  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.
