Why are event horizons so crisp??
Well if you think of the classical description of an event horizon as being a place where the escape velocity is the speed of light, then it’s crisp because a boundary like v = c is a crisp thing, like a line x = 1 on a coordinate axis. You can get arbitrarily close to that line and have x < 1. You can get arbitrarily close to an event horizon and have v < c, and escape. But there's a single point where v = c. That's the classical description. But from what I understand about the theory of Hawking radiation (and my understanding is, er, fuzzy), it involves tunneling from INSIDE the event horizon. So in the quantum mechanical description the horizon is not so crisp after all.
