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I What is the mechanism behind Hawking radiation?

  1. Nov 10, 2016 #1

    Grinkle

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    I thought Hawking radiation was a virtual particle pair emerging from nothing, one particle falling into the event horizon and the other particle tunneling out of the event horizon.

    Then I read this -

    https://www.physicsforums.com/insights/vacuum-fluctuation-myth/

    and now I think a virtual particle pair is always nothing, a pair never emerges from nothing in any observable manner.

    I am left wondering where the particles come from that constitute Hawking radiation.
     
  2. jcsd
  3. Nov 10, 2016 #2

    vanhees71

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    A black hole is far from being nothing. That's why there's also nothing just popping out of nothing (aka. the vacuum).
     
  4. Nov 10, 2016 #3

    phinds

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    As Hawking himself has said, the "virtual particle" description is NOT actually what's happening. It was just the best he could do to translate into English something that really can only be explained with math.
     
  5. Nov 10, 2016 #4

    A. Neumaier

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    At the end of the first paragraph of my article I gave a link that answers this.
     
  6. Nov 10, 2016 #5

    Grinkle

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    Thanks for the reminder. From the post -

    "The dry facts are that two real particles (e.g., two photons, or an electron and a positron) are created from the energy in the very strong gravitational field near the horizon of the black hole - from two gravitons or from an external gravitational field, not from the vacuum."

    Very digestible!
     
  7. Nov 11, 2016 #6
    The dry facts? Hardly so since gravitons are mentioned. They are still hypothetical particles since they haven't been experimentally proven to exist.
     
  8. Nov 11, 2016 #7

    A. Neumaier

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    That's why I gave two (in quantum gravity possibly equivalent) alternatives, and had added a postscript explaining more details. Quantum field theory in an external classical gravitational field (such as the one created by a classical black hole) has none of the problems of full quantum gravity, and is all needed! I now also edited the original sentence to make this even more clear!
     
  9. Nov 11, 2016 #8
    Yes but you need to have in mind:

    "The dry facts are that two real particles (e.g., two photons, or an electron and a positron) are created from the energy in the very strong gravitational field near the horizon of the black hole - from two gravitons or from an external gravitational field ..."

    This is an educated guess.

    "...not from the vacuum."

    This is a factual statement.

    where italic is a factual statement and bold is for educated guess, just to clear it up.

    Thank you for clearing up what your statements actually are, in great context.
     
  10. Nov 11, 2016 #9

    A. Neumaier

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    ''from a strong external field'' is not a guess. It is a prediction of traditional quantum field theory, and is experimentally verified in the case of strong external electromagnetic field. Strong external fields with energies significantly above the pair creation energy threshold necessarily create the corresponding particle pairs.
     
  11. Nov 11, 2016 #10

    vanhees71

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    That's very interesting! In which experiment is the Schwinger pair-production mechanism verified? I'd say that could lead to a Nobel prize!
     
  12. Nov 11, 2016 #11

    A. Neumaier

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    I though it was verified; need to check. In any case I had read some time ago quite a number of papers on this, though I must admit that I had only looked at the theoretical side of the problem.
     
    Last edited: Nov 11, 2016
  13. Nov 11, 2016 #12

    vanhees71

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    This would be a sensation! As far as I know, so far there's no experimental proof, although there are many groups trying to prove it using very strong laser fields. Of course, the Schwinger effect is an inevitable consequence of standard QFT. If it can be proven, it's another hint that QED is the right game (also in the strong-field case, which extends the usual empirical tests to this realm), if it's unanimously disproven it's the first hint that QED (and then perhaps any local QFT) is not the full truth!
     
  14. Nov 11, 2016 #13
    You forgot the "or...". The entire sentence is an educated guess since you wrote "two gravitons or...".
     
  15. Nov 11, 2016 #14

    Demystifier

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    Why would that be a sensation? A collision of the laser beams can also be thought of as a collision of photons, and I see nothing sensational about creation of electron-positron pairs from collision of sufficiently energetic photons.
     
  16. Nov 11, 2016 #15

    Haelfix

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    Note that the collision of photons that then output an electron positron pair is a decidedly perturbative problem in QED, whereas the Schwinger effect requires a strong field source and is a decidedly nonperturbative effect (it requires summing a whole class of Feynman diagrams). The result might look the same, but the fundamental mechanism is different.

    There would even be an ambiguity with lasers, as they really simulate a timevarying inhomogenous field at the focal point, and so one might complain about the results. In any event as far as I know, no such experiment has the necessary laser intensity to even probe such a thing, so it remains a moot point.
     
  17. Nov 11, 2016 #16

    mfb

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    Pair production from photon collisions is not new, but here the photons need a high energy (1 MeV invariant mass for a pair). Schwinger pair production would work at lower energies. The lasers are not there yet (the time variation should not be an issue if the laser frequency is reasonable), maybe in a few years.

    Photon-photon scattering is another effect waiting for experimental confirmation. The LHC ion collisions might make it possible.

    Edit: Oh, one day ago!
    I'll split the discussion if it continues.
     
    Last edited: Nov 12, 2016
  18. Nov 11, 2016 #17

    Vanadium 50

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  19. Dec 4, 2016 #18

    A. Neumaier

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  20. Dec 4, 2016 #19

    mfb

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    This has nothing to do with the Schwinger pair production - which itself is off-topic here.
     
  21. Dec 4, 2016 #20

    strangerep

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    ??? Afaik, pair production in superstrong EM fields has been experimentally known for decades in heavy ion collision experiments, where immense EM fields are created temporarily as the ions meet.

    One of the earliest papers is this one by the late Walter Greiner and colleagues. Greiner subsequently did a lot of research on heavy ion collision physics, and also authored several related books, e.g., this rather old book. IIRC, the subject is also mentioned in some of his later books. (Try googling for: greiner strong field heavy ion )

    Note that one needs to distinguish between:

    - Pair production from immense EM fields created in heavy ion collisions;

    - Delbruck scattering of electrons off the EM field of a nucleus;

    - Light-by-light scattering, which is related to Delbruck scattering, but more difficult to achieve experimentally, iiuc.

    Possibly, although it's not clear (at least to me) whether Hawking radiation (and its cousins) are primarily related to the strength of the curvature, or that many radiation-producing horizons seem to be Killing horizons. I.e., which is the more fundamental feature enabling the radiation effect? Strong curvature or Killing horizon? For Unruh radiation it's the latter, right?

    In both cases (Hawking/Unruh radiation, and Schwinger pair production), each effect arises (theoretically) via unitarily inequivalent representations (i.e., inequivalent vacua) of the basic quantum fields, iiuc.
     
    Last edited: Dec 4, 2016
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