Quantum hair and quantum gravity

A black hole may carry quantum numbers that are not associated with massless gauge fields, contrary to the spirit of the "no-hair" theorems. The "quantum hair" is invisible in the classical limit, but measurable via quantum interference experiments. Quantum hair alters the temperature of the radiation emitted by a black hole. It also induces non-zero expectation values for fields outside the event horizon; these expectation values are non-perturbative in ℏ, and decay exponentially far from the hole. The existence of quantum hair demonstrates that a black hole can have an intricate quantum-mechanical structure that is completely missed by standard semiclassical theory.