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Observational Loop QG-your opinion.

  1. Jun 27, 2009 #1

    marcus

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    Observational Loop QG--your opinion.

    Please read Bee Hossenfelder's recent post on this topic--you might also wish to glance at the papers/talks she discusses, if you haven't already done so.
    http://backreaction.blogspot.com/2009/06/constraining-modified-dispersion.html

    To what extent is the development of LQG (now primarily the spinfoam approach) being guided by astrophysics observations?

    To what extent is it being tested by astro and collider experiments? Is LQG falsifiable at current collider energies?

    ============

    Guidance is a looser less restrictive idea than falsifiability, and less technically stringent.
    It is pretty clear that Loop research is influenced by the connection with DSR (deformed special rel) and it's significant which type of DSR you make a bridge to. Recent observations of delayed high energy photons tend to favor the type of DSR in which very high energy particles see more bumps in the geometry and have a longer harder road to travel, so can be very very slightly delayed. Or more generally, for whatever reason, in those versions the high energy photons appear to propagate slower thru the quantum geometry.

    On the other hand, recent observations make less interesting the version(s) of DSR where higher energy photons have an advantage and arrive sooner. This gives Loop researchers some valuable clues or guidance in relation to DSR, but of course nothing here is confirmed yet. A lot more gammaray bursts need to be observed in order to establish that there is or is not a dispersion effect.

    Strict falsifiability by experiment is a more demanding and technical matter. Of course presentday LQG is empirically testable/falsifiable because it assumes there are no extra space dimensions accessible at any energy. I doubt that extra space dimensions will ever be detected (it seems like a silly made-up idea for which there is no evidence) but if they were then today's LQG theories would be instantly out the window. It's quite a robust test. :biggrin:

    To clarify the point by way of contrast, string extra dimensions are not being empirically tested because if one finds no evidence for them at a given level of energy one can imagine that you just have to look at some higher energy and they will appear. There is no falsifiability involved.

    But subjectively it seems a bit unsatisfactory because of the unliklihood of LQG being falsified this way. One would like a test that LQG has more chance of failing.

    ================

    So what do you think? I tend to agree with Bee Hossenfelder that what we are seen now is simple guidance. IOW that researchers in the field now have the potential to be guided by the gammaray observation data that is now coming in.

    Here's an earlier PF thread about the main paper Bee talks about:
    https://www.physicsforums.com/showthread.php?p=2248613#post2248613
     
    Last edited: Jun 27, 2009
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  3. Jun 29, 2009 #2
    Re: Observational Loop QG--your opinion.

    If somehow "extra-dimensions" are detected (something I doubt as well, but this is just a personal feeling), I wonder whether it is true that LQG would be instantly ruled out? I wonder whether it is possible to have a higher-dimensional LQG theory. I mean, start from a higher-dimensional Hamiltonian formulation of GR, do all the LQG quantization exercise, and see what comes up. I think such theoretical games have a lot of room to be played, the problem is how to rule them out according to observations, etc.
     
  4. Jun 29, 2009 #3

    turbo

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    Re: Observational Loop QG--your opinion.

    Bee's observations are well-reasoned. If high-energy gamma rays are delayed with respect to the bulk of the EM arriving from a GRB, we have to consider that Fotini Markopoulou-Kalamara was right, and that EM with shorter wavelengths should experience more interaction with the space through which they propagate. Of course, we will need a lot more GRB observations to see if this frequency-dependent slowing is a general feature of the bursts, and not a misinterpretation of some frequency-dependent evolution of EM at the source. Once we have enough observations (assuming more delays are observed), it will be interesting to see if the delays are proportional to the resdshift of the sources. That would be a very interesting development.
     
  5. Jun 29, 2009 #4
    Re: Observational Loop QG--your opinion.

    Yes, it is a major requirement to disentangle possible effects at the source, which could indeed happen, given that our knowledge of GRBs is still incomplete.
     
  6. Jun 29, 2009 #5

    nrqed

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    Re: Observational Loop QG--your opinion.

    Wow, you have insulted the intelligence of a large fraction of the theoretical particle physics community. It's no surprise that there are no string theorists spending time on this board to help discuss other things than LQG.
     
  7. Jun 29, 2009 #6

    marcus

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    Re: Observational Loop QG--your opinion.

    Just a personal feeling with me too--as I said, it's how it seems and that means seems to me :biggrin:. But I suspect a substantial number of string and former string theorists would tend to agree with us! The reason I suspect that is a trend for string-theorizers to get out of versions requiring extra spatial dimensions and to get into various approaches to 4D quantum gravity including Horava's new one--as well as allied research (e.g. Steve Giddings in black holes) not involving extra-D.

    This is a fascinating trend that we are watching happen right now. Maybe someone would like to comment. Large numbers getting into 4D, one way or another, and also into approaches focused primarily on the geometry (like LQG focuses) and only secondarily on the matter. (In LQG matter when present is mostly in some generic form, like a supporting-role player.) It's a question of priorities, a strategy of first getting background independent geometry right and then adding matter later. A growing number of researchers seem to be feeling that this could be the right way forward.

    Earlier confidence in both susy and extra-D may have eroded somewhat. So two good reasons that more string folks could be intested in LQG these days are:
    1. no extra dimensions
    2. primary focus on 4D geometry, with matter on hold.

    And one sees the interest because many of them are getting into research that has exactly these features.
     
    Last edited: Jun 29, 2009
  8. Jun 29, 2009 #7
    Re: Observational Loop QG--your opinion.

    I scanned the recent Smolin paper that is the subject of this thread, but I was alarmed to see that the second equation and much of the discussion is straightfowardly about predicting violations of Lorentz symmetry. According to this article any violation of Lorentz symmetry could be used to construct a perpetual motion machine:

    http://arxiv.org/abs/hep-th/0702124

    I didn't see any parts of the Smolin paper that address this issue. Any comments?
     
  9. Jun 29, 2009 #8

    MTd2

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    Re: Observational Loop QG--your opinion.

    Yes, I have a comment. I believe that any consistent Lorentz violating theory must not predict black holes, but its mimickers. That is, everything is the same until you get to the horizon, when you reach there, quantum regime predominates and geometry breaks down.
     
  10. Jun 29, 2009 #9

    atyy

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    Re: Observational Loop QG--your opinion.

    I wonder how this plays out in dumb holes.
     
  11. Jun 30, 2009 #10

    MTd2

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    Re: Observational Loop QG--your opinion.

    On second thought, maybe we should take the "naive" term of "naive Lorentz symmetry break", not as a break of the Lorentz symmetry, but as a quantity that measure the average deviation from the classical value of special relativity due to interactions of the particle with the space time foam. Think that a macroscopic effect of the space time foam is producing a extremely thin haze whose average refractive index increased with the frequency.
     
  12. Jun 30, 2009 #11

    atyy

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  13. Jun 30, 2009 #12

    MTd2

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    Re: Observational Loop QG--your opinion.

    Yes, I know him!

    See this one, for exemple, he lists several types of mimickers:

    http://arxiv.org/abs/0902.0346
     
  14. Jun 30, 2009 #13

    marcus

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    Re: Observational Loop QG--your opinion.

    MTd2 and atyy, the best recent review of modified Lorentz symmetry like DSR and also of outright Lorentz symmetry breaking is the paper by Liberati:
    http://arXiv.org/abs/0906.0681

    Even though Ted Jacobson has co-authored ten or more papers with Liberati, that particular paper of Jacobson http://arxiv.org/abs/hep-th/0702124 did not get mentioned in the review. Perhaps it is relevant only to the case where there is a preferred frame, and is therefore not of so much current interest (not relevant to the Smolin-Amelino paper for example) I really can't guess why it was not cited or mentioned.

    In any case Liberati talks about DSR (and Planck suppressed dispersion generally) around page 8 of the review paper. It's a good place to get a critical expert phenomenologist look at it. Liberati points out some of the unresolved problems. He also cites DSR papers, which the 2007 Jacobson et al does not, since apparently it is talking about something different.

    In DSR the speed c is the same for all observers. No preferred frame. I think most likely you both know that but mention it in case other readers do not and might wish to know.

    Liberati's article was prepared for the the Annual Review of Nuclear and Particle Science. He would be the person that Annual Reviews would ask :biggrin:
     
    Last edited: Jun 30, 2009
  15. Jun 30, 2009 #14

    MTd2

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    Re: Observational Loop QG--your opinion.

    Indeed, the speed of light is constant, but the point is that light does not move at the speed of light, but it is delayed like it was passing through a refractive media. I guess this analogy was why the case of birefringence was considered

    At first sight I seemed Smolin was going to case IV.a, but in the end of his article, I guess he was going to something different, that is, a large gaussian distribution of the delay for a given frequency.
     
  16. Jun 30, 2009 #15

    turbo

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    Re: Observational Loop QG--your opinion.

    Is this widely accepted? Do we understand the evolution of GRBs well enough to assume that delays of tens of seconds in highly energetic photons is indicative of frequency-dependent dispersion, and not due to some mechanism at the source?

    (Edit) I ask because there is a very large (and growing) body of work modeling the light-curves of GRBs and there seem to be lots of open questions.
     
    Last edited: Jun 30, 2009
  17. Jun 30, 2009 #16

    marcus

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    Re: Observational Loop QG--your opinion.

    I'm glad to hear that! this business we are discussing here is only one part of understanding GRB, the processes that cause them as revealed, for instance, by their different types of light-curves.

    What we are discussing here is something that has not been reliably observed yet---a delay proportional to the travel time (for photons of a given energy).

    We have nothing, no empirical grounds to say anything, until we have observed many GRB at various distances and can plausibly extract a travel-time effect.

    I would say that the point of Liberati's review article (for Annual Reviews) which I would urge anyone to read before trying to discuss the Smolin-Amelino is essentially that now that the Fermi spacecraft is up and running we do expect to see lots of GRB, and we will be able to tell, before very long, whether there is a travel-time effect. Or not!
     
  18. Jun 30, 2009 #17

    turbo

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    Re: Observational Loop QG--your opinion.

    And there is the rub. We have to make some assumptions about simultaneity of emission if we are to cite frequency-dependent time-of-flight effects.

    Some interesting stuff going on, including time-referenced analysis of light-curves, here.
    http://arxiv.org/abs/0904.4786
    Hopefully, astrophysicists will make enough progress in understanding GRBs to make time-of-flight effects measurable, even if only to rule them out to some level of confidence. Perhaps by constraining observations to gamma-rays only and binning them by energy and arrival time? ...
     
  19. Jun 30, 2009 #18

    MTd2

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    Re: Observational Loop QG--your opinion.

    Hey people, have you ever thought that photons can decay to lower wavelenghts and gravitons?
     
  20. Jun 30, 2009 #19

    marcus

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    Re: Observational Loop QG--your opinion.

    A charming thought! But isn't what you are imagining rather like the scattering process that we already know about with Xrays? In the Compton experiment the x-ray comes in and hits an electron and gives him some of her energy and then she goes away with a slightly longer wavelength.

    What I mean is, the photon does not really decay in the usual sense, it just gets its wavelength lengthened and its frequency lowered by some interaction.
    Perhaps I'm not getting something, and need you to explain a bit more.
     
  21. Jun 30, 2009 #20

    MTd2

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    Re: Observational Loop QG--your opinion.

    Hmm, it would be like this:

    photon comes and interacts with an inertial random fluctuation of space time, the photon loses a bit of energy and this random fluctuation is scattered, that is, it becomes a graviton. The higher the energy of a photon, the higher is the cross section of this interaction.
     
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