Black holes and positive/negative-energy particles

I was reading Brian Greene's "Hidden Reality" and came to the part about Hawking Radiation. Quantum jitters that occur near the event horizon of a black hole, which create both positive-energy particles and negative-energy particles, produce the so-called Hawking radiation. However, I do not understand why only the negative-energy particles are being absorbed into the black hole, while the positive-energy particles shoot outward. Shouldn't there be 50/50 chance that each type of particle is being absorbed by the black hole? Also, the book mentions that a negative-energy particle would appear to an observer inside the black hole as positive. Why?

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There are two ways to approach your question. The first is to explain what Brian Greene means, and the second is to point out that the "particles being swallowed" explanation is a metaphor and isn't actually how the calculation is done. I'll attempt both, but I'm outside my comfort zone so if others can expand or correct what follows please jump in!

When a pair of virtual particles are produced there isn't a negative energy particle and a positive energy particle. Instead the pair form an entangled system where it's impossible to distinguish between them. This entangled system can interact with the black hole and split, and the interaction guarantees that the emerging particle will be the positive one. NB "positive" and "negative" doesn't mean "particle" and "anti-particle" (for what it does mean see below), and the black hole will radiate equal numbers of particles and anti-particles.

Now onto the second bit, and I approach this with trepidation. When you quantise a field you get positive frequency and negative frequency parts. You can sort of think of these as representing particles and anti-particles. How the positive and negative frequencies are defined depends on your choice of vacuum, and in quantum field theory the vacuum is unambiguously defined. The problem is that in a curved spacetime, like the region near a black hole, the vacuum changes. That means observers far from the black hole see the vacuum as different from observers near the black hole, and the two observers see different numbers of particles (and antiparticles). A vaccum near the event horizon looks like excess particles to observers far away, and this is the source of the radiation.

Exactly the same maths gives the Unruh effect, i.e. the production of particles in an accelerated frame. The fact that the Unruh effect also produces particles shows that a black hole is not necessary for the radiation, so it can't simply be virtual particles being swallowed.

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The second part was utterly difficult to understand – James Kujareevanich Jun 22 '12 at 9:37
You're not alone in thinking that. The problem is you really need to understand quantum field theory to get more than a very approximate feel for what's going on. I guess that's why the "anti-particle falling into the black hole" analogy is so widely used. – John Rennie Jun 22 '12 at 9:43
This is a good answer, except for one thing: In curved spacetime, there isn't an unambiguous choice of vacuum. There's an unambiguous choice only when you have a Hamiltonian (in which case the vacuum is the lowest energy eigenstate), and this only happens when the time translation is an isomorphism of the spacetime. Without this symmetry, different observers may disagree about which state is really the vacuum, depending on which time axis they've chosen. – user1504 Jun 22 '12 at 14:08
Since particles are excitations from the vacuum, these observers will then disagree about the particle content of a system. So, for example, an observer falling freely into a black hole will see empty space, while an observer off at infinity will see space filled with blackbody radiation. – user1504 Jun 22 '12 at 14:10
Re Danu's comment: the draft appears to be titled Introduction to Quantum Fields in Classical Backgrounds, but a quick comparison with the Google Books copy of Quantum effects in gravity suggests they are the same book. Thanks Danu, another addition to the large pile of books to be read one day. – John Rennie Nov 28 '14 at 8:47

Also, the book mentions that a negative-energy particle would appear to an observer inside the black hole as positive. Why?

Very roughly speaking and in as simple terms as possible, inside the black hole, gravity is so intense that the time coordinate and one of the spatial coordinates (the radial coordinate) swap "roles". That's one way to see why you can't get "back up" and exit the hole. "Back up" is now in the reverse time direction and you can't go back in time.

Anyhow, since energy is associated with the time coordinate and momentum with the spatial coordinates, the energy and radial momentum of a particle also swap "roles" when crossing the horizon. The negative energy of the particle becomes negative momentum and the positive momentum of the particle becomes positive energy.

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