Restoring predictability in semiclassical gravitational collapse

Abstract

Hawking showed that the radiation emitted from a classical collapsing shell is thermal. Here, we show that a semiclassical collapsing shell emits radiation that is only approximately thermal, with small but significant deviations. The most important difference is the presence of small off-diagonal elements in the radiation density matrix with a magnitude of order $1/\sqrt{S_{BH}}$, $S_{BH}$ being the Bekenstein-Hawking entropy of the incipient black hole. The off-diagonal elements store the correlations between the collapsing shell and the emitted radiation and allow information to continuously leak from the collapsed body. The rate of escape of information is initially very small, increasing to order one by the Page time, when the black hole has lost half its original entropy. We show that, until the Page time, the radiation is almost exactly thermal and that, from this time on, it begins to purify at an increasing rate.