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The end of String Theory and Loop Quantum Gravity ?

  1. Aug 26, 2011 #1
    If spacetime is composed of tiny quantum "grains," the gamma-ray photons' polarization should change from random polarization (at the GRB source) to biased toward a certain polarization when received by the Integral spacecraft.
    The Integral polarization results depend on spacetime being constructed from discrete quanta that behave in a way that fits with quantum theory. The holographic universe hypothesis goes one step further, constructing 3-dimensional spacetime from projections of a 2-dimensional "shell" -- perhaps gamma-ray photons behave differently in this fuzzy, projected, quantum world, and this could be why no polarization difference between gamma-ray photons are detected.
    "This is a very important result in fundamental physics and will rule out some string theories and quantum loop gravity theories," said Laurent in the ESA press release.
    http://news.discovery.com/space/we-might-not-live-in-a-hologram-after-all-110701.html
     
  2. jcsd
  3. Aug 26, 2011 #2

    tom.stoer

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    I don't know what the current string theory an covariant loop quantum gravity approaches currently say regarding polarization - and whether we already a have conclusive results from these theories.
     
  4. Aug 26, 2011 #3
    This ESA result was already discussed in this thread
    https://www.physicsforums.com/showthread.php?t=511288"

    and I can also add that what Philippe Laurent actually said was badly reported by the journalist.

     
    Last edited by a moderator: May 5, 2017
  5. Aug 26, 2011 #4
    see also:

     
  6. Aug 26, 2011 #5

    atyy

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    LQG may be compatible with Lorentz-invariance - but is it compatible with the ESA result?

    In particular, the classical limit so far seems to be discuss Regge gravity, which is grainy.
     
  7. Aug 26, 2011 #6
    Hi Atyy!
    One should be careful not to confuse two kinds of discreteness.
    The discreteness introduced by the fact that the theory is approximated on a graph, as in Regge calculus, is different from the fundamental quantum discreteness of the theory, that comes from the discrete spectrum of area and volume. The first is like saying that you see a certain numbers of (discrete) modes for a field in a box, the second is related to the actual quantization of the field.
    A discussion about this and further references can be found in the http://arxiv.org/abs/1102.3660" [Broken].
    Now, look at the figure at page 21: to recover full GR, one has to remove both discreteness, that correspond to take two distinct limits: respectively these are the "continuous limit" and the "classical limit".
     
    Last edited by a moderator: May 5, 2017
  8. Aug 26, 2011 #7

    atyy

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    Yes, that's exactly the figure I had in mind. So far the continuous limit hasn't been shown, has it?
     
    Last edited by a moderator: May 5, 2017
  9. Aug 27, 2011 #8

    tom.stoer

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    I think there are different experiments, namely regarding dispersion relations and regarding polarizations, respectively. That's different. Does LQG say anything regarding polarizations?
     
  10. Aug 27, 2011 #9
    I didn't know it was discussed before. Thank you for comments.
    The journalist wrote about a space as something material made of particles and antiparticles. There is something as the polarisation of the space but it doesnt mean there is a material stoff.
    Due to Quantum Decoherence there aren't the particles, distance, space nor time on a quantum fundamental level.
    http://www.decoherence.de/
    There is a superposition of the quantum information which are creating the Wave Function. The Wave Function is not material it is a mathematical description. Therefore the space also might be something mathematical (relation between the information on a 2D screen) only. The Integral satellite project will not detect the particles of the space then if it is not real.
    Do I understand it properly ?
     
  11. Aug 27, 2011 #10
    I would not say that the continuos limit has not been proven. We know that the continuos limit of Regge calculus is GR. Therefore in LQG you can take first the classical limit, and you get Regge, and then you take the continuous limit, and you get GR. Every thing seems fine to me.

    The discretization of Regge calculus is an approximation. Therefore you may ask which is the regime in which this approximation is valid. A good physicist would choose the appropriate discretization depending on the system that s/he wants to describe.
     
  12. Aug 27, 2011 #11
    I'm not aware of any discussion about the polarization of light in LQG. The only polarization that I know it is interesting for LQG is the polarization in the CMB, because this could let us detect possible effects coming from the big bounce.
     
  13. Aug 27, 2011 #12

    atyy

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    But does the Regge-GR limit exist within LQG?

    First the truncated to full QG limit must exist, then the classical and continuous limits must commute (for the scenario you bring up - but I think it's also fine if they don't commute, as long as one can take continuous then classical starting from truncated?)
     
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