Gravity seminar - Bernard Kay Seminar

Abstract: Since Hawking's 1974 discovery, we expect a black hole formed by collapse will emit radiation and eventually disappear. One aspect of the information loss puzzle is the challenge to define an objective notion of physical entropy which, in accordance with the Second Law, increases for this, and other, closed unitarily evolving systems. We question whether coarse-grained entropy can meet this challenge but argue that matter-gravity entanglement entropy can. We argue that due to (usually neglected) photon-graviton interactions, if the evaporation is slowed down by putting the black hole in a slightly permeable box, the radiation remaining after a large black hole has evaporated will (be pure and) mainly consist of roughly equal numbers of photons and gravitons entangled with one another -- with a (matter-gravity entanglement) entropy greater than that of the freshly formed black hole. We also give arguments that, in the absence of such a box, the final state would be similar. If it is so, this would seem to improve the prospects for the resolution of the firewall puzzle since late emitted photons/gravitons would not be needed to purify early emitted photons/gravitons; instead they would purify one other.