The Kerr

metric is the

solution to

Einstein's equations for a non-

spherically symmetric vacuum, that is a

rotating black hole. As well as an

event horizon, it also possesses an

ergo surface, which is a

surface within which it is not possible to remain at a

constant angle; in other words, within the

ergo surface, one must

rotate about the

origin. Effectively, the

rotating black hole is dragging

spacetime along with it, forcing

particles falling into it to

rotate.

The Kerr metric is exceptionally important, as it can be shown that **all** black holes eventually become Kerr.