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The MAGIC gammaray flare result, if true, favors Spinfoam LQG

  1. Aug 23, 2007 #1


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    Energy dependent speed of gammaray photons may have been observed, by the MAGIC collaboration at its Canary Islands observatory.

    Lee Smolin has been making the point for several years now that if an energy dependent speed of gammarays is ever observed it will be an enormous stimulus to LQG.

    AFAIK it has never been proven that any full 3+1D LQG-related theory NEEDS the slight bending of special relativity involved here-----"deformed special relativity" (DSR).
    But DSR is very "Loop-friendly" and if some deformation of special relativity---with, for instance, a slight energydependent speed of light---were confirmed that would facilitate several research lines in the Loop community.

    By itself, this doesnt PROVE anything. It strikes me as something analogous to the relation of the string community to SUPERSYMMETRY. Observing SUSY at LHC would be a big shot in the arm for stringfolks. But they aren't predicting it. Or maybe a better analogy would be detectable extra dimensions. They'd like it but they aren't betting serious money or professional honor on it.


    But I just want to remind whoever's around that Laurent Freidel et al DID PROVE THAT deformed relativity WAS NECESSARY IN SPINFOAM in the 2+1D case.
    This is an important result and it was reported in a Physical Review Letters piece by Freidel and Livine last year.

    IF A SIMILAR RESULT happens to be proved for the next higher dimension case then that Spinfoam model will be falsifiable and if the MAGIC result is confirmed it will have passed a test. It is always possible that such a result for the 3+1D case will be proved.
    They may even be working on it. So we should check out the Freidel Livine paper.

    3D Quantum Gravity and Effective Non-Commutative Quantum Field Theory
    Laurent Freidel, Etera R. Livine
    Phys.Rev.Lett. 96 (2006) 221301
    4 pages
    (Submitted on 9 Dec 2005 (v1), last revised 17 May 2006 (this version, v2))

    "We show that the effective dynamics of matter fields coupled to 3d quantum gravity is described after integration over the gravitational degrees of freedom by a braided non-commutative quantum field theory symmetric under a kappa-deformation of the Poincare group."

    the idea is if you take a spinfoam picture of gravity, and couple matter to it so that you now have a combined quantum spacetime-and-matter picture, and then if you TURN OFF GRAVITY then you get back a certain quantum field theory, of the matter, on a spacetime that is now FLAT because you turned off gravity. And the symmetry of that quantum field theory turns out to be a deformed special relativity.

    There are some people around who are more familiar with this work and can interpret it better. I hope they correct me if I've made any major blunders here.

    What I'm saying is that IF by good fortune Freidel and or some others should happen to extend this result to the full 4D, then it would be an example of a spinfoam QG theory that MUST have deformed relativity in the flat case.
    And that is very nearly tantamount to saying that it should have energydependent speed of gamma, in the flat case of little or no gravity.
    And that flatness is what prevails most of the way between the sources of those gammaray flares and where we are observing them.
    Then there would be a really falsifiable QG that would be shot down if the MAGIC observation is not confirmed. and would survive if it was confirmed.
    Last edited: Aug 23, 2007
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  3. Aug 23, 2007 #2


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    Got links to the data, Marcus? Fotini has been hanging her hat on this energy-dependent c for a while, and I would love to see her idea supported or negated, since either outcome would represent a net increase in our understanding of the nature of the vacuum.
  4. Aug 23, 2007 #3


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    Hi Turbo,
    I don't have their data. here was some earlier comment on the MAGIC gammaray flare news

    I will summarize here, without wasting so much space.

    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)

    The thing is at redshift z = 0.034 meaning light travel time of 0.46 billion years
    and there was a delay of 4 minutes
    between two batches or "bins" of photons, a high-energy bunch 1.2-10 TeV and a lower-energy bunch 0.25-0.6 TeV

    the rough order of magnitude is that the higher-energy ones traveled about 16 quadrillionths slower

    the flare-up at the AGN (active galactic nucleus) only lasts a few minutes*, so the size of the delay is about the same as the length of the flare.

    it is the sort of thing that BEGS to be replicated with other galaxies and at greater distances.

    they put a linear model to it and they estimated that there was a coefficient which I would write as 1/6 of the reduced Planck energy (the Planck energy with 8 pi G instead of G is about 2.43 E18 GeV)

    in their linear model the fractional reduction in speed, for a photon with energy E, was E/M where M is 1/6 Planck energy.

    Of course at that rate if the linear model was accurate to much higher energy then if you made a photon with 1/6 Planck energy then it would just sit there and have speed zero.

    speed (E) = 1 - E/M

    but we are talking about linear approximation in the regime where E/M is about 10-14

    *they say doubling time about 2 minutes---gives idea of attack and decay timescale
    Last edited: Aug 24, 2007
  5. Aug 23, 2007 #4


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    Thanks, Marcus. Thanks to Google I was able to find:


    as well. The results fit with the idea that more energetic (higher-frequency) EM must interact more frequently with the propagating medium (space) than lower frequencies, resulting in a lower speed of propagation through the same medium. If this effect can be confirmed, it will open up inquiries into the fine structure of the vacuum, and eventually into the proposition that the fundamental properties of the vacuum can be conditioned by the mass embedded in it - a key concept in Einstein's view of space as a refractive medium.

    Nice catch.
  6. Aug 24, 2007 #5


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    To my knowledge one can also heuristically derive modified dispersion relations in string theory. So does this MAGIC result really give any hint to any special research direction?
  7. Aug 24, 2007 #6


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    Aren't we being a bit premature in jumping all over this?

    I mean, besides the fact that it has to be (i) published first, (ii) verified by other observations, etc., even the authors are cautioning that there could be other factors involved here that could explain their results. I've seen string theorists grasping even the smallest hint of "experimental evidence" out of desperation, but I didn't t know it applies to LQG theorists as well!

  8. Aug 24, 2007 #7
    I hope so, even if, as you said Marcus, by itself, this doesnt PROVE anything.:uhh:
    Well, QG people are arguing so far about LI breaking, but I was told that this would be a secondary question if you conder that the whole theory needs to reach a more complete structure. I'm aware of more predictions about it from the stringy side that from the QG side... isn't it so? take a look to the Trieste workshop's program...(slides available)
    I hope that Etera would like to say someting about... :wink:

    Actually the conprension of the astrophysical process of the flare is worth. I think that in the analysis they assume that tha flare is almost istantaneus.
    "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."
    The way to have a better knowledge about it is first to study more sources applying the same analysis.
    "This pioneering study demonstrates clearly the potential scientific value of such an analysis."
    That's to say: always more work for esperimentalists...:grumpy:
  9. Aug 24, 2007 #8
    I did not read the paper. But of course there is a caveat. One should know exactly what is happening at the source. High energy and "low" energy photons must have been emitted simultaneously. There is a lot of astrophysical stuff to be learned and cleaned up before making any firm conclusions about possible quantum gravity effects. I hope all this can be improved more and more in the years to come....
  10. Aug 24, 2007 #9


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    Certainly, the importance of an observed delay rests on the assumption that a burst emits all frequencies simultaneously, and that there is no evolution in the energy of the photons populating the burst with time. We don't know that is the case. If the effect is real, the magnitude of any delay should be proportional to the distance that the EM from the burst traveled, too. More observations are needed. GLAST is scheduled for launch in Feb 2008 and its observations may help clarify some of this.
  11. Aug 24, 2007 #10


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    As several people have indicated already, nothing is proven so far.
    Fortunately gammaray flares and gammaray bursts (GRB) happen often so the key idea will be to replicate.

    An additional instrument that is expected is GLAST (gammaray large area space telescope)
    the latest date given for its launch is here:

    when I checked the schedule it had been updated 23 August and the date for GLAST was 5 February 2008 (but it was not a firm launch date, just a "no earlier than")

    To my mind until they replicate by timing other flares it is not even clear that there is a delay. Once it is established that there is a regular delay, if that happens, then people can try to pin down the cause. Obviously the cause could be an unknown effect at the source as well as QG dispersion.

    The main people in this case are the 130+ experimentalists called the MAGIC collaboration. The paper is very short, 5 pages, because it is written for Physical Review Letters.

    The idea of publishing in PRL is when one has an important result of wide interest, that one wants to get out to other physicists quickly. So they have submitted this as a letter to PRL and they will doubtless be bringing out a LONGER paper that gives more details and data, to submit for publication elsewhere.

    The over a hundred people in the MAGIC collaboration (very similar to what one sees in accelerator research :smile:) are concerned with the instruments, the data collection, the statistical analysis etc. I would say that the collaboration is the principle author. It is not a theoretical paper but primarily a practical one, which
    the paper comparatively easy to read. They want to tell you how they binned the data and about their 1000 Monte Carlo computer simulations with artificial data that they used to check their analysis, so as to make sure they were not being fooled.

    Also they have ideas about effects at the SOURCE which might interest you.
    they estimate the source plasma diameter to be about 109 kilometers (see the bottom of page 4). Which is about 50 light minutes. The delay they found was about 4 light minutes. One would think that just going thru the plasma could introduce some delay which might be energy dependent. They did some analysis of this. In other words they have been trying to exclude source effects. But the main way to exclude source effects is to replicate with AGN at different distances (redshifts)---because the hypothesized QG dispersion delay depends directly on the distance the light travels.
    Last edited: Aug 24, 2007
  12. Aug 24, 2007 #11


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    Blog reaction.
    SciAm blog by George Musser one of the editors of Scientific American
    http://blog.sciam.com/index.php?title=hints_of_a_breakdown_of_relativity_theor&more=1&c=1&tb=1&pb=1 [Broken]

    exerpt from George Musser's post:
    "this is one of those "extraordinary claims require extraordinary evidence" situations. But if the high-energy gammas really did lose the cosmic race, we're talking Big Discovery. It could be a way to constrain string theory, loop quantum gravity, and other bleeding-edge theories.

    The basic picture is that high energies might cause small-scale fluctuations in the shape of spacetime, which would act as subatomic lenses. The higher the photon energy, the more it might induce such lensing and the slower it would cover large distances. Four minutes isn't much of a delay over a half-billion-year journey, but then again, you don't expect much. From the lag, you can deduce where quantum gravity kicks in."

    Ars Technica blog by Chris Lee
    Last edited by a moderator: May 3, 2017
  13. Aug 24, 2007 #12


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    "if true, favors Spinfoam LQG"
    The string people are already getting their public relation/publicity working into high gear to get there first with their claims.
    Spinfoam LQG better hurry up if they do not want to face an uphill battle.
    Reuters paper would be a good place to start from.
  14. Aug 24, 2007 #13
    Hints of a breakdown of relativity theory?

    The MAGIC gamma-ray telescope team has just released an eye-popping preprint (following up earlier work) describing a search for an observational hint of quantum gravity. What they've seen is that higher-energy gamma rays from an extragalactic flare arrive later than lower-energy ones. Is this because they travel through space a little bit slower, contrary to one of the postulates underlying Einstein's special theory of relativity -- namely, that radiation travels through the vacuum at the same speed no matter what?
  15. Aug 24, 2007 #14
    maybe the gamma rays passed through a nebula or something and the low energy rays were absorbed and reemitted slowing their effective velocity while the high energy rays passed right on through, or maybe differences in deflection... in any case, it would take a lot more than this to cause a breakdown in relativity, even if this is a valid exception, if it can't be explained away it will probably just be chalked up to "technical difficulties"
    Last edited: Aug 24, 2007
  16. Aug 25, 2007 #15


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    Peter Woit has a post about reactions to the MAGIC paper

    Among other things he quotes a calming wait-and-see comment from J.L. Kelley an avowed astrophysicist

    here's the link to J. Kelley's comment
    Last edited: Aug 25, 2007
  17. Aug 25, 2007 #16


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    Einstein stated here:


    that the speed of light through a vacuum must be variable to permit the deflection of light expected from gravitational effects. He explained that the constancy of the speed of light in a vacuum is a concept that is peculiar to special theory of relativity and not to the general theory of relativity. Until the end of his life Einstein tried to determine what properties of space were modified by the presence of embedded matter. I think that he would have been quite excited about some aspects of LQG, including the premise that the vacuum is not smooth and monolithic, but has a fine-scale structure.

    The person quoted on Peter Woit's blog has exactly the right mind-set on follow-up.
    It would also be valuable to compare the observations to those of other programs - GLAST in particular. Could be exciting times.
    Last edited: Aug 25, 2007
  18. Aug 25, 2007 #17


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    YES!!! Either way, up or down, it's going to be good.

    This was the attitude expressed in Smolin's book (p. 237)

    It would be wonderful to get a real prediction out of a quantum theory of gravity and then have it shown to be false by an unambiguous observation. The only thing better would be if experiment confirmed the prediction. Either way we would be doing real science.

    It looks as if gammaray astronomy is going to be able to test quantum gravity. and that's the main thing---to have theories confronted by observation. It matters much less whether this or that particular theory is falsified or not falsified.

    It is interesting to read about how the IACT (imaging air cherenkov telescope) type of telescope works
    Here is a general page from the MAGIC observatory site
    http://magic.mppmu.mpg.de/introduction/index.html [Broken]
    here is a special page on how IACT works
    http://magic.mppmu.mpg.de/introduction/iact.html [Broken]

    they can actually photograph the electron-positron SHOWER that a single TeV gamma photon makes in the atmosphere (10-20 km altitude)
    by the brief Cherenkov glow that the shower makes
    and from that they can tell the direction the original gamma photon came from and also the energy of the original photon.

    this is what that 17 meter diameter optical mirror dish is doing

    TURBO, you asked about a link to their data.

    this is a 51 page paper they posted earlier about the same observations, where they did not offer a quantum gravity explanation (very little attempt at explanation of any sort)
    in this paper they give a lot of data, plotting it various ways. It may be the kind of thing you were interested in seeing.
    Last edited by a moderator: May 3, 2017
  19. Aug 27, 2007 #18


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    Bee's blog on MAGIC


    Bee usually has something sensible to say. Figuring out how theories could be tested is her specialty. She has a long informative discussion but
    here is the essential conclusion:
    "This is the summary, status Sunday evening. In case I loose one or the other reader here, let me give you my opinion up front: there is no experimental evidence for quantum gravity, and none of the present theories can be tested with currently available data. If you're looking for sensations, you're on the wrong blog. I suggest you try one of those mentioned above instead."
    Last edited: Aug 27, 2007
  20. Aug 27, 2007 #19


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    Yes, that's what I wanted to see. Thanks for the link.
  21. Aug 27, 2007 #20
    Hi Marcus,

    I agree 100% with Sabine on those matters. As I have pointed out elsewhere and perhaps a couple of times, there is a lot of astrophysics to be learned before having a solid basis to move on to other territories. This is not excessive prudence, but simply the usual scientific method. This does not mean, however, that one should not be open to new ideas and possibilities. These should always be put into perspective.
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