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Map of QG (Perimeter view, January 2008 update)

  1. Jan 1, 2008 #1

    marcus

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    At Perimeter Institute, Bianca Dittrich recently gave a survey introduction to (non-string) QG with Lee Smolin and Leonard Susskind asking questions among other.
    The talk video is online PIRSA 07120030
    and the title was Introduction to Quantum Gravity

    I think of this as a kind of QG map from the Perimeter perspective. It looks like there was a big audience, as P.I. seminars go. She was laying out the whole field, including the history from the 1930s on. Giving an overview of 70 years of QG, and the present, and HIGHLIGHTING THE MOST SALIENT APPROACHES as she saw them.

    So it's interesting what she did and did not mention and what she emphasized by spending time on.

    0-12 minutes history and general ideas

    12-20 eight minutes on Triangulations (CDT) approach. She herself is not working in this but according to her two other people at P.I. are----Joe Henson and Dario Benedetti.
    It was remarkable that she put it first and devoted so much time. It is a good intro to basic CDT.

    20-28 canonical LQG+Spinfoam+Loop Cosmology. Eight minutes on that whole line of research, which is what she is working in herself.

    28-32 four minutes on efforts to derive the low energy limit and testable predictions
    (passing mention of several other approaches but no details on them.)

    32+ Open questions. Summary of conclusions. Discussion from the audience.

    http://pirsa.org/07120030
    to get the video, click on "windows media"
     
    Last edited: Jan 1, 2008
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  3. Jan 1, 2008 #2

    marcus

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    I just started watching a second time to Dittrich's half-hour overview talk of Quantum Gravity. I think she's very good.

    It was interesting how she narrowed the focus. She didn't mention Causal Sets (Sorkin, Dowker...) or Asymptotic Safety (except maybe an oblique reference in connection with Triangulations). Joe Henson used to be doing Causal Sets, maybe still is, but she mentioned his current work in Triangulations at Perimeter (which I didn't know of)

    She covered a wide expanse including 70 years of history, but she also focused on two main QG approaches
    1) Triangulations---the CDT path integral
    2) the LQG-Spinfoam-LoopCosmology combination

    And even though the latter is producing roughly 50X as many papers which are getting 50X the citations that the former gets (order of magnitude) she devoted equal time to each of them.

    The focus was in a way similar to what we saw in the GR18 Sydney programme where there were over 600 GR, gravity, cosmology people. No string person was invited to give a plenary talk, and two QG people were invited: Renate Loll and Laurent Freidel. One of these represents topic 1) above, and one topic 2). That was the balance.

    In quantity of research Triangulations is much less important than Loop-Spinfoam, and it is a more recent arrival on the scene. But in the perspective of the GR18 conference and in Dittrich's talk they look on par----whether or not that is significant, or just an accident.
     
  4. Jan 2, 2008 #3

    marcus

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    As I see it a major theme emerging for 2008 is different QG approaches CONVERGING.
    Like different climbing teams making their ways up different sides of the same mountain.
    A good reason to have a map in the first place is to be able to anticipate convergences.

    It looks like in 2007, with the new Marseille spinfoam, the spinfoam approach made solid contact with canonical LQG-----the immirzi showed up, and discrete geometric spectra, and spinfoam seemed to work as a time-evolution joining an initial final spinnetwork state of geometry.
    So it's fair of Dittrich to lump all these approaches together in one:
    LQG-spinfoam-LQC, the last being application to cosmology and lately also to study what goes on inside the black hole horizon and what replaces the singularity.

    The way Dittrich laid it out there were just two main areas Triangulations and Loop.
    (somehow other things like Causal Sets and Asymptotic Safety did not get emphasized)
    So I want to be alert to anything that looks like a BRIDGE between Triangulations and Loop.

    Some PF posters f-h and francesca and maybe also Alejandro Satz have clued us to watch out for Eugenio Bianchi. Here is a December 2007 talk of his at Utrecht:
    http://www1.phys.uu.nl/wwwitf/Seminars/Quistabstracts.htm#Bianchi

    E. Bianchi (Scuola Normale Superiore, Pisa) - 13 December 2007

    Simplicial Geometry from Loop Quantum Gravity

    "The relation between Loop Quantum Gravity and simplicial geometry has been pointed out many times in the literature, both at the kinematical and at the dynamical level. In the first part of this talk I will present some new results about the curvature operator in Loop Quantum Gravity and show that appropriate superpositions of spin-network states have curvature with support on one-dimensional hinges, reproducing a three-dimensional simplicial geometry. The second part of the talk is devoted to the dynamics, introduced in terms of a spinfoam model. I will present an analysis of 2- and 3-spin correlation functions on a semiclassical state peaked on flat space and show that they can be reproduced computing in perturbative-Regge-calculus correlations of two and three area-fluctuations around flat space. This analysis is meant to be an intermediate step towards understanding if Loop Quantum Gravity admits a regime which can be described in terms of perturbative (continuum) quantum gravity on flat space."

    This talk was at a seminar series that Renate Loll runs at the Utrecht ITF.

    Around the same time (mid December) Bianchi and Modesto got a paper about this accepted for publication in Nuclear Physics B, here is the preprint
    http://arxiv.org/abs/0709.2051
    The perturbative Regge-calculus regime of Loop Quantum Gravity
    Eugenio Bianchi, Leonardo Modesto
    43 pages, accepted by Nucl.Phys.B
    (Submitted on 13 Sep 2007, revised 11 Dec 2007)

    "The relation between Loop Quantum Gravity and Regge calculus has been pointed out many times in the literature. In particular the large spin asymptotics of the Barrett-Crane vertex amplitude is known to be related to the Regge action. In this paper we study a semiclassical regime of Loop Quantum Gravity and show that it admits an effective description in terms of perturbative area-Regge-calculus. The regime of interest is identified by a class of states given by superpositions of four-valent spin networks, peaked on large spins.
    As a probe of the dynamics in this regime, we compute explicitly two- and three-area correlation functions at the vertex amplitude level. We find that they match with the ones computed perturbatively in area-Regge-calculus with a single 4-simplex, once a specific perturbative action and measure have been chosen in the Regge-calculus path integral. Correlations of other geometric operators and the existence of this regime for other models for the dynamics are briefly discussed."

    I am picturing Loll Triangulations and Rovelli Loop as two amoebas beginning to grope for each other and the question is Who Eats Whom?

    I think I see about 20 times more researchers and talent on the Loop side. When those two bodies of research get together what does the result look like? More like T'ation or more like Loop?

    Maybe it doesn't matter. I think it looks more like Triangulation in spite of the 20-fold imbalance.

    I'm impressed that in their model they have found quantum spacetime foam---not putting it in by hand. A foamy, fractally geometry has been expected for 50 years. It came naturally up for them

    I'm impressed by the extreme simplicity. They just continue the Quantum Field Theory program inventing nothing and changing as little as possible. Perturbative has been shown not to work, so they go nonperturbative (as QCD did and moved to a lattice) and to avoid the lattice dependence on a fixed background they moved to a free triangulation.
    The bare minimum of concepts and of changes to established method.

    All path integral method must use some regularization. Theirs is as similar to Feynman's as i can imagine. it is called piecewise linear. Instead of line segments for a particle path, triangulations describing a path thru piecewise linear geometries. I am impressed by the mathematical naturalness and conservatism.

    I'm impressed that they are running computer sims using the full model. Small universes pop into existence, swell up, shrink down, and disappear (back into the minimal geometry state). In Loop cosmology they run simulations but this is not done using the full LQG-spinfoam. It is done with a symmetry-reduced model with only a few degrees of freedom. Triangulations is the only approach where they do full-model simulations of universes.
     
    Last edited: Jan 3, 2008
  5. Jan 4, 2008 #4

    marcus

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    I misread the clock the first time I reported on Dittrich's talk. So I'll correct that now. Just now I listened to her talk a third time. It is valuable. It gives a clear professional look at the whole Quantum Gravity picture.

    So it's interesting what she did and did not mention and what she emphasized by spending time on.

    0-12 Twelve minutes history, problems, and general ideas

    12-20 Eight minutes on Triangulations (CDT) approach. She herself is not working in this but according to her two other people at P.I. are----Joe Henson and Dario Benedetti.
    It was remarkable that she put it first and devoted so much time. Just that brief talk provides a good intro to basic CDT.

    20-28 Eight minutes for canonical LQG.

    28-36 Eight minutes for Spinfoam+Loop Cosmology.

    36-37 One minute on Group Field Theory and deriving matter Feynman diagrams from the Spinfoam formalism.

    37-42 Five minutes on efforts to derive the low energy limit and testable predictions
    (passing mention of several other approaches but no details on them.)

    42+ Open questions. Summary of conclusions. Discussion from the audience.
     
    Last edited: Jan 4, 2008
  6. Feb 20, 2008 #5

    marcus

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    I think theres a need to keep updating the map of research directions in QG, so I want to revise this or add to it. I was strongly influenced by Bianca Dittrich's view in the talk she gave in December. That talk had a narrow focus, and there were things left out that she might have mentioned had she been giving a longer talk, or writing a survey paper:

    1. Krasnov's Self-Dual approach, that he just gave an ILQGS presentation on Slides and audio here:
    http://relativity.phys.lsu.edu/ilqgs/krasnov020508.pdf
    http://relativity.phys.lsu.edu/ilqgs/krasnov020508.mp3

    2. Astrup et al, two recent papers combining Noncommutative Geometry with LQG, a bid to include the Standard Model of particle physics in LQG
    http://arxiv.org/abs/0802.1783
    http://arxiv.org/abs/0802.1784
    On Spectral Triples in Quantum Gravity I and II
    Johannes Aastrup, Jesper M. Grimstrup, Ryszard Nest
    https://www.physicsforums.com/showthread.php?t=215282
    Ashtekar et al also came out with a paper studying the possible tie-in of LQG with NCG.

    3. The E8 gambit, also a unification bid. Among other things interesting in view of talks given recently by Bertram Kostant.

    Here's the announcement of the most recent talk:
    Here's the video streamer of the talk:
    http://mainstream.ucr.edu/baez_02_12_guest_stream.mov
     
    Last edited: Feb 21, 2008
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