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Marcel Twelve redefines mainstream

  1. Jul 14, 2009 #1


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    The Twelfth Marcel Grossmann meeting is taking place this week in Paris. The meetings started in 1975 and occur every three years, to review recent developments in gravitation and general relativity.
    The expressed focus of the current MG12 is "recent developments in theoretical and experimental general relativity, astrophysics, and relativistic field theories."

    Normally no single event, book, lecture, experiment, or conference would by itself redefine the scientific mainstream. And yet what constitutes the mainstream of research is constantly subject to redirection, and various things like a major international conference can help define it for us.

    One has to look at the line-up of invited plenary talks, and how that compares with 3 years ago. Also there are the various parallel sessions---how well populated they are and what the titles of the papers indicate, again compared with MG11 in Berlin, three years back.

    There has been a subtle change in the physics character and emphasis of the Marcel meeting. I have to run to make an appointment, but will continue this post later this afternoon.

    BTW today (Tuesday 14 July) it was Abhay Ashtekar who chaired at the session of invited plenary talks. Two speakers of note at today's plenary session were Laurent Freidel, Juan Maldacena, and Herbert Hamber. To give a clue about Freidel's MG12 talk here's the abstract:

    Spin Foam models: models of quantum dynamical space time

    Abstract: In this talk I will give an overview of spin foam models which describe the dynamics of quantum gravity in a background independent context. I will focus especially and the recent developments which concerns the construction of these models in 4 dimensional gravity and present some of the key results obtained in this context like the construction of the model, the proof of the semi-classical limit and the relationship with loop quantum gravity and SU(2) spin network states.

    Likewise to give a taste of Maldacena's talk:
    Black holes as a source of information

    Abstract: We will review the duality between gravity in negatively curved spacetimes and quantum field theories on the boundary. Black holes in these spacetimes represent finite temperature configurations for the theory on the boundary. The classical dynamics of these black holes can be used to get some insight into strongly coupled quantum systems.

    And as a further sample, Hamber's talk:
    Ultraviolet Divergences and Scale-Dependent Coupling in Quantum Gravity

    Abstract: I will discuss how non-perturbative approaches to the problem of ultraviolet divergences in quantum gravity, similar in spirit and methods to what is done in the modern renormalization group treatment of non-abelian gauge theories, point to a possible weak scale dependence of Newton's constant at very large distances. I will then discuss ways by which such a scale dependence could in principle be verified observationally.

    Last edited: Jul 14, 2009
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  3. Jul 14, 2009 #2


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    If you go back to 2006, Marcel Eleven, http://www.icra.it/MG/mg11/ you see these among the invited plenary talks:

    Thibault Damour
    Cosmology and string theory

    Sasha Polyakov
    The structure beyond spacetime
    (The abstract, though not the title, explicitly says strings.)

    Eva Silverstein
    Cosmological singularities in string theory

    Igor Klebanov
    Gauge Theories, Strings and Cosmology

    Joe Polchinski
    Cosmic superstrings

    Dieter Luest
    String theory and the standard model of particle physics

    By contrast in this year's MG12 plenary line-up there is this one talk:
    Gabriele Veneziano
    Transplanckian string collisions: an update

    Abstract: I will review recent progress on transplanckian scattering in string theory and, in particular, the issue of determining the parameter-space region where quantum phenomena associated with classical gravitational collapse are expected to occur.

    In the other plenary talks, string theory is not mentioned by name either in the title or in the online abstract. What is sometimes mentioned instead is the mathematical relation called AdS/CFT, which has broad applications and is not identical with any of the various string theories.
    You could consider this as a subtle nuanced shift in style, or tone. Intellectual fashions change like dress fashions and a lot can happen in three years.
  4. Jul 14, 2009 #3


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    You see the real substance of the conference when you look at the Parallel Sessions. Actually this is where it dawned on me that the character had changed. The Berlin MG11 had a "The Quantum and Gravity" section and within that there were 4 subsections of which "QG4" was Loop---it had fewer recognized leaders and senior people than what one sees in this year's line-up.

    The Paris MG12 is to have two Loop-and-allied sessions, one on Thursday and one on Friday. Loaded with new research much of which we have listed and/or discussed here at PF, and many of the speakers being ones you would recognize. Jerzy Lewandowski is chairing both sessions and people are speaking like Abhay Ashtekar, Martin Bojowald, Fernando Barbero, Alejandro Perez, Jorge Pullin. None of these people spoke at the corresponding parallel session three years ago in Berlin!
    And also some great young people: Hanno Sahlmann, Emanuele Alesci, Andrei Goerlich, Florian Conrady, Eugenio Bianchi, Benjamin Bahr, and a half dozen others I recognize. Goerlich for example has given what is, in my opinion, the best exposition of Loll's Triangulation QG method with the possible exception of Loll herself. Several of these new generation QG people have been Rovelli Phd students, or are currently. Sahlmann worked with Thiemann and recently moved up from postdoc to the next rung.


    To find the corresponding list for MG11, look here:
    http://www.worldscibooks.com/etextbook/6997/6997_toc.pdf [Broken]
    and scroll way down at the end of the proceedings table of contents to where it says pages
    Again it is more of a subtle nuanced change in the Marcel weather. There are other ways you can gauge this, for instance by looking at the tone of Ashtekar's MG11 talk, which is on arxiv. But I won't belabor the point. In 2006 some central mainstream forces were trying to marginalize nonstring QG at the Marcel, and in 2009 there is no sign of this (that I can see.)

    So what changes in Loop-and-allied physics have we seen that could either explain, or at least correlate with this shift towards inclusiveness in the mainstream? In particular what about Hamber's work? He recently published this:
    Quantum Gravity on the Lattice
    Herbert W. Hamber
    63 pages, 12 figures; Gen.Rel.Grav.41:817-876,2009
    (Submitted on 8 Jan 2009)
    "I review the lattice approach to quantum gravity, and how it relates to the non-trivial ultraviolet fixed point scenario of the continuum theory. After a brief introduction covering the general problem of ultraviolet divergences in gravity and other non-renormalizable theories, I cover the general methods and goals of the lattice approach. An underlying theme is the attempt at establishing connections between the continuum renormalization group results, which are mainly based on diagrammatic perturbation theory, and the recent lattice results, which apply to the strong gravity regime and are inherently non-perturbative. A second theme in this review is the ever-present natural correspondence between infrared methods of strongly coupled non-abelian gauge theories on the one hand, and the low energy approach to quantum gravity based on the renormalization group and universality of critical behavior on the other. Towards the end of the review I discuss possible observational consequences of path integral quantum gravity, as derived from the non-trivial ultraviolet fixed point scenario. I argue that the theoretical framework naturally leads to considering a weakly scale-dependent Newton's costant, with a scaling violation parameter related to the observed scaled cosmological constant (and not, as naively expected, to the Planck length)."

    This of Hamber's has interesting points of similarity with the abstract Rovelli posted some time ago for his lectures at the September Corfu school:

    Covariant loop quantum gravity and its low-energy limit
    Carlo Rovelli
    "I present a new look on Loop Quantum Gravity, aimed at giving a better grasp on its dynamics and its low-energy limit. Following the highly succesfull model of QCD, general relativity is quantized by discretizing it on a finite lattice, quantizing, and then studying the continuous limit of expectation values. The quantization can be completed, and two remarkable theorems follow. The first gives the equivalence with the kinematics of canonical Loop Quantum Gravity. This amounts to an independent re-derivation of all well known Loop Quantum gravity kinematical results. The second the equivalence of the theory with the Feynman expansion of an auxiliary field theory. Observable quantities in the discretized theory can be identified with general relativity n-point functions in appropriate regimes. The continuous limit turns out to be subtly different than that of QCD, for reasons that can be traced to the general covariance of the theory. I discuss this limit, the scaling properties of the theory, and I pose the problem of a renormalization group analysis of its large distance behavior."

    I think everyone who follows the subject knows that Loop QG now has several formulations, which either employ conventional quantization methods proven useful elsewhere or do not, as the case may be, and also has some idea of the progress made towards checking the largescale GR limit, especially since 2007. The main approaches agree on some basic results like the spectra of area and volume operators, so that the research program is cumulative. New ways of calculating geometric transition amplitudes, as they are developed, should give the same results as the old methods, where applicable.
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  5. Jul 14, 2009 #4


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    The questions that begin forming for me go like this: when Rovelli says "covariant LQG" he means spin foam LQG. there was a breakthrough in covariant LQG in 2007 when a slight change in the amplitude formula for a foam vertex was made, like changing the amplitude formula in a Feynman diagram, and some quick advances followed. And we know that a spin foam, as a 2-complex, can be dual to a simplicial complex. So the LQG path integral is a close relative of simplicial QG path integral (like Loll's Triangulations QG). And so although LQG is not limited to working with a regular lattice it shouldn't surprise anyone that a 4D lattice formulation should come along. The aim would naturally be to keep the backgroundlessness of LQG---its independence from any fixed background geometry---and still be able to employ lattice field theory methods.

    Now Rovelli may be talking about having coupling constants like G and Lambda run. He is explicitly talking about some similarities with lattice QCD, or QCD generally.

    And he is explicitly keeping LQG diffeomorphism invariant (aka "general covariance") which is where he says the differences with QCD arise.

    So how close is he to what Hamber is talking about? Hamber is a longtime co-author with Ruth Williams at Cambridge, who was Daniele Oriti's PhD advisor. Williams is the guru for Regge gravity, or simplicial gravity in general. The thing that stands out about Hamber is his belief in Asymptotic Safety---the UV fixed point of gravity's renomalization flow--and the fact that he has worked that into a simplicial or lattice approach. How does this mesh with LQG?

    a bit of detail from Hamber's UC Irvine page:
    "Professor Hamber earned his high school Abitur from the Deutsche Schule Mailand, and his undergraduate physics degree, in theoretical physics, from the University of Milano, Italy. As a Fulbright Fellow he obtained his Ph.D. from the University of California at Santa Barbara in 1980. After a postdoctoral appointment in the theory division of DoE's Brookhaven National Laboratory , he became a member of the Institute for Advanced Study in Princeton NJ, before joining the Physics Department of the University of California at Irvine in 1985. During the academic year 1993-1994 he has held a visiting appointment at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland. During the academic year 2006-2007 he has held a visiting appointment at the Albert Einstein Institute (AEI) in Berlin, Germany."

    The AEI is a center for Loop-and-allied QG. Thomas Thiemann, Daniele Oriti, Bianca Dittrich are there now. Loll and Bojowald were there earlier, and numerous others as well. There is some fascinating stuff about Hamber's research interests on the same page:

    Also notice that the International Organizing Committee for Marcel XII contains Steven Weinberg. There is probably some connection between the vision of the new mainstream you get from Weinberg's July 7 talk at Cern and the vision you get from Marcel XII.
    We have a thread about the significance of things pointed out in Weinberg's talk here:
    It's the whole Asymptotic Safe QG business.

    Maybe this will all seem clear and simple to someone else. Would you like to explain it?
    I'm struggling to get it in focus.
    Last edited: Jul 14, 2009
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