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Experimental Limits on Spin-Foam Effects From Quasars

  1. Jun 3, 2015 #1


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    Both string theorists and loop quantum gravity theories have proposed that space-time may be something other than the perfectly smooth, perfectly local space of classical general relativity, which can potentially lead to path dependent phase alteration of light from a common source due to the cumulative effect of a discrete or non-uniform space-time over the course of the journey that affects photons taking different paths differently.

    These effects should be easiest to see in high frequency photons that have traveled long distances, such as light from distant quasars.

    A new experimental search but boundaries on a parameter alpha (with potential values from 0 to 1) which is a function of how much space-time differs from classical general relativity which has a value of 1. The lower bound for alpha from experiment is now about 0.76 for the highest frequency gamma rays, and 0.46 in measurements from infrared light.

    Two proposals that have an alpha that is independent of wavelength, a "random walk" hypothesis that would imply alpha = 0.5, and a "holographic" model which would imply alpha = 2/3 are ruled out by the results, but the parameter space still leaves plenty of room for space-time with a character different from quantum gravity so long as the alpha predicted is in the range from 0.77-.99 or so.

    A discussion of the experiment is found at: http://www.science20.com/quantum_gr...antum_spacetime_foam_from_quasar_light-155928
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  3. Jun 3, 2015 #2


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    Author of the Science 2.0 article does not seem to understand what he is talking about. Bases his article on the Perlman et al article which has no connection with LQG Spin Foam.
    The article talks about Wheeler's idea of "space-time foam", does not mention LQG or cite LQG sources, which is intelligent because space-time foam is entirely different from the LQG spin foam method of calculating transition amplitudes. LQG does not postulate that spacetime is "made" of little triangles and simplexes, or that light has to thread its way thru some kind of lattice. So the whole analysis does not apply.

    People periodically make this mistake. The Science 2.0 author mistakenly mentions LQG spin foam because he or she does not understand. But Perlman et al do not mention LQG or Spin foam QG or cite relevant papers or connect in any way. People (many, not just the 2.0 author) can understandably get confused by the occurrence of the word "foam" in two separate contexts. But Perlman et al, as it happens, don't seem to have made that mistake.

    BTW a good article about Lorentz covariance in LQG and Spin Foam would be by Rovelli and Speziale searching by the two names should get it.
    Lorentz covariance of loop quantum gravity
    Carlo Rovelli, Simone Speziale
    (Submitted on 8 Dec 2010)
    The kinematics of loop gravity can be given a manifestly Lorentz-covariant formulation: the conventional SU(2)-spin-network Hilbert space can be mapped to a space K of SL(2,C) functions, where Lorentz covariance is manifest. K can be described in terms of a certain subset of the "projected" spin networks studied by Livine, Alexandrov and Dupuis. It is formed by SL(2,C) functions completely determined by their restriction on SU(2). These are square-integrable in the SU(2) scalar product, but not in the SL(2,C) one. Thus, SU(2)-spin-network states can be represented by Lorentz-covariant SL(2,C) functions, as two-component photons can be described in the Lorentz-covariant Gupta-Bleuler formalism. As shown by Wolfgang Wieland in a related paper, this manifestly Lorentz-covariant formulation can also be directly obtained from canonical quantization. We show that the spinfoam dynamics of loop quantum gravity is locally SL(2,C)-invariant in the bulk, and yields states that are preciseley in K on the boundary. This clarifies how the SL(2,C) spinfoam formalism yields an SU(2) theory on the boundary. These structures define a tidy Lorentz-covariant formalism for loop gravity.
    6 pages, 1 figure. Published Physical Review D, (2011) 43 citations
    Last edited: Jun 3, 2015
  4. Jun 3, 2015 #3


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    AFAICS the Science 2.0 article is based entirely on (partial misunderstanding of) this Perlman et al article concerned with spacetime foam effects.
    I am not sure which QG models this would apply to. But there are a number of references to string theory papers. Maybe if one is interested one could look at what Perlman et al cite/refer to and get an idea what they are actually talking about. Here's the paper:
    New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations
    Eric S. Perlman (FIT), Saul A. Rappaport (MIT), Wayne A. Christensen (North Carolina), Y. Jack Ng(North Carolina), John DeVore (Visidyne), David Pooley (Sam Houston St.)
    (Submitted on 26 Nov 2014 (v1), last revised 13 Mar 2015 (this version, v5))
    One aspect of the quantum nature of spacetime is its "foaminess" at very small scales. Many models for spacetime foam are defined by the accumulation power α, which parameterizes the rate at which Planck-scale spatial uncertainties (and thephase shifts they produce) may accumulate over large path-lengths. Here α is defined by theexpression for the path-length fluctuations, δℓ, of a source at distance ℓ, wherein δℓ≃ℓ1−αℓαP, with ℓP being the Planck length. We reassess previous proposals to use astronomical observations ofdistant quasars and AGN to test models of spacetime foam. We show explicitly how wavefront distortions on small scales cause the image intensity to decay to the point where distant objects become undetectable when the path-length fluctuations become comparable to the wavelength of the radiation. We use X-ray observations from {\em Chandra} to set the constraint α≳0.58, which rules out the random walk model (with α=1/2). Much firmer constraints canbe set utilizing detections of quasars at GeV energies with Fermi, and at TeV energies with ground-based Cherenkov telescopes: α≳0.67and α≳0.72, respectively. These limits on α seem to rule out α=2/3, the model of some physical interest.
    11 pages, 9 figures, ApJ, in press
    Last edited: Jun 5, 2015
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