  Hawking showed that a black hole formed by collapse will emit radiation and
eventually disappear. We address the challenge to define an objective notion of
physical entropy which increases throughout this process in a way consistent
with unitarity. We have suggested that (instead of coarse-grained entropy)
physical entropy is matter-gravity entanglement entropy and that this may offer
an explanation of entropy increase both for the black hole collapse and
evaporation system and also for other closed unitarily evolving systems. For
this to work, the matter-gravity entanglement entropy of the late-time state of
black hole evaporation would have to be larger than the entropy of the freshly
formed black hole. We argue that this may possibly be the case due to (usually
neglected) photon-graviton interactions.
