Event horizon

An event horizon, in general relativity, is a boundary in spacetime, most often an area surrounding a black hole, beyond which events cannot escape the gravitational pull. Light emitted from beyond the horizon can never reach the observer, and any object that approaches the horizon from the observer's side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses. The traveling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time.