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Virtual particles and Hawking radiation
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Hawking's 1975 paper ''Particle creation by black holes'', available at 

   http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.cmp/1103899181

mentions virtual particles precisely once, in the middle of p. 202:

''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 
black hole 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''

But he says at the bottom of the page:

''It should be emphasized that these pictures of the mechanism 
responsible for the thermal emission and area decrease are heuristic
 only and should not be taken too literally.''


What happens is that the high energy of the gravitational field of the 
black hole creates real particle-antiparticle pairs -- that before 
that event it could be viewed in terms of Feynman diagrams as virtual 
is completely irrelevant to his argument. 

One of these real particles is swallowed by the black hole, the other 
is radiated away. As a result, the black hole loses radiation, hence 
total energy, and its effective mass decreases because of mass-energy 
equivalence.