Physical observability of horizons

TL;DR

This paper argues that apparent horizons are physically observable and more relevant for astrophysical black holes than event horizons, which are generally unobservable except in idealized stationary cases.

Contribution

It clarifies the physical observability of different types of horizons and challenges the use of event horizons for defining evolving black holes in astrophysics.

Findings

Apparent horizons are generically physically observable.

Event horizons are not observable except in idealized stationary cases.

In dynamic scenarios, apparent horizons are the relevant physical boundaries.

Abstract

Event horizons are (generically) not physically observable. In contrast, apparent horizons (and the closely related trapping horizons) are generically physically observable --- in the sense that they can be detected by observers working in finite-size regions of spacetime. Consequently event horizons are inappropriate tools for defining astrophysical black holes, or indeed for defining any notion of evolving}black hole, (evolving either due to accretion or Hawking radiation). The only situation in which an event horizon becomes physically observable is for the very highly idealized stationary or static black holes, when the event horizon is a Killing horizon which is degenerate with the apparent and trapping horizons; and then it is the physical observability of the apparent/trapping horizons that is fundamental --- the event horizon merely comes along for the ride.