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Quantum grains (String theory & LQG in trouble?)

  1. Jul 1, 2011 #1
  2. jcsd
  3. Jul 1, 2011 #2


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    Irrelevant to presentday LQG. The results only concerns theories (I don't know which) that have been shown to be Lorentz violating. LQG has not been and is not in that class of theories.

    I looked at the Philippe Laurent et al paper on arxiv yesterday. It does not mention LQG as far as I could see. What you quote is Science Daily---pop journalism. Can't rely on it.

    The technical paper is

    I just checked, and in the scholarly paper I can't find any citation to any of the standard LQG sources at all! Nor, of course, does it mention LQG. Philippe Laurent is quoted by the Science Daily reporter as saying something which, if he actually said it, just shows he does not know what he is talking about LQG-wise. But in any case the actual scholarly paper avoided that.
    Last edited: Jul 1, 2011
  4. Jul 3, 2011 #3
    Exact Marcus, LQG is compatible with local Lorentz invariance.
    Two explicit references about this are the old paper (2003)

    ``Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald contraction",
    C Rovelli, S Speziale, Physical Review D67 064019; http://arxiv.org/abs/gr-qc/0205108" [Broken]

    and a more recent one, in the covariant theory:

    ``Lorentz covariance of loop quantum gravity",
    C Rovelli, S Speziale, Physical Review D83 104029; http://arxiv.org/abs/1012.1739" [Broken]

    There are interesting proposals on the possibility of Lorentz violations (Pullin, Gambini, Smolin, Amelino Camelia...). These are still viable and compatible with the observations so far. But notice that these possibilities are not necessarily implied by LQG.

    Last edited by a moderator: May 5, 2017
  5. Jul 3, 2011 #4


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    I think it's very missleading:
    suggests that the "grains" themselves must be much smaller - which is not necessarily the case. Instead the effects of these grains need to be much smaller. So if there is a theory which is compatible with Planck-space grains but w/o any violation or deformation of Lorentz invariance at all (like LQG) then this theory remains to be a perfectly valid candidate theory for quantum gravity.

    In that sense science daily does not make a good job.

    Has anybody written a comment on their web page?
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