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Does this paper prove GR emerge from LQG?

  1. yes

    25.0%
  2. no

    75.0%
  3. maybe, unsure, other

    0 vote(s)
    0.0%
  1. Dec 5, 2009 #1
    Does this paper prove GR emerge from LQG with matter?

    A common criticism of LQG is that despite intense research, no semiclassical limit has been shown to connect GR with LQG in the low energy limit. And hence, it is not even a candidate theory of quantum gravity.

    This paper


    http://arxiv.org/abs/0912.0554
    Emergence of General Relativity from Loop Quantum Gravity
    Chun-Yen Lin (University of California at Davis)
    7 pages, 2 figures
    (Submitted on 3 Dec 2009)
    "I show that general relativity emerges from loop quantum gravity, in a relational description of gravitation field in terms of coordinates defined by matter. Local Dirac observables and coherent states are constructed for an explicit evaluation of the dynamics. The dynamics of large scales conforms with general relativity, up to the corrections near singularities."
     
    Last edited: Dec 6, 2009
  2. jcsd
  3. Dec 6, 2009 #2

    marcus

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    He makes the problem easier by introducing matter.
    That is not necessarily bad, but it makes the problem different. In fact, it looks to me like a smart move, making for an interesting paper!
    After all, Cosmologists use the microwave background, the light from near-evenly dispersed ancient matter in a somewhat similar way---it gives a criterion for something being at rest, and a kind of mark to measure motion relative to. It helps describe a preferred time-slicing.
    Maybe it was a good idea of Chun-Yen to introduce matter.

    But that said, it does change the problem. He solved an easier problem than the one you seem to be talking about.

    I think it's great, but I don't see any way I can answer on your poll. There is no place where it says he solved an easier version of the problem. None of the answers you have listed seem right or to make sense.

    Anyway Chun-Yen is looking good. For now he is just a PhD student at Davis working on his thesis. I hope he gets invited to give a seminar talk here (Davis is just 50 miles up highway 80 from us). I'd certainly go.
     
    Last edited: Dec 6, 2009
  4. Dec 6, 2009 #3
    Ok, but does he solve then the easier version, which is
    LQG + matter has GR in low-energy regime, and quantum corrections that match LQC in the planck regime + matter?
     
  5. Dec 6, 2009 #4

    atyy

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    I actually think geometry cannot make sense without matter, so I like that part, at least naively. I of course don't know if the claims of this paper are right, since I haven't followed the mathematical details. But I think the paper itself points out a gap in its argument, which is the unproven assumption of the existence of a certain state, so I checked "no".
     
  6. Dec 6, 2009 #5

    MTd2

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    The derivation of that paper is to appear in reference 12. But I believe he derived correctly, otherwise the name of his advisor would be a little bit torn because of the kind of acknowledgments he made.
     
  7. Dec 6, 2009 #6
    I agree -- since our universe has matter, if he shows LQG + matter = GR in the low energy limit, it is relevant to our universe. If LQG + matter makes predictions that are testable say on CMB that deviate from either SM or GR alone, and these predictions are born out, the other case is just hypothetical.


    What would be the advantage in solving the harder problem of LQG without matter gives rise to GR without matter in the low energy limit?
    Do we know GR applies in a universe without matter? Maybe matter is needed for spacetime to shape and curve in accordance with GR, not the other way around.
     
    Last edited: Dec 6, 2009
  8. Dec 6, 2009 #7

    atyy

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  9. Dec 6, 2009 #8
    I think Ed Witten's objections might apply regarding kodama state -- Andrew Randanomo had a couple of papers which generalizes immirizi to a real number which he argues then that Witten's objections in analogy to chern-simons wavefunction

    # ^ Hideo Kodama (1988). "Specialization of Ashtekar's Formalism to Bianchi Cosmology". Progress of Theoretical Physics 80 (6): 1024. doi:10.1143/PTP.80.1024.
    # ^ a b Lee Smolin (2002). "Quantum gravity with a positive cosmological constant". arΧiv:hep-th/0209079 [hep-th].
    # ^ Edward Witten (2003). "A Note on the Chern-Simons and Kodama Wavefunctions". arΧiv:gr-qc/0306083 [gr-qc].
    # ^ a b c Andrew Randono (2006). "Generalizing the Kodama State I: Construction". arΧiv:gr-qc/0611073 [gr-qc].
    # ^ a b c Andrew Randono (2006). "Generalizing the Kodama State II: Properties and Physical Interpretation". arΧiv:gr-qc/0611074 [gr-qc].
     
  10. Dec 6, 2009 #9
    If the poll stated "Does this paper prove GR emerge from LQG with matter?"

    how would you answer?

    thanks
     
  11. Dec 6, 2009 #10

    MTd2

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    ensebelah, all coherent formulations of horava gravity do not yield GR at IR, but GR + dark cold matter. What do you think of that?
     
  12. Dec 6, 2009 #11
    Since our universe could be described as GR + dark cold matter in the Lambda-CDM I'd say it's consistent with known observations. So definitely promising. Possibly economical.
     
  13. Dec 6, 2009 #12

    marcus

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    I'd say that this paper is a 7-page paper which states that the details will appear in reference [12], a longer paper by Chun-Yen that is in preparation. This is mathematics, so details are essential.

    So I'd conclude that you are groping in the right direction. The poll question should include *with matter*, and also the poll is premature---the right time to ask would be when you see the complete proof, and people have had a chance to react. Like when reference [12] comes out Perimeter will probably invite Chun-Yen to visit and give a talk, and people like Laurent will be sitting listening and asking questions, and we will get a kind of stereoscopic in depth idea. Right now all one can have is a hunch.

    My hunch is....well you could make a poll about hunches at this point :biggrin:
    My hunch is that Steve Carlip is really tops, and he is advising and directing ChunYen research, and my hunch is that reference [12] WILL appear within 8 or 9 months and that it will be solid work, and (now counting chickens before hatched) that ChunYen will go present a seminar talk somewhere visible, and quite likely it will go over well.

    So we will know in 9 or 10 months, and the reason I am feeling so optimistic about this now is that from my perspective ChunYen gets a lot of initial credibility just by being a PhD student of Carlip.
    I heard Carlip give a seminar at Berkeley, in a seminar run by Horava, with Horava drilling him with questions and sounding a bit miffed from time to time. That was back in September or so, about 3 months back. I got a really good impression of Carlip.
    And John Baez sent his PhD student Derek Wise, after Derek got his PhD, up to Davis to postdoc with Carlip. He could have gone to Europe or Canada or Penn State. It says something. Plus there are all of Carlip's respected book-and-paper outputs.
    I am registering that Davis is a place where good work is being done.

    You know they are also doing Loll-style CDT research there? With their own software, not Loll's, which means they get some algorithmic implementation independence.
    Anyway at present we can just poll our hunches, but I'm feeling optimistic.

    ===EDIT TO REPLY TO NEXT POST===
    MTd2 I don't know the physical meaning of Immirzi smaller than one! That is the range where the EPRL spinfoam model agrees with the FK spinfoam model. People often specify that 0 < immirzi < 1 because it makes things simpler, because it doesnt make any difference whether you are Freidel-Krasnov or Engle-Pereira-Rovelli-Livine in that case, the models agree and give the same result. But I don't know if there is a clear physical meaning. What did you have in mind?
     
    Last edited: Dec 7, 2009
  14. Dec 6, 2009 #13

    MTd2

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    I want to make an observation, but first I have to ask a question:

    What is the meaning of Immirzi parameter modulus smaller than 1?
     
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