View Full Version : Hawking radiation
2008-Dec-06, 03:56 PM
I have just started listening to the astronomy cast podcasts and they have discussed Hawking radiation a couple of times (I'm backlogged to episode 20). The discussion deals with events happening just inside the event horizon. My question is: what happens in the space just outside the horizon? Are pairs of particles being created outside the horizon just as they are inside the horizon? In the space outside of the event horizon could one of the particles be created on the inside the horizon and hence add to the BH and this would then balance some of the evaporation from the BH?
2008-Dec-06, 04:44 PM
I don't listen to the podcasts, but there seems to be some confusion here.
The pair production that accounts for Hawking radiation occurs just outside the event horizon: one member of the pair escapes, the other falls into the black hole. In another formulation of the same phenomenon, this can be viewed as a single particle successfully "tunnelling" from the inside the event horizon to the outside.
2008-Dec-06, 08:05 PM
As Grant said, the pair production which results in Hawking radiation
occurs just outside the event horizon. Pair production takes place
everywhere, but only at a black hole's event horizon can it result in
detectible radiation. The energy required to produce the radiation
comes from the black hole's gravity. When pair production happens
to result in one particle falling into the black hole and the other
escaping, the mass of the black hole is reduced by an amount equal
to the mass-energy of the escaping particle. The mass-energy of
both particles comes from the black hole's gravity, so the particle
which falls in doesn't add any more mass to the black hole. Half
the total mass-energy of the pair escapes as Hawking radiation.
-- Jeff, in Minneapolis
2008-Dec-06, 09:34 PM
Excellent, Jeff. I've noticed that the quality of explanations from certain posters is getting up towards publication standards!
2008-Dec-07, 03:58 PM
Hawking Radiation is disputed conjecture with fundamental theoretical problems and does not or may not exist according to several papers including .
 http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.cmp/1103899181, Particle Creation by Black Holes, S. W. Hawking (12 Apr 1975)
 arxiv.org/abs/gr-qc/0008016, Trans-Plankian Modes, Back-Reaction, and the Hawking Process, Prof. Dr. Adam D. Helfer (2000)
2008-Dec-07, 04:28 PM
I must forewarn you that it is not a safe practice to try and imagine the physics of an event horizon.
For example, if you could somehow hold your position at the event horizon, you would be cooked by radiation. Not Hawking radiation, but Unruh radiation. Hawking radiation is truly the radiation that results in evaporation. Unruh radiation is just a very good oven.
If you could somehow fall into a back hole(not too hard), you would not see Unruh radiation, but Hawking radiation would still exist.
The radiation of a BH is a very ugly state of affairs, indeed.
Just to confirm what others have said, the black hole mass equivalent to the energy of both particles created just outside the event horizon. The one that falls in gives its mass back. The particle that escapes takes away that amount of mass from the BH.
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