Where does the time asymmetry come from in Hawking Radiation?

Question

Taking General Relativity and Quantum Field Theory, Hawking predicts radiation emitted from a black hole.

Both GR and QFT are time CPT symmetric.

Taking just GR by itself, a black hole will stay the same forever. Taking QFT by itself a vacuum will stay the same forver (on average).

So why, when combining the two do we get something time asymmetric: small black holes evaporating?

Or equally a very large black hole will increase in size due to in coming radiation from the background.

I mean, in classical mechanics I can see how a clump of atoms will generally tend to disperse due to random processes. But a black hole is not a clump of atoms... at least not in terms of GR.

How does time asymmetry enter the equations? Do we have to take into account the history of the black hole and it's collapse from a star?

e.g. it is also described as an electron-positron pair created near the horizon with one falling into the black hole and one shooting off to infinity.

The time reverse of this would be an electron coming from infinity and annihilating a positron emitted near the horizon. So does this suggest different boundary conditions for past and future?

Perhaps on the microscopic scale it is time symmetric with black holes randomly being created and evaporating?

Equally, what is the time reverse of the Unruh effect?

Answer 1

The generic answer to this kind of thing is that the asymmetry comes from the choice of boundary conditions. Here, I imagine the boundary conditions are chosen to be that there are only outgoing photons, no incoming ones.

The justification for this choice of boundary conditions would be that we're really talking about an astrophysical black hole, which formed by gravitational collapse, not a Schwarzschild black hole, which is eternal. The logic is no different from the logic behind the fact that the sun is predicted to emit light but not absorb it.