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New papers implies the Kodama state gives LQG a good semiclassical limit

  1. Mar 18, 2007 #1
    LQC is a subset, a symmetry reduced minisuperspace of LQG, which appears to have a Hamiltonian with a good semiclassical limit that reproduces GR.

    First, Randano papers addresses Witten's earlier paper on Kodama's state being nonrenormalizable.

    Now, thanks to Marcus, it appears Eyo Eyo Ita paper, Existence of generalized semiclassical Kodama states 1-3, http://arxiv.org/abs/gr-qc/0703052, that LQG apparently with the presence of matter, has GR as part of its semiclassical limit which is deSitter spacetime. If this is true it would appear that LQG has completed a major milestone, and is now a "theory of quantum gravity" -- the Hamiltonian is now able to reproduce GR at distances much larger than the planck distance.

    The claim has been that string theory is the only game in town, in that its long distance graviton scattering amplitude matches that of GR reframed as a QFT. There are doubts, such as that it is not background independent, that its calculations are down in 10 dimensions with unbroken supersymmetry, but LQG it is alleged does not even reproduce classical spacetime.

    It appears in these papers that the Kodama state gives LQG reproduction of GR.
     
    Last edited: Mar 18, 2007
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  3. Mar 18, 2007 #2

    marcus

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    As you point out, that allegation is in process of being refuted.
    Remarkable advances been made in just the past two years---work by Rovelli and by Bojowald as well as papers you cited. I should also mention Ashtekar and Thiemann.

    Yes that part is well-established. What has been happening recently is the removal of restrictions and an extension of those results to more general cases verging on the full theory.

    You are referring to the three-part paper of the Cambridge (DAMTP) graduate student E.E. Ita.

    In my opinion this is one of the most interesting QG papers that has appeared so far in 2007, so I'm very glad you brought it up for discussion, Ensabah.

    I want to list some trivia about the Ita paper to put it in context of random non-essential information (stuff of daily life). I will, and hope that others will as well, comment on the paper itself. BTW I think Ita is brilliant.
     
  4. Mar 18, 2007 #3
    For BI folks it probably is the most important. This problem was emphasized by
    arxiv.org/pdf/hep-th/0501114 "LQG a string theorist point of view"

    Ita cites the famous Randamo paper on the Kodama state and Witten's objection, by generalizing the Imirizi paramter to be real.

    Larry Yaffe and the string theory community critics of LQG (i.e Motl, Distler, Susskind etc.,) have stated

    http://www.math.columbia.edu/~woit/wordpress/archives/000069.html

    "The oft-repeated argument that string theory is the most promising framework we have for combining quantum mechanics and gravity remains true. Even though there is no real non-perturbative definition of string theory, I don’t think one can dispute this assertion. (As an aside, so-called “loop quantum gravity” is an interesting one-parameter family of statistical mechanics models, but has not been shown to have anything to do with gravity. Does it have a large-volume limit? Does it have long distance dynamics described by some effective field theory plus classical GR? Who knows…)"

    If Ito's paper holds up, and in future papers he wishes to compare the canonical Hamiltonian with the path-integral (i.e spinfoam) approach, LQG may finally have a semiclassical limit, or what he called semiclassical quantuam correspondence, and it may support Ashketar's Bojaworld's work on LQC.

    Ito appears to be the first to cite Andrew Randono paper on Kodama.
     
    Last edited: Mar 18, 2007
  5. Mar 18, 2007 #4

    marcus

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    heh heh :biggrin:

    In contrast to that rather sweeping statement made in 2004, which I suppose a sufficiently out-of-touch string theorist might still subscribe to, we have this cautiously qualified announcement from the abstract of a recent Bojowald paper:

    http://arxiv.org/gr-qc/0611112

    Our results demonstrate that loop quantum gravity has the correct classical limit, at least in its sector of cosmological perturbations around flat space, in the sense of perturbative effective theory.
     
    Last edited: Mar 18, 2007
  6. Mar 19, 2007 #5
    Yes, very interesting set of papers by Ita:

    http://arxiv.org/find/gr-qc/1/au:+Ita_E/0/1/0/all/0/1

    and I can't find more information on his previous work, so if these
    are his first papers, he started very well!:biggrin:

    Christine
     
  7. Mar 19, 2007 #6

    marcus

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    Ita's PhD supervisor is P. D. D'Eath. This line of research seems to be a new one for him, and not similar to anything I know of having been done by anyone in Hawking's group (Relativity and Gravitation group/DAMTP) at Cambridge.

    Daniele Oriti was there---his supervisor was Ruth Williams, whose specialty is simplicial gravity (Regge GR)---and his work was in spinfoam models and group field theory: related to LQG but not smack on central to it the way E.E. Ita's is.

    The emphasis in QG at Cambridge has tended to be on path integral approaches (quite possibly reflecting a preference on the part of Stephen Hawking, as group leader). I wonder if this signals a change in direction for Ita's advisor P. D. D'E. or I suppose it could reflect shift in the group. To an outsider it looks a bit like "diversifying the research portfolio" the investment analogy that Smolin sometimes talks about.

    what I mean is, they didn't require E.E.Ita to aim his PhD research in some direction where THEY were already going. They apparently let him choose a field in which nobody at the DAMTP group was working.

    This is entirely speculation on my part. I know zero about it. Arivero, who sometimes posts here, is at Cambridge and may have met some of these people.
    http://www.damtp.cam.ac.uk/
    http://www.damtp.cam.ac.uk/user/gr/about/members/staff.html
    http://www.damtp.cam.ac.uk/user/gr/about/members/death.html
     
    Last edited: Mar 19, 2007
  8. Mar 19, 2007 #7
    Yes, as a future direction of research he suggests to compare results of teh Hamiltonian canonical approach with the path-integral spin foam approach.
    To date, the two have not been shown to be dual to one another, although spin foam apparently can be coupled to QFT of your choice (i.e Freidel)

    Hopefully with the Kodama state, a version of Spin Foam can be shown to be equivalent, and both have "semiclassical quantum correspondence" that is GR.

    It appears that the Kodama state, with a positive Imirizi parameter, should be a major thrust of BI research and offer a description of gravity from planck scale to GR, and that prior research in LQC can be salvaged.

    I agree with Lubos Motl, Jacques Distler, Aaron Bergman, etc., and other string theory that being BI is useless if you cannot reproduce classical GR spacetime where GR is well tested.
    String theory's claim as being the only one that reproduces GR where GR is valid may no longer be true, finally, and being a quantum theory as well.

    I'm tempted to ask Lubos what he thinks of these more recent developments.
     
  9. Mar 19, 2007 #8

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    (As an aside, so-called “loop quantum gravity” is an interesting one-parameter family of statistical mechanics models, but has not been shown to have anything to do with gravity. Does it have a large-volume limit? Does it have long distance dynamics described by some effective field theory plus classical GR? Who knows…)

    That's actually a perfectly fine statement to me. It has *not* been shown. There is evidence but we do not yet know. Hamiltonian LQG represents the exact observable algebra quantum mechanically and in the only physical non abelian gauge theory known the low energy limit is not given by the "classical" Hamiltonian theory one quantized to start with.

    It also has not been shown that the Landscape contains the Standard Model, that String Theory is perturbatively finite at higher genus, that the AdS/Cft conjecture is true, etc. (Each of these conjectures is supported by more evidence then the semiclassical limit of LQG variants.)
     
    Last edited: Mar 19, 2007
  10. Mar 19, 2007 #9

    You do not feel that the Kodama state gives a good low energy large volume limit?
     
  11. Mar 19, 2007 #10

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    This question hinges on wether you think that Smolin convincingly refuted Wittens criticism of this state. I think the jury is out on that. Even if so a single state does not a semiclassical limit make. It would be very strong evidence in favour of it existing, but I'm by no means an expert on this.
     
  12. Mar 19, 2007 #11
    Do you mean Randano?
     
  13. Apr 4, 2007 #12

    jal

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    Well paper # 4 has been put out.
    http://arxiv.org/abs/0704.0367
    Existence of generalized Kodama states. IV. The search for a quantization of 4-dimensional gravity
    Eyo Eyo Ita III

    It's enought to make my head buzz.
    He has not finished setting up the scenery.
    I hope that he will have visual effects to accompany his description of the dynamics.
    It should be interesting to see his model in action.
    jal
     
  14. Apr 4, 2007 #13

    marcus

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    this is what concerns me a little about the series of papers.

    In one sense this is very good. He has set out. He doesnt know for sure where it will get him, or if he will be able to reach his target destination. He keeps plowing ahead.

    Maybe that is how it is supposed to be. But at the moment I feel less confident about ultimate success here than in some other cases of work in progress.
     
    Last edited: Apr 4, 2007
  15. Apr 4, 2007 #14

    jal

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    Hi marcus!
    I think that every attempt at addressing the dynamics will help those coming later.
    Everybody is stuck in the starting gate. I would expect that there would be a few false starts.
    jal
     
  16. Apr 4, 2007 #15

    marcus

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    your buoyant optimism goes well with coffee this morning.
    thanks (I mean it.)
     
  17. Apr 6, 2007 #16
    With all due respect, what seems to an outsider to be the main focus of the "group" at Cambridge has precious little to do with what actually goes on here. There is no pressure of which I am aware placed on grad students to work in a particular area. In fact, the amount of leeway given to students here to pursue their own ideas is one of the hallmarks of the department.
     
  18. Apr 6, 2007 #17

    marcus

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    That is good to hear! Cambridge should be emulated by other departments in this respect!

    I strongly believe that support should be given to mentally independent PhD students and they should be allowed to branch off the beaten pathways if they choose. The problem is finding an advisor, if you want to pursue something not covered by anyone on the faculty.

    Coalquay I'm curious to know if you have ever encountered PD D'Eath. Any notion of his current research interests?
    I looked up his papers and don't recall seeing any Loop-or-related quantum gravity.
     
    Last edited: Apr 6, 2007
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