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Major developments LQG a look back 1998-2009

  1. Jun 30, 2009 #1
    Hey Marcus,
    what have been the most important results in LQG from the last ten years, say 1998-2009? A variety of ways from citations to number of papers/researchers, to solving outstanding problems.

    TO put it another way what does LQG tell us in 2009 that was not known in 1998? Are we any closer now in solving its outstanding issues from 1998?
     
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  3. Jun 30, 2009 #2

    marcus

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    As I would imagine you are well aware, LQG was reformulated in 2007 and given a dynamics and the main effort now is showing that with the 2007 dynamics the theory has the right largescale limit.

    We cannot see into the future, especially with research, so it would be naive to talk about a distance from an endpoint. Obviously we cannot see or know the endpoint. The field has come a long ways, especially I think since 2006. But we cannot talk about "distance to endpoint" because it is not at present defined.

    We will have an endpoint of sorts when LQG has been shown to reproduce GR in the limit
    (there is visible progress towards this goal but it ain't over till it's over) and when whatever LQG is at that point will have been observationally tested. Then we will be able to look back and judge how near or far we were from the endpoint in 2009.

    Rovelli (probably the most representative central person to talk about LQG) gave an honest forthright account of the current situation and progress in LQG at the Strings 2008 conference. The Strings 2008 invited him to give exactly that kind of overview and status report. I don't believe he gave any indication that he is guessing a prediction like what you ask for.

    My own wildass guess about what to expect in 2009 is already on record in a thread "LQG: to expect in 2009." :biggrin: I put it out partly playfully so you can call me names when the year is out and what I said would happen didn't. I see so much progress, so many new young researchers posting about the lowenergy limit, that I actually predict the lowenergy limit will get done this year. And I think substantial progress toward observational testability will be made. So far things are going about as I anticipated in that thread. But someone's wildass guess is not an authoritative source! :biggrin: So my reaction to your post is that there is not any answer to your question---the question is not well-posed.
     
    Last edited: Jun 30, 2009
  4. Jul 1, 2009 #3

    Demystifier

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    Can you point out the crucial papers of that reformulation? (For people like me who are not up to date in LQG.)
    Thanks in advance!
     
  5. Jul 1, 2009 #4

    marcus

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    Thanks for asking! I will make a special post about this in another thread as soon as I have time, but to respond immediately here is a list of the top 20 recent LQG papers:
    https://www.physicsforums.com/showthread.php?p=2104022#post2104022

    The ones you are asking about are #1, #3, #4 on that list.

    Maybe #8, which appeared later in the year (November 2007) would serve to sum up what happened, and could replace having to read the other three I mentioned. I'm not sure if that would work. In any case the most highly cited papers about the new formulation are numbers 1, 3, and 4.
     
    Last edited: Jul 1, 2009
  6. Jul 1, 2009 #5
    I infer you speak of reformulation of SF from BC to EPRL -- so from 1999-2009, the major development appears to be a discontinuation of the search of a hamiltonian in quantized Ashketar variables with good semiclassical limit, and a major shift to SF for semiclassical, and simplified LQC for phenomenology but that the original LQG has reached a dead end.
     
  7. Jul 2, 2009 #6

    Demystifier

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    Thanks! :smile:
     
  8. Jul 2, 2009 #7

    cristo

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    Let's keep this thread on-topic. Remember that this is a technical forum, and not general discussion. All off-topic posts have been removed. Penalties will be given for this in future.
     
  9. Jul 2, 2009 #8

    marcus

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    I should point out that 2007-2009 saw a major confirmation and revitalizing of the earlier LQG (the old version that was developed before 1998)

    The spinfoam approach came in around 1995-1996 as I recall, and was in full swing by 1998. Most of the community shifted over to spinfoam.
    That seems to be the natural place to handle dynamics, whereas the earlier spinnetwork formalism was good for more descriptive stuff---referred to as kinematics--like area and volume operators.

    The problem, until 2007, was that it was not clear they were the same theory. Spinnetwork and spinfoam are just two formalisms, two ways of calculating. They are not ontological, they do not say what is there (general fact about quantum theories--have to do with measurement not imaginary microscopic cogwheels, Bohr warned about that kind of pretense). Well the question was are these two ways of calculating compatible!
    And until 2007 it looked bad, because the Spinnetwork approach had an immirzi parameter which played a very important role and occurred in the spectra of the geometric operators, and the Spinfoam approach did not!

    Also the Spinfoam approach did not reproduce the results about the discrete spectra of the area and volume operators at all. You couldn't even calculate them in Spinfoam, so there was nothing to compare with the other approach.

    In 2006 a trouble was discovered with the Spinfoam formula that had been in use for some 8-10 years. In 2007 a new (vertex amplitude) formula was developed. And it turned out to confirm the earlier spinnetwork approach. And it also had a natural place for the immirzi parameter. As I recall this mostly came out last year.

    So now the two theories are welded together into one, and have gotten a new impetus, a new boost of energy. And there is only one LQG, which simplifies talking. Both spinnet and spinfoam formalisms are in active use because they are good for different things. One can place more reliance because one knows that where they overlap they give the same answers so they represent the same QG theory.
     
    Last edited: Jul 2, 2009
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