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No quantum foam found.

  1. Aug 31, 2011 #1

    wolram

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    arXiv:1108.6005 (cross-list from gr-qc) [pdf, ps, other]
    No quantum gravity signature from the farthest quasars
    Fabrizio Tamburini (1), Carmine Cuofano (2), Massimo Della Valle (3,4), Roberto Gilmozzi (5) ((1) Dept. of Astronomy, University of Padova, Italy, (2) Dept. of Physics, University of Ferrara, Italy, (3) INAF - Osservatorio Astronomico di Capodimonte, Naples, Italy, (4) International Center for Relativistic Astrophysics Network, Pescara, Italy, (5) European Southern Observatory, Garching bei Muenchen, Germany)
    Comments: 5 pages 6 figures
    Journal-ref: Astronomy & Astrophysics, 533, A71 (2011)
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)

    Context: Strings and other alternative theories describing the quantum properties of space-time suggest that space-time could present a foamy structure and also that, in certain cases, quantum gravity (QG) may manifest at energies much below the Planck scale. One of the observable effects could be the degradation of the diffraction images of distant sources.
    Aims: We searched for this degradation effect, caused by QG fluctuations, in the light of the farthest quasars (QSOs) observed by the Hubble Space Telescope with the aim of setting new limits on the fluctuations of the space-time foam and QG models.
    Methods: We developed a software that estimates and compares the phase variation in the interference patterns of the high-redshift QSOs, taken from the snapshot survey of HST-SDSS, with those of stars that are expected to not be affected by QG effects. We used a two-parameter function to determine, for each test star and QSO, the maximum of the diffraction pattern and to calculate the Strehl ratio.
    Results: Our results go far beyond those already present in the literature. By adopting the most conservative approach where the correction terms, that describe the possibility for space-time fluctuations cumulating across long distances and partially compensate for the effects of the phase variations, are taken into account. We exclude the random walk model and most of the holographic models of the space-time foam. Without considering these correction terms, all the main QG scenarios are excluded. Finally, our results show the absence of any directional dependence of QG effects and the validity of the cosmological principle with an independent method; that is, viewed on a large scale, the properties of the Universe are the same for all observers, including the effects of space-time fluctuations.
     
  2. jcsd
  3. Aug 31, 2011 #2
    Wow, that's quite a blow.
    Thanks for the reference wolram.
     
  4. Aug 31, 2011 #3
    http://arxiv.org/abs/1108.6005

    It would rule out QG "scenarios", if those QG models were very specific at the "quantum foam" level.

    This seems to rule out the universe being an anarchic buzzy structure at microscopic levels, which is nice to know, but I doubt it rules out anything important.
     
  5. Aug 31, 2011 #4

    tom.stoer

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    It rules out approaches where the quantum foam in the mathematical definition of the theory manifests itself in experimental effects. But in many cases this is not be the case. For example string theory LQG do not predict anything like that.
     
  6. Aug 31, 2011 #5
    You mean it rules out only those approaches that actually predict something empirically verifiable, right?
    But not those that are unfalsifiable in principle or don't predict anything, obviously.
     
  7. Aug 31, 2011 #6

    marcus

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    A QG approach can predict effects which are empirically verifiable and yet NOT predict the particular effect which was looked for in this paper.


    For instance there is a LQG phenomenology literature which explores this possibility. I would advise sticking to recent papers. Here are some that have appeared since 2008:
    http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+INFLATION+OR+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

    AFAIK the kind of image degradation described in this paper (1108.6005 ) has never been derived as a prediction in the LQG context. The paper does not seem relevant to LQG.
    However it may be relevant to some other QG approach(es) with which I'm not familiar.

    It is possible that some people are confused by the word "foam". In LQG if you were to look at space with a powerful microscope you would not see a foam. The quantum states of geometry are described by labeled networks, but it is not imagined that this is what space is "made" of. It is more like what our measurements of geometry are made of :biggrin:.

    In LQG what is called a spinfoam is a method to calculate transition amplitudes for example from an initial state of spatial geometry to a final state. It is not supposed that a spinfoam exists in any naive substantive way as a model of spacetime. I guess you could say that what exists are the transition amplitudes between states of geometry.

    And these states are described by labeled networks, that is to say by GRAPHS, not by FOAMS. That is quite different. Space itself is not imagined to be discrete, but rather the geometric measurements made on it have discrete spectra.

    Using a recent (say post 2007) formulation of LQG, I don't recall any talk of image degradation such as they imagined and tested for here. Can't say about earlier---there was quite a variety of models. Plane waves were studied in LQG context in a recent paper. Gravitational waves in this case. You can check if you want. Here's the paper:

    ]http://arxiv.org/abs/1106.1448
    Towards Loop Quantization of Plane Gravitational Waves
    Franz Hinterleitner, Seth Major
    (Submitted on 7 Jun 2011)
    The polarized Gowdy model in terms of Ashtekar-Barbero variables is further reduced by including the Killing equations for plane-fronted parallel gravitational waves with parallel rays. The resulting constraint algebra, including one constraint derived from the Killing equations in addition to the standard ones of General Relativity, are shown to form a set of first-class constraints. Using earlier work by Banerjee and Date the constraints are expressed in terms of classical quantities that have an operator equivalent in Loop Quantum Gravity, making space-times with pp-waves accessible to loop quantization techniques.
    14 pages
     
    Last edited by a moderator: May 5, 2017
  8. Aug 31, 2011 #7

    tom.stoer

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    No.

    afaik string theory predicts conservation of Lorentz covariance (maybe it fails to predict other things, but here it definitly predicts something). Quantum mechanics predicts discrete angular momenta but (depending on the symmetry of a problem) nevertheless predicts conservation of rotational invariance. I wouldn't call QM unfalsifiable simply b/c its prediction regarding the rotational symmetry agrees with the prediction of its classical counterpart :-)

    The problem is on the experimental side: A theory predicts the absence of a certain effect. In order to falsify the theory (using this specific aspect) the experiment has to prove the presence of this effect. If the experiment is only able to show its absence (or suffers from insufficient measurement accuracy) then this does not mean that the theory has been verified, but only that the experiment failed to falsify the theory. And it does not mean that the theory has no predictive power.

    So in this specific case it is not the theory that doesn't say anything specific (it does) but it's the experiment that fails to constrain, disprove or falsify the theory.
     
  9. Aug 31, 2011 #8
    I don't think what you explain applies here, at least from reading the article it can be deduced that by the experiment they rule out "something" predicted by some QG models, not "the absence of something" as you claim.
    Do you agree with the conclusions? where it says "Our results indicate that most of the models of space-time foam consistent with the holographic principle and, consequently, with black hole entropy, can be ruled out for most of the values of the fluctuation amplitude parameter a0."
     
  10. Sep 1, 2011 #9

    tom.stoer

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    I do not claim that they prove the absence of something; they do. And I do not understand the difference between "ruling out something predicted by some QG models" and "proving the absence of something ...".
     
  11. Sep 1, 2011 #10
    Marcus wrote:
    "It is possible that some people are confused by the word "foam". In LQG if you were to look at space with a powerful microscope you would not see a foam. The quantum states of geometry are described by labeled networks, but it is not imagined that this is what space is "made" of. It is more like what our measurements of geometry are made of ."

    It is very important to notice it as above. The photon doesn't move in a space. The photon is an information which relates to another information and we observe it as a certain relation of the information in an device. According to Quantum Decoherence there isn't a distance nor space at all. There are relations between the information only.

    Due to Mathematical model of hologram The amplitude and phase of the light are represented by the absolute value and angle of the complex number.
    http://en.wikipedia.org/wiki/Holography

    The distance is encoded as a number of the relations between two Quantum States. Each relation encodes the Planck time dilation and the time is derived from that program. THE NUMBER OF THE RELATIONS DOESN'T DEPEND ON THE FREQUENCE OF THE PHOTON. The infrared photon and the gamma ray will have the same number of the relations with a background created by the wave functions, calculated mathematically as a Compton wave length of a proton ussually.
    http://www.hologram1.glt.pl/
     
    Last edited: Sep 1, 2011
  12. Sep 1, 2011 #11
    No, you didn't claim that, you claimed that they ruled out the absence of something.
    In the article afaik they claimed none of this, I quoted what they claimed and asked you if you agreed with that conclusion, do you or don't you?
     
  13. Sep 1, 2011 #12

    tom.stoer

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    Can you please again post what exactly I should agree or disagree with?
     
  14. Sep 1, 2011 #13
    Do you agree with the conclusions? where it says "Our results indicate that most of the models of space-time foam consistent with the holographic principle and, consequently, with black hole entropy, can be ruled out for most of the values of the fluctuation amplitude parameter a0."
     
  15. Sep 4, 2011 #14

    tom.stoer

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    I checked the paper again: I agree that they rule out models predicting what they are studying :-) But I disagree with "most of the models of space-time foam consistent with the holographic principle and, consequently, with black hole entropy".

    The two major research directions as of today are string theory and loop quantum gravity. Both indicate some "foamy", "stringy" or discrete structure of spacetime. Both are compatible with the holographic principle and related black hole entropy. But both theories as of today do (afaik) NOT predict what is subject of this paper.
     
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