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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.

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