A Rotating Black Hole

In 1963, Roy C. Kerr solved Einstein's equations for a black hole that is rotating. The structure of a rotating black hole is a bit different from that of a stationary black hole. There is an additional zone called the ergosphere (from the Greek word ergon, meaning "work"), from within which it is theoretically possible to extract energy and matter from the black hole.

Effectively, the event horizon of a rotating black hole is smaller than the event horizon of an otherwise identical but non-rotating one.

In a rotating black hole, the ergosphere is associated with the stationary limit, the location at which space-time is flowing at the speed of light, making stationary particles that would be travelling at the speed of light. Within the stationary limit, no particles can remain at rest, even though they are outside the event horizon.

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