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Does string theory/LQG predict Hawking radiation?

  1. Aug 11, 2010 #1
    Actual experimental aparatus, measurements and all that, see figure 2 on page 6
    Measurement of stimulated Hawking emission in an analogue system
    Silke Weinfurtner, Edmund W. Tedford, Matthew C. J. Penrice, William G. Unruh, Gregory A. Lawrence
    (Submitted on 11 Aug 2010)
    "There is a mathematical analogy between the propagation of fields in a general relativistic space-time and long (shallow water) surface waves on moving water. Hawking argued that black holes emit thermal radiation via a quantum spontaneous emission. Similar arguments predict the same effect near wave horizons in fluid flow. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include wave horizons. Long waves propagating upstream towards this region are blocked and converted into short (deep water) waves. This is the analogue of the stimulated emission by a white hole (the time inverse of a black hole), and our measurements of the amplitudes of the converted waves demonstrate the thermal nature of the conversion process for this system. Given the close relationship between stimulated and spontaneous emission, our findings attest to the generality of the Hawking process."

    It should be possible to use the above system to experimentally observe the characteristic emission spectrum and then modify it and apply it to Hawking radiation (done using QFT on curved spacetime) to see if Hawking's calculation is correct. If it is, then what about string/LQG?

    String theory offers calculations of entropy for extremel and near-extremel BH, LQG for all BH, up to a parameter.

    Does either or any QG offer detailed predictions for Hawking radiation, and do they agree with Hawking's calculation or disagree? As QG theories, do they improve upon Hawking's calculations?

    Has there been any astrophysical observation of micro-BH end-stage gamma ray bursts?

    If string theory is unable to reproduce Hawking radiation, then is its description of BH entropy complete?

    It would be interesting to compare BH Hawking radiation using analogue empirical observation (as described in paper) generalized to fiber optic and bose einstein condensates, to LQG prediction vs string theory.

    Wiki doesn't cite papers (perhaps Marcus could help) where LQG reproduces Hawking radiation, up to LQG corrections. No mention for string theory.
    If LQG does this, isn't this an independent confirmation of LQG BH-entropy robustness? Any criticism or skepticism of this claim (i.e channeling Lubos)

    wiki offers this claim


    Large extra dimensions

    Formulae from the previous section are only applicable if laws of gravity are approximately valid all the way down to the Planck scale. In particular, for black holes with masses below Planck mass (~10−5 g), they result in unphysical lifetimes below Planck time (~10−43 s). This is normally seen as an indication that Planck mass is the lower limit on the mass of a black hole.

    In the model with large extra dimensions, values of Planck constants can be radically different, and formulas for Hawking radiation have to be modified as well. In particular, the lifetime of a micro black hole (with radius below the scale of extra dimensions) is given by

    \tau \sim {1 \over M_*} \Bigl( {M_{BH} \over M_*} \Bigr) ^{(n+3)/(n+1)}

    where M * is the low energy scale (which could be as low as a few TeV), and n is the number of large extra dimensions. This formula is now consistent with black holes as light as a few TeV, with lifetimes on the order of "new Planck time" ~10−26 s.

    Deviation from Hawking radiation in loop quantum gravity

    A detailed study of the quantum geometry of a black hole horizon has been made using Loop quantum gravity. Loop-quantization reproduces the result for black hole entropy originally discovered by Bekenstein and Hawking. Further, it led to the computation of quantum gravity corrections to the entropy and radiation of black holes.

    Based on the fluctuations of the horizon area, a quantum black hole exhibits deviations from the Hawking spectrum that would be observable were x-rays from Hawking radiation of evaporating primordial black holes to be observed. The deviation is such that the Hawking radiation is expected to be centered at a set of discrete and unblended energies.
    Last edited: Aug 12, 2010
  2. jcsd
  3. Aug 12, 2010 #2


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    Both string theory and LQG are compatible with Hawking radiation. In addition, in principle they both may also be used to calculate small corrections to it.
  4. Aug 12, 2010 #3
    do you have a reference for string?
  5. Aug 12, 2010 #4


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