On the proximity of black hole horizons: lessons from Vaidya

In dynamical spacetimes, apparent and event horizons do not coincide. In this paper we propose a geometrical measure of the distance between those horizons and investigate it for the case of the Vaidya spacetime. We show that it is well-defined, physically meaningful, and has the expected behaviour in the near-equilibrium limit. We consider its implications for our understanding of black hole physics.