Radiation from a collapsing object is manifestly unitary

Understand

The process of gravitational collapse excites the fields propagating in the background geometry and gives rise to thermal radiation.

• We demonstrate by explicit calculations that the density matrix corresponding to such radiation actually describes a pure state.
• While Hawking's leading order density matrix contains only the diagonal terms, we calculate the off-diagonal correlation terms.
• These correlations start very small, but then grow in time.