There are four known, exact, black hole solutions to the Einstein field equations, which describe gravity in general relativity. Two of those rotate: the Kerr and Kerr-Newman black holes. It is generally believed that every black hole decays rapidly to a stable black hole; and, by the no-hair theorem, that (except for quantum fluctuations) stable black holes can be completely described at any moment in time by these eleven numbers:
These numbers represent the conserved attributes of an object which can be determined from a distance by examining its electromagnetic and gravitational fields. All other variations in the black hole will either escape to infinity or be swallowed up by the black hole. This is because anything happening inside the black hole horizon cannot affect events outside of it.
In terms of these properties, the four types of black holes can be defined as follows:
|Non-rotating (J = 0)||Rotating (J > 0)|
|Uncharged (Q = 0)||Schwarzschild||Kerr|
|Charged (Q ? 0)||Reissner-Nordström||Kerr-Newman|
Note that most black holes have a large (non-zero) angular momentum and negligible charge as they get stable some time after any big event (creation, merger etc). So in practice, the Kerr solution is the simpler and more useful of all the solutions.
Rotating black holes are formed in the gravitational collapse of a massive spinning star or from the collapse or collision of a collection of compact objects, stars, or gas with a total non-zero angular momentum. As all known stars rotate and realistic collisions have non-zero angular momentum, it is expected that all black holes in nature are rotating black holes. Since observed astronomical objects do not possess an appreciable net electric charge, only the Kerr solution has astrophysical relevance.
In late 2006, astronomers reported estimates of the spin rates of black holes in The Astrophysical Journal. A black hole in the Milky Way, GRS 1915+105, may rotate 1,150 times per second, approaching the theoretical upper limit.
A rotating black hole can produce large amounts of energy at the expense of its rotational energy. This happens through the Penrose process in the black hole's ergosphere, an area just outside its event horizon. In that case a rotating black hole gradually reduces to a Schwarzschild black hole, the minimum configuration from which no further energy can be extracted, although the Kerr black hole's rotation velocity will never quite reach zero.
A rotating black hole is a solution of Einstein's field equation. There are two known exact solutions, the Kerr metric and the Kerr-Newman metric, which are believed to be representative of all rotating black hole solutions, in the exterior region.
Rotating black holes have two temperature states they can exist in: heating (losing energy) and cooling. In 1989, Paul Davies argued that the transition between the two states occurs when the square of the black hole's mass-to-angular-momentum ratio, in Planck units, equals the golden ratio. This claim was later found to be incorrect and in contradiction with Davies' earlier work.