Event Horizon

The event horizon is the boundary around a black hole beyond which nothing can escape, not even light. It is not a physical surface you could touch, but a one-way line in space: cross it, and every possible path leads inward, toward the centre. From outside, the event horizon is the closest thing a black hole has to an edge.

The name captures the idea well. An "event" in physics is anything that happens at a particular place and time. The horizon is the limit beyond which events can no longer be seen by the outside universe, just as the horizon on Earth marks the limit of what you can see across the surface. Anything that happens inside is permanently hidden from us.

Why it mattered to Hawking

The event horizon was central to Hawking's most important work. His area theorem showed that the total area of an event horizon can never decrease, a result that hinted at a deep link between black holes and entropy. And his most famous discovery, Hawking radiation, comes from applying quantum theory to the empty space just outside the horizon, revealing that black holes are not perfectly black after all. The horizon is also where the information paradox plays out: the puzzle of what happens to information that crosses it.

For the fuller picture, see black holes.

See how an event horizon's size depends on mass with the black hole calculator.