apparent horizon

The apparent horizon is the surface surrounding a black hole on which outgoing light rays are just trapped, and cannot expand outward. The apparent horizon thus satisfies a stronger condition than that of the event horizon, and the apparent horizon always lies inside the event horizon, or coincides with it.


To appreciate the difference between the apparent horizon and the event horizon, imagine that a man is running through a corridor filled with doors. He is trying to run outward, but the doors are closing in sequence from the outside in. How many doors will he be able to pass through before he is blocked by a closed door? The door that is closest to him that is currently closed is analogous to the apparent horizon. The door that he will actually reach before he cannot travel further is analogous to the event horizon.