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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:

Hope this helps,
Georgia & Koji
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