Center for Gravitation and Cosmology, Yangzhou University (May 16 2022)
There were several good questions at the end, and a discussion of the following rather fundamental topic.
In the conventional description of quantum measurement a pure state evolves into a mixed state, with probabilities of distinct outcomes (non-unitary von Neumann projection).
Against Measurement (John Bell)
What Hawking suggested is that a black hole (i.e., gravity) causes pure states to evolve into mixed states. But if pure states already evolve into mixed states in ordinary quantum mechanics, why is it problematic for black hole physics (gravity) to have this effect?
Title: Quantum Hair and Black Hole Information
Abstract: I discuss recent results concerning the quantum state of the gravitational field of a compact matter source such as a black hole. These results demonstrate the existence of quantum hair, violating the classical No Hair Theorems. I then discuss how this quantum hair affects Hawking radiation, allowing unitary evaporation of black holes. Small corrections to leading order Hawking radiation amplitudes, with weak dependence on the external graviton state, are sufficient to produce a pure final radiation state. The radiation state violates the factorization assumption made in standard formulations of the information paradox. These conclusions are consequences of long wavelength properties of quantum gravity: no special assumptions are made concerning short distance (Planckian) physics.