perceived problem, and then at the end the problem would be resolved and there would be another sharp rise.

    So his graph was a kind of staircase of sagging steps and at the end he had a dotted-line UPTURN with a questionmark. Beginning around 2008.

    So he basically talked to that one slide, that "QFT stock price" graph for the first 58 minutes. And then at minutes 58-70 he talked about the dotted-line expected upturn and AsymSafe QG.

    In the history part he explained why he thought of AsymSafe back in 1976-1979, what the problem was with QFT that caused him to think of it. And how it would solve the problem, if the UV critical surface is finite dimensional.

    The UV critical surface (now conjectured to be only 3D!) is the surface (more precisely hypersurface) in theory space that if you get on that surface then the RenormGroup flow will pull you in to the UV fixed point.

    If the theory is not merely "effective" but is actually fully predictive at high energy and "fundamental" then this UV critical surface being finite dimensional means you only have to experimentally determine a finite number of free parameters. Once you have determined those values, and plugged them in, then you are on the surface and the flow takes over and you are home safe, and good to go for unlimited high energy.

    The history is that around 1979 Weinberg and others tried but could not find evidence of a UV fixed point and a finite dimensional critical surface. If they had succeeded presumably there would have been no string theory because (as he calls it) "good old QFT" would have been nonperturbatively renormalizable in the A.S. sense. Percacci, Weinberg and others use this term "nonperturbatively renormalizable" to describe the aim of A.S.

    I had an earlier thread here at PF called "What if Weinberg had succeeded in 1979." Talking about the same history.

    Then Weinberg talked about how Martin Reuter got things going again in 1998 with the "exact renormalization group flow" approach which, however required a truncation. And Weinberg explained how he, Weinberg, ignored it until the Percacci Codello Rahmede paper came along, because until that paper it was not clear that you might have a finite dimensional UV critical surface.

    PCR gave it a chance to be 7D and it turned out to be only 3D. They truncated less drastically than Reuter originally did, so the flow had more room to screw up in, so to speak, but the method still worked. And there was the coincidence of dimension = 3 in both cases.

    At this point it is minute 58 in the talk and Weinberg is through with the history. He is starting to cite recent work, and other people's papers and get into what he really wanted to talk about and what he prepared the audience for.

    Since you don't have the necessary bandwidth, I should probably listen to those last 10 minutes again and try to give some more detail just on them alone.

    For me the most exciting was where he said his own work along somewhat the same lines as Bonanno Reuter finds a natural explanation of an inflation episode simply in the running of the coupling constants, basically just Newton's G and Lambda, the cosmo constant. I will try to do some more on this later.

    For ready reference, and in case anyone reading the thread has not watched the video of Weinberg's talk, here is the link:
    http://cdsweb.cern.ch/record/1188567/
     
    Last edited: Jul 13, 2009
  7. Jul 13, 2009 #6

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    When you are as old as I am you will understand why I congratulate you. The details do not matter, looking back. It's all good, even the waiting.
    But if you want something to read that does not require bandwidth:

    http://arxiv.org/abs/0803.2546
    Primordial Entropy Production and Lambda-driven Inflation from Quantum Einstein Gravity
    Alfio Bonanno, Martin Reuter
    12 pages, 4 figures, IGCG-07 Pune
    (Submitted on 17 Mar 2008)
    "We review recent work on renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values. In particular we argue that QEG effects can account for the entire entropy of the present Universe in the massless sector and give rise to a phase of inflationary expansion. This phase is a pure quantum effect and requires no classical inflaton field."

    Reuter talked about his explanation of inflation/graceful exit in his invited plenary talk at Loops 2007 (Morelia, Mexico). And then he gave this talk at the IGCG conference at Poona, India later that same year. So this is really 2007 work, which was written up for the IGCG proceedings and posted on arxiv in 2008.

    I am skeptical of their idea about primordial entropy. But my attention is riveted by the part of the paper dealing with inflation. This is what I want to see Weinberg nail down.
    It would be great to have inflation without needing any "inflaton" and that "slow roll" and "eternal inflation bubble multiverse" fairyland stuff. Just simple running of the constants, period.
     
    Last edited: Jul 13, 2009
  8. Jul 13, 2009 #7

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    In his 7 July Cern talk, Weinberg refers to his first formulation of the Asymptotic Safe idea as having been "my Erice lectures" and writes the reference "S.W. 1976".

    Those Erice lecture notes are unpublished but are sometimes cited in the literature as
    "S. Weinberg Erice lecture notes C76-07-23.1 HUTP-76/160 Erice subnuclear physics."
    I think there is a Harvard preprint series called HUTP-76/160. Anyway HU would be harvard university and TP would be theoretical physics, Weinberg was there, and they probably archived the notes.

    The first regular publication of the A.S. proposal was in a 1979 book edited by Stephen Hawking and Werner Israel called "General Relativity: An Einstein Centenary Survey" and published by the Cambridge U.P. This was a collection papers one of which Weinberg contributed.

    Erice is a beautiful spot on the north coastal Sicily with a medieval fortress. It has long been the site of a series of theoretical physics lectures by eminent people. I believe it was possibly at Erice that Dirac, near the end of his life, is supposed to have made the brief remark: "Heisenberg was right" (with no further explanation). Not entirely scrutable but nevertheless remembered to this day. If this bit of folklore is correct, then perhaps he meant by contrast to Schroedinger. For some reason Weinberg prefers to cite his Erice lectures rather than his chapter of the Hawking/Israel book, and we can respect this preference by adopting the practice of giving the seminal AsymSafe reference as "Weinberg (1976)".
    http://cerncourier.com/cws/article/cern/38704/1/CCant1_04_09

    eh ree chay
     
    Last edited: Jul 14, 2009
  9. Jul 13, 2009 #8

    atyy

    User Avatar
    Science Advisor

    The nice thing about AS is it has, I think, a well-defined stopping point, since proving or disproving AS is mathematically well-defined and possible in principle using known techniques - just that the calculations are very, very tortuous. BTW, is AS related to http://arxiv.org/abs/0902.3765?
     
  10. Jul 13, 2009 #9

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    That paper is about N=8 SUGRA. I think N=8 supersymmetry gravity is a quite different topic from ordinary 4D non-supersymmetric General Relativity, which is what we are dealing with here.

    I mentioned how important the Percacci Codello Rahmede paper is, in this whole story. How it brought Weinberg, for one, back into the research that he had started back in 1976-1979, by giving strong evidence that not only does the UV fixed point exist for gravity but the UV critical surface is finite dimensional, and in fact could be just three dimensional.

    In case anyone wants a link to that pivotal paper:

    http://arxiv.org/abs/0705.1769
    Ultraviolet properties of f(R)-Gravity
    Alessandro Codello, Roberto Percacci, Christoph Rahmede
    4 pages; International Journal of Modern Physics A23:143-150,2008
    (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 renormalizable.

    There is a follow-up where they include matter and show results in more detail.

    http://arxiv.org/abs/0805.2909
    Investigating the Ultraviolet Properties of Gravity with a Wilsonian Renormalization Group Equation
    Alessandro Codello, Roberto Percacci, Christoph Rahmede
    86 pages, 13 figures; Annals of Physics 324:414-469,2009
    (Submitted on 19 May 2008)
    "We review and extend in several directions recent results on the asymptotic safety approach to quantum gravity. The central issue in this approach is the search of a Fixed Point having suitable properties, and the tool that is used is a type of Wilsonian renormalization group equation. We begin by discussing various cutoff schemes, i.e. ways of implementing the Wilsonian cutoff procedure. We compare the beta functions of the gravitational couplings obtained with different schemes, studying first the contribution of matter fields and then the so-called Einstein-Hilbert truncation, where only the cosmological constant and Newton's constant are retained. In this context we make connection with old results, in particular we reproduce the results of the epsilon expansion and the perturbative one loop divergences. We then apply the Renormalization Group to higher derivative gravity. In the case of a general action quadratic in curvature we recover, within certain approximations, the known asymptotic freedom of the four-derivative terms, while Newton's constant and the cosmological constant have a nontrivial fixed point. In the case of actions that are polynomials in the scalar curvature of degree up to eight we find that the theory has a fixed point with three UV-attractive directions, so that the requirement of having a continuum limit constrains the couplings to lie in a three-dimensional subspace, whose equation is explicitly given. We emphasize throughout the difference between scheme-dependent and scheme-independent results, and provide several examples of the fact that only dimensionless couplings can have 'universal' behavior."

    Roberto Percacci kicks butt. This is why you have to respect the Italian "Institute for Advanced Studies" at Trieste. They have people like him. We have to learn to recognize when it says SISSA: "school italian superior studies advanced". Think of it as the Trieste Institute for Advanced Studies, just they say it differently.

    Here is Oriti's new book on Quantum Gravity Approaches, where Percacci wrote the chapter on Asymptotic Safe QG.
    http://www.amazon.co.uk/approaches-Quantum-Gravity-Toward-Understanding/dp/0521860458
    The Cambridge Press page allows freeer browsing, more look inside.
    http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521860451
    The 1976 idea at Erice Sicily that Reuter re-started in 1998 has gone bigtime.
     
    Last edited: Jul 14, 2009
  11. Jul 14, 2009 #10

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    It turns out that Renate Loll is also speaking at the AsymSafe conference, on November 5.

    Loll is another person that Steven Weinberg cited in the last 10 minutes of his July 7 talk at Cern. Interesting how the lists coincide. Obvious reason is that both Loll Triangulations and Reuter AsymSafe QG discovered unexpectedly that in their respective models the dimensionality of spacetime goes from 4D at large scale down to around 2D at very small. The spacetime dimensionality is an observable, not an assumed given, and it runs.
    It's possible that the world could actually be the way that these two non-string QG methods found independently. It makes sense for Loll to be on hand. As well as her co-author Jan Ambjorn, who we already saw is on the speakers list.

    The November conference is called Asymptotic Safety, Thirty Years Later.
     
    Last edited by a moderator: May 4, 2017
  12. Jul 14, 2009 #11

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    there is a little paper of Weinberg's he posted on arxiv recently where he puts in about Asymptotic Safety near the end. A simple explanation of it in a couple of paragraphs.
    http://arxiv.org/pdf/0903.0568
    ==quote page 13,14==
    It is possible and perhaps likely that the ingredients of the underlying theory are not the quark and lepton and gauge boson fields of the Standard Model, but something quite different, such as strings. After all, as it has turned out, the ingredients of our modern theory of strong interactions are not the nucleon and pion fields of Källén’s time, but quark and gluon fields, with an effective field theory of nucleon and pion fields useful only as a low-energy approximation.

    But there is another possibility. The underlying theory may be an ordinary quantum field theory, including fields for gravitation and the ingredients of the Standard Model. Of course, it could not be renormalizable in the Dyson sense, so to deal with infinities every possible interaction allowed by symmetry principles would have to be present, just as in effective field theories like the chiral theory of pions and nucleons. But it need not lose its predictive power at high energies, if the bare coupling constants gn(Λ) for an ultraviolet cut-off Λ (multiplied by whatever positive or negative powers of Λ are needed to make the gn dimensionless) approach a fixed point gn
    as Λ → ∞. 31

    This is what happens in quantum chromodynamics, where g∗ = 0, and in that case is known as asymptotic freedom.32 In theories involving gravitation it is not possible for all the gn∗ to vanish. In this more general case where gn∗ is not necessarily zero, the approach to a fixed point is known as “asymptotic safety,” because the theory is safe from the danger that dimensionless couplings like ggrav = GΛ2 (where G is Newton’s constant) might run off to infinity as Λ goes to infinity.

    For asymptotic safety to be possible, it is necessary that βn(g∗ ) = 0, where
    βn (g (Λ)) ≡ Λ dgn(Λ)/dΛ. It is also necessary that the coupling constants gn (Λ) at any finite cut-off lie on a trajectory in coupling constant space that is attracted rather than repelled by this fixed point. There are reasons to expect that, even with an infinite number of coupling parameters, the surfaces spanned by such trajectories have finite dimensionality, so such a theory would involve just a finite number of free parameters, just as for ordinary renormalizable theories. The trouble, of course, is that there is no reason to expect the gn∗ to be small, so that ordinary perturbation theory can’t be relied on for calculations in asymptotically safe theories. Other techniques such as dimensional continuation33, 1/N expansions34, lattice quantization35, and the truncated “exact” renormalization group equations36, have provided increasing evidence that gravitation may be part of an asymptotically safe theory 37. So it is just possible that we may be closer to the final underlying theory than is usually thought.
    ==endquote==

    Weinberg's footnotes:
    31 S. Weinberg, in Understanding the Fundamental Constituents of Matter – 1976 Erice
    Lectures, ed. A. Zichichi (Plenum Press); and in General Relativity, ed. S. W. Hawking
    and W. Israel (Cambridge University Press, 1979) 790.
    32 D. J. Gross and F. Wilczek, Phys. Rev. Lett. 40, 1343 (1973); H. D. Politzer, Phys.
    Rev. Lett. 30, 1346 (1973).
    33 S. Weinberg, ref. 31 (1979); H. Kawai, Y. Kitazawa, & M. Ninomiya, Nucl. Phys. B
    404, 684 (1993); Nucl. Phys. B 467, 313 (1996); T. Aida & Y. Kitazawa, Nucl. Phys. B
    401, 427 (1997); M. Niedermaier, Nucl. Phys. B 673, 131 (2003) .
    34 L. Smolin, Nucl. Phys. B208, 439 (1982); R. Percacci, Phys. Rev. D 73, 041501
    (2006).
    35 J. Ambjørn, J. Jurkewicz, & R. Loll, Phys. Rev. Lett. 93, 131301 (2004); Phys. Rev.
    Lett. 95, 171301 (2005); Phys. Rev. D72, 064014 (2005); Phys. Rev. D78, 063544 (2008);
    and in Approaches to Quantum Gravity, ed. D. Orıti (Cambridge University Press).
    36 M. Reuter, Phys. Rev. D 57, 971 (1998); D. Dou & R. Percacci, Class. Quant. Grav.
    15, 3449 (1998); W. Souma, Prog. Theor. Phys. 102, 181 (1999); O. Lauscher & M.
    Reuter, Phys. Rev. D 65, 025013 (2001); Class. Quant. Grav. 19. 483 (2002); M. Reuter
    & F. Saueressig, Phys Rev. D 65, 065016 (2002); O. Lauscher & M. Reuter, Int. J. Mod.
    Phys. A 17, 993 (2002); Phys. Rev. D 66, 025026 (2002); M. Reuter and F. Saueressig,
    Phys Rev. D 66, 125001 (2002); R. Percacci & D. Perini, Phys. Rev. D 67, 081503 (2002);
    Phys. Rev. D 68, 044018 (2003); D. Perini, Nucl. Phys. Proc. Suppl. C 127, 185 (2004);
    D. F. Litim, Phys. Rev. Lett. 92, 201301 (2004); A. Codello & R. Percacci, Phys. Rev.
    Lett. 97, 221301 (2006); A. Codello, R. Percacci, & C. Rahmede, Int. J. Mod. Phys. A23,
    143 (2008); M. Reuter & F. Saueressig, 0708.1317; P. F. Machado and F. Saueressig, Phys.
    Rev. D77, 124045 (2008); A. Codello, R. Percacci, & C. Rahmede, 0805.2909; A. Codello
    & R. Percacci, 0810.0715; D. F. Litim 0810.3675; H. Gies & M. M. Scherer, 0901.2459; D.
    Benedetti, P. F. Machado, & F. Saueressig, 0901.2984, 0902.4630; M. Reuter & H. Weyer,
    0903.2971.
    37 For reviews see M. Niedermaier & M. Reuter, Living Rev. Relativity 9, 5 (2006);
    M. Niedermaier, Class. Quant. Grav. 24, R171 (2007); M. Reuter and F. Saueressig,
    0708.1317; R. Percacci, in Approaches to Quantum Gravity, ed. D. Orıti (Cambridge
    University Press).

    I hadn't realized that Smolin's 1982 paper was published in Nuclear Physics B. And I had not realized that Plenum Press had published the 1976 Erice Lectures.
     
    Last edited: Jul 14, 2009
  13. Jul 23, 2009 #12

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Highlighting part of the Steven Weinberg quote in the previous post
    The kicker here is being able to give an elegant minimalist explanation for inflation. Weinberg indicates that he has a paper in preparation about this. Bonanno and Reuter already did the groundbreaking work in 2008, applying AsymSafe QG to inflation. (No need for an inflaton field or a fantasized "slow roll" down a special potential. Inflation starts and stops naturally with the running couplings.)
    http://arxiv.org/abs/0803.2546
    The elegance and economy has got to be a plus.

    To understand where Weinberg is coming from, look at his recent papers. They are all puzzling about inflation, how would it work? He had been beating his brains on "inflaton" style inflation for two years or more---2007 and 2008---so when he got a similar idea to Bonanno and Reuter of letting the running Lambda do it this must have seemed attractive.

    BTW here's the talk Smolin will be giving at the November AsymSafe workshop:
    Lee Smolin, Perimeter Institute
    Asymptotic safety in the light of our modern understanding of quantum geometry
     
    Last edited: Jul 23, 2009
  14. Jul 23, 2009 #13

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    With a lot of people now focusing on running couplings in QG (especially newton G and cosmo constant Lambda) one can expect Loop and Spinfoam QG researchers to be seeing how running couplings fit into the Spinfoam picture.

    Evidently Smolin is talking about this at the AsymSafe workshop in November. Rovelli has something about it in the abstract of his lecture series planned for the Corfu school in September. Kirill Krasnov posted something today. He has been involved in Spinfoam LQG from the start (1998 as I recall.)

    Here's from the conclusion of the paper Krasnov just posted on arxiv:
    ==quote==
    ...Translated into the language of spin foams this renormalization motivation may be formulated as follows. In spin foam approach to quantum gravity one obtains an amplitude for a manifold by “gluing” together amplitudes for the individual spacetime simplices, see e.g. 3 and references therein for more details. Let us consider the “renormalization” in the context of spin foams, i.e. analyze what happens when one computes the simplex σ amplitude as the result of integration over the labels of the “smaller” simplices that are glued together to make σ (in a technical jargon this corresponds to an e.g. 5 → 1 move). When the elementary simplex amplitudes are built as dictated by the Plebanski action (5) (or its SO(4) version), the new simplex amplitude – the result of the spin foam “renormalization group flow” – is of a different type, not anymore describable as coming from the original Plebanski action. This is, we believe, how the non-renormalizability of GR manifests itself in the spin foam context. Thus, the spin foam renormalization group flow does not preserve the classical action (5) one starts from. As we have already said, we find this entirely natural, and having to do with the non-renormalizability of the underlying theory.

    It is however possible (but quite non-trivial to show) that some larger class of theories may be closed under such a renormalization group flow. In the discrete setting of spin foams this would manifest itself in the simplex amplitude given by the result of the 5 → 1 move being of the same type as one started from, but with all the coupling constants – parameters of the theory – being changed in some subtle way. Should one find the class of theories with such a property, one can then see whether its UV completion exists by determining whether there is some non-trivial UV fixed point of the flow. This fixed point, if exists, would then provide the sought UV theory. It is then clear that the first step in the direction of this program is to enlarge the class of gravity theories that is being considered. We would like to propose the class (6) as a viable and natural arena for these ideas in the spin foam context.
    ==endquote==
    http://arxiv.org/abs/0907.4064
    Gravity as BF theory plus potential
    Kirill Krasnov
    7 pages, published in Proceedings of the Second Workshop on Quantum Gravity and Noncommutative Geometry (Lisbon, Portugal)
    Int.J.Mod.Phys.A24:2776-2782,2009
    (Submitted on 23 Jul 2009)
    "Spin foam models of quantum gravity are based on Plebanski's formulation of general relativity as a constrained BF theory. We give an alternative formulation of gravity as BF theory plus a certain potential term for the B-field. When the potential is taken to be infinitely steep one recovers general relativity. For a generic potential the theory still describes gravity in that it propagates just two graviton polarizations. The arising class of theories is of the type amenable to spin foam quantization methods, and, we argue, may allow one to come to terms with renormalization in the spin foam context."
     
    Last edited: Jul 23, 2009
  15. Aug 14, 2009 #14
  16. Aug 22, 2009 #15

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Finbar, thanks for posting the link!

    The written version is lacks some of the content of the talk, I see.
    He doesn't mention his own current work---applying asymptotic safe QG to cosmology of the early universe. He doesn't include the warnings to string theorists that ST "might not be how the world is". That the world might be more along the lines of "good old quantum field theory."

    Also the written version does not mention the November conference on AS to be held at Perimeter---he is listed as one of the participants. He referred to that in the talk.
    In a lot of ways the last 12 minutes of the video talk are more feisty, more emphatically challenging, and the paper is abbreviated and toned-down.

    The slides to his 6 July talk are here:
    http://itpworkshop.unibe.ch/MaKaC/getFile.py/access?contribId=126&sessionId=19&resId=0&materialId=slides&confId=2 [Broken]

    Although they lack a lot of the interpretive verbal comment that gave punch to the talk, they have more equations, and in particular they have a discussion of his own recent work in AS.
    He gives some 3 slides of equations, previewing his own research, on how running G and Lambda could give a natural explanation for both inflation and a graceful exit from inflation.

    If anyone looks at the PDF file of the slides, the part about AS is slides #17-32. It really gets interesting in slides #24-32 where on #24 he says "good news" and reports the Percacci Codello Rahmede papers, and then his own recent work on AS early universe is discussed on #30-32.
    The video was more understandable but as I recall it glossed over the very last bit---guess he was running out of time.
     
    Last edited by a moderator: May 4, 2017
  17. Aug 26, 2009 #16

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    In the initial post I gave the list of confirmed invited speakers for the November AsymSafe conference as it was at that time. Three more of those invited have confirmed, including Loll and Rivasseau. Time to update.

    The new confirmed list is:

    Jan Ambjorn, Utrecht University
    Alfio Bonanno, INAF, Catania
    Daniel Litim, University of Sussex
    Holger Gies, ITP, Jena University
    Renate Loll, Utrecht University
    Max Niedermaier, University of Sussex
    Martin Reuter, Mainz University
    Vincent Rivasseau, University Paris-Sud XI
    Frank Saueressig, CEA, Saclay
    Lee Smolin, Perimeter Institute
    B.F.L. Ward, Baylor University
    Steven Weinberg, University of Texas, Austin
    Jean Zinn-Justin, CEA, Saclay

    Rivasseau is one of the five giving lecture series in September at the Corfu QG School.
    (Ashtekar, Baez, Barrett, Rivasseau, Rovelli)

    Smolin wrote one of the early papers about asymsafe QG, back in the 1980s. The talk he is listed as giving at the conference is titled:
    Asymptotic safety in the light of our modern understanding of quantum geometry

    This is a theme that must be on the minds of a number of people. How to relate QG to the running of constants like Newton G and the cosmo Lambda.

    Having these run---so G starts out almost zero and Lambda very large---is an elegant way to get inflation. And then as G gets larger and Lambda smaller, inflation naturally (or as is said "gracefully") stops. One doesn't have to "make up stuff" so that the proper things happen.

    But how does this fit into Loop and Triangulations QG? Apparently Smolin plans to talk about that.

    Ted Jacobson already raised this issue almost stridently in a 2007 paper. Kirill Krasnov used the phrase "come to terms" in his recent paper. He offered a way that spin foam could come to terms with renormalization. It is possible to see this as a challenge that will contribute to making the next couple of years interesting---to see how it plays out.

    Having this AsymSafe conference at Perimeter helps to make it a high-stakes game.

    Daniel Litim had a less visible conference at Sussex about renormalization and QG last September. He has an established interest in asymptotic safety. Ward just posted a paper about A.S. on arxiv within the past month. Martin Reuter of course very familiar---plenary talks at Loops 05 and 07---started the resurgence of A.S. by working persistently on it for a decade nearly alone for some of that time. Nice to see the different factors that contribute to a field suddenly becoming hot. How they come together.
     
    Last edited by a moderator: May 4, 2017
  18. Aug 26, 2009 #17

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    How does renormalization compat with spinfoam? This paper was posted not long ago:
    http://arxiv.org/abs/0907.4064
    Gravity as BF theory plus potential
    Kirill Krasnov
    7 pages; published in Proceedings of the Second Workshop on Quantum Gravity and Noncommutative Geometry (Lisbon, Portugal), Int.J.Mod.Phys.A24:2776-2782,2009
    (Submitted on 23 Jul 2009)
    "Spin foam models of quantum gravity are based on Plebanski's formulation of general relativity as a constrained BF theory. We give an alternative formulation of gravity as BF theory plus a certain potential term for the B-field. When the potential is taken to be infinitely steep one recovers general relativity. For a generic potential the theory still describes gravity in that it propagates just two graviton polarizations. The arising class of theories is of the type amenable to spin foam quantization methods, and, we argue, may allow one to come to terms with renormalization in the spin foam context."

    A basic problem relating background independent QG to renormalization was presented in (I think) a comprehensible way by Ted Jacobson in this 2007 paper:

    http://arxiv.org/abs/0707.4026
    Renormalization and black hole entropy in Loop Quantum Gravity
    Ted Jacobson
    8 pages; Class.Quant.Grav.24:4875,2007
    (Submitted on 26 Jul 2007)
    "Microscopic state counting for a black hole in Loop Quantum Gravity yields a result proportional to horizon area, and inversely proportional to Newton's constant and the Immirzi parameter. It is argued here that before this result can be compared to the Bekenstein-Hawking entropy of a macroscopic black hole, the scale dependence of both Newton's constant and the area must be accounted for. The two entropies could then agree for any value of the Immirzi parameter, if a certain renormalization property holds."

    It wouldn't be surprising, to me at least, if these two people showed up at the Perimeter conference in November (the full participant list has not yet been posted, we only know the confirmed speakers.)
     
    Last edited: Aug 26, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook