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About hawking radiation

  1. Mar 7, 2010 #1
    1)when hawking first propounded this thermal radiation, he took into acount the the gravitational cllapse.
    so I am not sure, if we consider a stationary system, then there won't be any particle creation. Is this the case? because I think there will not be any mixing of posive and negative frequencies.

    2)From Unruh effect, we can also get the temperature.the freely-falling observer and the static observer on the event horizon will have different definition of vacuum, then the temperature can be deduced from the Bogolubov coefficients. But I cann't see why the event horizon is so significant in this case?
    what happen on the event horizon exactly, when we consider Hawking radiation?
    Last edited: Mar 8, 2010
  2. jcsd
  3. Mar 8, 2010 #2
    1.) Remember that the event horizon changes the outcome for particles, not their initial behaviour. Pair creation is going to occur (if the BH is 'hotter' than the Background), and the theory doesn't take place in a stationary system.

    2.) Do you mean static or stationary? A BH is neither. A Schwarzschild BH is non-rotating, not static or stationary.
  4. Mar 8, 2010 #3

    What I know about Hawking radiation is that there are inequivalent vacuums, as Unruh effect. so, would you please explain in this respect to more details? Thank you so much!
  5. Mar 8, 2010 #4
    while you are at it, please explain how a particle can have "negative energy".
  6. Mar 8, 2010 #5
    I'm not advocating either theory, just pointing out some basics. The stuff about the Unruh effect was edited in by the OP after I posted. EDIT: To clarify, the quote in my first post was the OP's Original Post if you care for a reference.
  7. Mar 9, 2010 #6
    The negative energy is to the infinite observer. In Penrose process, the negative energy is necessary to extract energy from a BH.
    This negative energy is non-observable locally, according to Hawking's original paper.
  8. Mar 9, 2010 #7
    I should add that negative energy of a single particle vs. negative energy densities in the region adjecent to the EH (part of the ergoregion) is not hard to imagine. I don't believe that QFT requires that individual particles have negative energy.

    I don't know if what I said was accurate, so if a mentor or such wants to clarify this, I'd be happy to learn.
  9. Mar 9, 2010 #8

    a particle has a probability to be outside of the blackhole it is part of. When this probability is fulfilled, the particle radiates.

    The end (?)
  10. Mar 9, 2010 #9
    That isn't HR, or at least, that ignores many other factors such as the nature of the emission and the worldline of the emitted photon.
  11. Mar 9, 2010 #10
    i do not understand what you mean by an "infinite observer". seems a particle would represent some positive value of energy regardless of the location or ref frame of any observer.
  12. Mar 11, 2010 #11

    Actually, this process is not quite clear to me , I may just as well refer to Hawking's paper
    "Particle Creation by Black Holes", Commun.math.Phys.43,

    "One might picture this negative energy flux in the following way. Just outside the event horizon there will be virtual pairs of particles, one with negative energy and one with positive energy. The negative particle is in a region which is classically forbidden but it can tunnel through the event horizon to the region inside the BH where the Killing vector which represents time translations is spacelike. In this region the particle can exist as a real particle with a timelike momentum vector even though its energy relative to infinity as measured by the time translation Killing vector is negative."
  13. Mar 11, 2010 #12


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    Tunneling from inside to outside the event horizon is equivalent to a particle tunneling outside its own future light cone (i.e tunneling FTL)...I think it's debatable whether this can happen, certainly tunneling can't carry any information FTL (see this thread along with https://www.physicsforums.com/blog.php?b=1113 [Broken] for some discussion of the issue).
    Last edited by a moderator: May 4, 2017
  14. Mar 11, 2010 #13
    Thanks for those links. Heat transfer doesn't need to tunnel though (?). Normal photon or particle emission due to thermal sources can be described by quantum effects - does this mean the thermal emission from a black hole is un-quantum (strictly classical)? Or is HR only allowed in a classical blackbody mechanics?
    Last edited by a moderator: May 4, 2017
  15. Mar 11, 2010 #14
    Here is how Wikipedia describes pair production:


    and also

    I assume participants here know that black holes are colder than our universe so Hawking radiation has NOT been observed....

    For one thing, the fee falling observer sees no event horizon...and passes thru that theoretical boundary without ever knowing it....and sees a perfect vacuum (no photons) and is surrounded by virtual photon pairs...in contrast the static observer near the horizon will be burned almost instantaneously by radiation which appears to her as thermal radiation...She sees a single thermal photon and has no way of knowing its one of a pair of virtual photons...

    Leonard Susskind THE BLACK HOLE WAR, page 173...
  16. Mar 11, 2010 #15
    HR is a purely quantum effect that only MIMICS thermodynamic radiation. Really, it's... quantum evaporation through single particles tunneling: i.e. single particles with worldlines extending both into and outside of the event horizon, but carrying no information. The "trick" is that the portion within the EH is formulated as the past worldline of the "escaping" particle as part of a virtual pair. The part of the pair with "negative energy" from the POV outside the EH/Ergosphere, is the antiparticle that never escapes. EDIT: Clarification: it's also the same particle formulated as the past history of the "escaped" particle. It's very odd.

    EDIT: @Naty1: Not to mention that for the HR to be detectable it would need to be a VERY low-mass bh and a COLD universe.
    Last edited: Mar 11, 2010
  17. Mar 11, 2010 #16
    I agree..I have never seen that as a description...

    My post above actually address this, but more can be understood by reading about the Unruh effect. Background DOES appear warmer to an accelerating observer!!!!

    (which does seem rather crazy at first...and I don't think this has ever been experimentally verified...)
  18. Mar 11, 2010 #17
    That would be one hell of an experiment! If you ever figure out how to do it... take pictures? :wink:
  19. Mar 11, 2010 #18


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    Yes, the positive energy particle escapes off to infnity, whereas the negative energy (relative to infinity) one is trapped on a timelike path , thus taking energy away from the gravitational field and consequently shrinking the hole (technically it is this absorption of negative energy, rather than emmission of quanta that shrinks the hole).

    The average wavelength of this emission process is of order the mass of the blackhole, thus its inherently nonlocal as it makes no sense to try to localize the quanta, since the curvature of the hole is roughly the same magnitude as the wavelength of the particle. Here, the concept of a 'particle' is strictly speaking global in nature.

    Heurestically we can speak of 'tunneling' but that is somewhat hazy. For one, this is not strictly speaking an instanton process, at least not in the traditional way. People have tried to make the identification, but there are analytic continuation problems that tend to crop up.
  20. Mar 11, 2010 #19
    ...And of course all of this leads to The Information Paradox. There is a lot of work yet to be done in this area, and with something as odd on its face as a BH, some odd theories have chances they wouldn't elsewhere.

    I maintain that the study of earthbound analogues such as sonic BHs is the key to understanding HR if it eixsts. Beyond that, it's going to be just a theory hovering out there, until an understanding of quantum gravity changes the field.
  21. Mar 11, 2010 #20
    i'm sorry, but i still have no idea what you mean when you say a particle has negative energy relative to infinity. when a spontaneous particle pair is created from the quantum foam, you have a particle and an antiparticle - not a positive energy particle and a negative energy particle.

    also, it seems remarkably counterintuitive that when a particle pair is created near the EH of a BH, when they surely must have the same momentum and trajectory, and are exceedingly close together, that somehow, within a ridiculously small amount of time, one gets sucked in, while the other miraculously manages to escape the enormous gravitational field extant near a BH, when it cannot [possibly have enough momentum to do so.

    i know all this must just be just so far beyond my poor little brain, but it just makes no rational sense to me whatsoever that hawking radiation can be a real thing.
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