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The Question
(Submitted November 25, 2001)
I have a little bit of a problem understanding Hawking radiation and
black hole evaporation. So far I have heard two different explanations
for hawking radiation. One being the seperation of particle/antiparticle
pairs due to the intense gravitational field of a black hole. The other
being the escape of some particles because of quantum uncertainties in the
velocity of subatomic particles which can allow a particle to briefly travel
faster than light.
Also, my current understanding of black hole evaporation is that due to
Hawking radiation black holes actually emit energy and will evetually
evaporate away due to this loss of energy. This doesn't make sense in the
context of the partricle/antiparticle type of Hawking radiation. It seems
the black hole should actually increase in size because it is actually
gaining matter/energy through this process. The particle/antiparticle
pair is not actually part of the black hole but borrowed energy. Can you
please make sense of all this for me?
The Answer
Thanks for your question. You are correct that there are multiple
ways to visualize the generation of Hawking radiation. The first is
indeed the separation of virtual matter/antimatter pairs by the intense
gravitational force exerted by the black hole and the other is the
quantum tunneling of a particle, such as a photon out of the black hole
event horizon.
Both cases describe black hole evaporation. The best way to think of
why indeed the black hole is losing energy in the matter/antimatter
scenario is that particles are actually created at the expense of
gravitational energy (which is then related to the properties of
space-time). Consider the following example. Say there's a particle
moving toward a large mass --the particle feels the gravitational pull of
the mass. The gravitational field exerts a potential energy on the
particle which in turn is converted to kinetic energy of the moving
particle. To conserve total energy the potential energy of the
mass gets more negative. Indeed, the mass experiences a loss of potential
energy as the particle experiences a gain in kinetic energy. So in the
same way, at the black hole event horizon, two virtual particles --say
photons --separate due to the gravitational force of the black hole; one
adds negative energy as it falls into the black hole, the other (now a
real particle) escapes.
You might want to check out:
http://library.thinkquest.org/C007571/english/advance/english.htm
Hope this helps,
Georgia & Koji
For "Ask an Astrophysicist"
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