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Stuff about GRB observations (dispersion?)

  1. Aug 23, 2007 #1


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    stuff about gammaray flare observations (dispersion?)

    Probing Quantum Gravity using Photons from a Mkn 501 Flare Observed by MAGIC
    J. Albert, et al., for the MAGIC Collaboration, John Ellis, N.E. Mavromatos, D.V. Nanopoulos, A.S. Sakharov, E.K.G. Sarkisyan
    5 pages, 3 figures, submitted to Phys. Rev. Lett
    (Submitted on 21 Aug 2007)

    "We use the timing of photons observed by the MAGIC gamma-ray telescope during a flare of the active galaxy Markarian 501 to probe a vacuum refractive index ~ 1-(E/M_QGn)^n, n = 1,2, that might be induced by quantum gravity. The peaking of the flare is found to maximize for quantum-gravity mass scales M_QG1 ~ 0.4x10^18 GeV or M_QG2 ~ 0.6x10^11 GeV, and we establish lower limits M_QG1 > 0.26x10^18 GeV or M_QG2 > 0.39x10^11 GeV at the 95% C.L. Monte Carlo studies confirm the MAGIC sensitivity to propagation effects at these levels. Thermal plasma effects in the source are negligible, but we cannot exclude the importance of some other source effect."

    website for the "magic" collaboration
    recent news

    "We cannot exclude the possibility that the delay we find,
    which is significant beyond the 95% C.L., may be due
    to some energy-dependent effect at the source.

    As far as I know there was NOT a firm prediction to expect planckscale dispersion based on any of the main nonstring QG models.

    Smolin and others have published several papers showing how a very slight dispersion effect MIGHT show up (due to QG) in the course of a very long time while a pulse travels distances on the order of a billion lightyears. Smolin has argued for a small but observable energydependence of the speed of light. But I don't know of any QG paper where there is a clear firm prediction.

    Also I don't know if this "MAGIC" collaboration actually did observe a real dispersion.
    If someone has more clear perception of this, please help get it straight.

    This particular gammaray flare that they observed had traveled for an estimated 0.46 billion years by the time it reached us. That would be a long enough time for some faster photons to get out ahead of some slower photons----as Smolin and others have proposed---so that the receiver could pick up two spikes.

    this kind of dispersion, if it exists, is too slight to be observed in light traveling on ordinary timescales----it is only a four minute delay after a half-billion year travel.

    quite possibly, as they say in the paper, the observed delay could have been caused by some effect AT THE SOURCE. so conventional physics might apply and one would not actually be seeing a dispersion effect associated with the spatial medium.
    Last edited: Aug 23, 2007
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  3. Aug 23, 2007 #2


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    they divided the photons up into 5 or so batches and found the hi-energy batch was delayed about 4 minutes relative to a low-energy batch

    high energy: 1.2 - 10 TeV
    lo energy: 0.25 - 0.6 TeV

    the delay of 4 minutes that accumlated over the half-billion year (more precisely 0.46 billion) travel time translates into a fraction reduction in the speed of light

    delta c/c, which is roughly 10-14

    More exactly, if you want to check it, 4 minutes is this fraction of 0.46 billion years:
    1/6 x 10-13
    to check you can just multiply to see how many minutes in a year
    Anyway the order of magnitude fractional change in the speed the photon goes is 10-14

    there are two cases. either delta c/c depends LINEARLY on the energy or it depends on the SQUARE. (the venerable physics attitude, either first order or second order dependence)

    In the linear case, they estimate

    delta c/c = - E/ (0.4 x 1018 GeV)

    so now you can predict how fast photons will travel. say you take one that is 4 TeV
    that would be from the high-energy bunch, that is 4000 GeV so you can predict
    delta c/c = - 4/(0.4 x 1015) = - 10-14

    just doing a little cancelation of ten-powers and being sloppy. and that was what it was supposed to be, about.

    In the secondorder case, they estimate

    delta c/c = - (E/ (0.6 x 1011 GeV))2

    We could try out another high-energy photon like 6 TeV (all we need is one in the range 1.2-10 TeV) and if you put that in for E you get

    delta c/c = - (6000/ (6 x 1010))2 = - (10-7)2 = - 10-14

    and that is about what it is supposed to be also. So either way, first or second order, works fine.
    Last edited: Aug 23, 2007
  4. Aug 23, 2007 #3


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    now the usual Planck energy is 1.2 x 1019 GeV

    but many people including John Baez have recommended replacing G by 8piG
    because it is 8 pi G that occurs in the Einstein equation and it may be more sound than saying G just like it is generally better to say hbar instead of h.

    and then you get the REDUCED Planck energy 2.43 x 1018 GeV
    The Wikipedia article says, approvingly, "Particle physicists and cosmologists often use the reduced Planck energy."

    OK so lets look at the coefficient that the authors of this paper got for the LINEAR case and see if it is ORDER ONE TIMES REDUCED PLANCK.
    The number they came up with that works, i.e. FITS their observation is 0.4 x 1018 GeV
    and the reduced Planck energy is 2.43 x 1018 GeV

    and those two, with no reason to expect it, are within a factor of SIX of each other so they are about same magnitude.

    Some of the authors are famous people or have a certain notoriety at least, like CERN's John Ellis, and Nick Mavromatos, and Giovanni Amelino Camelia.
    they and their crowd of collaborators have in effect turned a dish to the sky and inspected the tealeaves and came up with an unexpected experimental number (describing variation in speed of light with energy) and FOR NO EXPLAINABLE REASON that number turned out to be within a factor of six of
    cosmologists favorite energy---the reduced Planck energy. The only reaction I have at the moment is WTF.

    Of course Lee Smolin and Joao Magueijo and Giovanni Amelino Camelia have written papers saying that something like that should happen, and apparently so have other people, but it was never clear to me why and certainly I was not expecting it. Also one has to still be very cautious about accepting this paper.
    Last edited: Aug 23, 2007
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