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Landmark paper from Freidel and Krasnov

  1. Aug 13, 2007 #1

    marcus

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    For the first time the Immirzi has found its way into spinfoam.
    they also got a Lorentzian version. I think this is an important paper.
    both these guys are top QG researchers since like 1998 (when spinfoam and groupfieldtheory started to happen). this paper has to be major

    http://arxiv.org/abs/0708.1595
    A New Spin Foam Model for 4d Gravity
    Laurent Freidel, Kirill Krasnov
    40 pages
    (Submitted on 13 Aug 2007)

    "Starting from the Plebanski formulation of gravity as a constrained BF theory we propose a new spin foam model for 4d Riemmanian quantum gravity that generalises the well-known model of Barrett-Crane and resolves the ultralocality problem that this model is known to possess. It is well known that the BF formulation of 4d gravity possesses two sectors: one corresponding to gravity and the other topological. The model presented here is shown to give a quantisation of the gravitational sector. The present model is dual to the recently proposed spin foam model of Engle et al. which, we show, corresponds to the topological sector of the theory. One important outcome of our approach is that it also allow us to introduce the Immirzi parameter into the framework of spin foam quantisation. We generalize some of our considerations to the Lorentzian setting and obtain a new spin foam model in that context as well."
     
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  3. Aug 13, 2007 #2

    marcus

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    The third quarter of 2007 is only half over and I am already thinking what are going to be the most important most influential papers in non-string QG this quarter.
    It is pretty clear what three of them are going to be.

    this Freidel-Krasnov is one (setting spinfoam on a firm basis, including Immirzi)

    the Dittrich-Thiemann is one (challenging a core 1990s result of the canonical LQG)

    and the Bonanno-Reuter paper with all the Cosmology results (because cosmology is the PROVING GROUND for quantum geometry/gravity)

    The Bonanno-Reuter paper actually came out in early June but I MISSED IT because it coincided with a blissful week snorkling and hiking in Hawaii away from the computer and suchlike modern contraptions. So we have to include it with the July-September batch.

    Can anyone think of another non-string QG paper from July-September that is on par with these three? Please suggest candidates. I will get the links for the three I just mentioned:

    f-h reminds me of a very important fourth paper which I will insert here

    http://arxiv.org/abs/0708.0883
    The complete LQG propagator
    Emanuele Alesci, Carlo Rovelli

    "Some components of the graviton two-point function have been recently computed in the context of loop quantum gravity, using the spinfoam Barrett-Crane vertex. We complete the calculation of the remaining components. We find that, under our assumptions, the Barrett-Crane vertex does not yield the correct long distance limit. We argue that the problem is general [...]"

    see the next post by f-h


    http://arxiv.org/abs/0708.1595
    A New Spin Foam Model for 4d Gravity
    Laurent Freidel, Kirill Krasnov


    http://arxiv.org/abs/0708.1721
    Are the spectra of geometrical operators in Loop Quantum Gravity really discrete?
    Bianca Dittrich, Thomas Thiemann


    http://arxiv.org/abs/0706.0174
    Entropy signature of the running cosmological constant
    Alfio Bonanno, Martin Reuter

    This is already three with f-h addition, four and the quarter is only half through.
    If we get many more between now and the end of September that have comparable impact then it will be a surprising time in QG. Could happen. Who knows?
    We didn't hear from Martin Bojowald yet---his Nature Physics July article "What happened before the Big Bang" is not available online except to subscribers so we can't include it in our M.I.P. poll. Maybe he will put something on arxiv that discusses his bounce-with-limited-foreknowledge finding that was the basis of the July article.

    BTW one reason it's important to get the Immirzi in there (with spinfoam for instance) is that several papers have showed it to have a real physical significance. this is quite apart from blackhole entropy and horizon area. If you set aside all that stuff about blackholes you still have an Immirzi that should be measurable by experimental particle physicists---in principle for now, but maybe for real measurable later. If the Immirzi is a real physical parameter of nature, then ANY THEORY OF QUANTUM GRAVITY OUGHT TO HAVE IT, at least if the theory is fundamental. So it was a bit worrisome that spinfoam did NOT have the Immirzi.
    this tentative---I'm not sure of it. But this is my take on it at present, so I am delighted that Freidel Krasnov discovered it shows up.

    If anyone is interested in links about the physical significance of the Immirzi there are some papers by Freidel and also some by Rovelli and others. Arxiv search should turn them up, or ask me.

    ============
    UPDATE
    just now checked arxiv and saw

    http://arxiv.org/abs/0708.1915
    Consistently Solving the Simplicity Constraints for Spinfoam Quantum Gravity
    Etera R. Livine, Simone Speziale
    6 pages, 2 figures
    (Submitted on 14 Aug 2007 (v1), last revised 14 Aug 2007 (this version, v2))

    "We give an independent derivations of the Engle-Pereira-Rovelli spinfoam model for quantum gravity which appeared in arXiv:0705.2388. Using the coherent state techniques we introduced in arXiv:0705.0674, we show that the EPR model realizes a consistent imposition of the simplicity constraints."

    which is part of a convergence. Apparently the original Engle Pereira Rovelli vertex can be improved by FLIPPING A SIGN and not only did the authors realize this and bring out a followup paper, but also Freidel Krasnov did (and gave an interesting two-sector explanation) but also Livine Speziale realized it and brought out this paper. So together with the original authors Engle Pereira Rovelli there is an exciting convergence from three directions on the same spinfoam model. Or so it seems at the moment.

    thanks to f-h for helpful comment in the next post.
     
    Last edited: Aug 14, 2007
  4. Aug 14, 2007 #3

    f-h

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    Easy:

    arXiv:0708.0883
    Alesci, Rovelli

    The complete LQG propagator

    "Some components of the graviton two-point function have been recently computed in the context of loop quantum gravity, using the spinfoam Barrett-Crane vertex. We complete the calculation of the remaining components. We find that, under our assumptions, the Barrett-Crane vertex does not yield the correct long distance limit. We argue that the problem is general [...]"

    Or how about:
    arXiv:0708.1236

    which Freidel and Krasnov build upon.The breakthrough papers for this new development as far as I read it were:
    arXiv:0705.0674
    arXiv:0705.2388

    That said, I'm not even half way through the new paper, so I don't know how much further they take this. The Alesci/Rovelli paper is clearly the most important though. Without it all the others are just more models. They extract physics and show that models are *wrong*.

    Falsified.
     
  5. Aug 15, 2007 #4
    Just wanted to point out that there should soon appear another alesci-rovelli paper, a "complete lqg propagator II", in preparation if you believe the references of the first opus. The second one, instead of arguing that the Barrett-Crane vertex is wrong, should explain how kind of asymptotics a spinfoam vertex should have in order to lead to the right tensorial structure of the graviton propagator... that should be even more interesting in principle, no?

     
  6. Aug 15, 2007 #5

    Kea

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    And BTW, Laurent mentioned to me that the new Baratin/Freidel work would be out soon.
     
  7. Aug 16, 2007 #6

    f-h

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    What's that going to be about? 2-Spinfoams?
     
  8. Aug 16, 2007 #7

    Kea

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    Yes, 2-spinfoams. Basically, the representation theory of the Poincare 2-group for 4d spinfoams, which is the flat space case for QFT. (For gravity you really need tricategories and higher).
     
  9. Aug 16, 2007 #8

    marcus

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    Would one of you be kind enough to explain?

    Kea says to expect a Baratin-Freidel paper.
    f-h says is it about "2-spinfoams"?
    Kea says yes.
    What do the representations of the Poincaré 2-group look like?
    Are they being used to label some faces or some other parts of the spinfoam?
    Are you (f-h, Kea) serious about this? My first impression was that you were joking but now I'm not sure :bugeye:
     
  10. Aug 16, 2007 #9

    Chronos

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    Dang, I still like the Dittrich paper [bearing in mind I am the dull knife in the drawer]. The Freidel paper is, however fascinating [and difficult to follow at times]. Incorporating the Immirizi parameter appears to be an astounding result. I look forward to the discussion. I have a nagging suspicion this is more complicated task than it appears in the paper, but am receptive to the idea.
     
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