Arotating black hole is ablack hole that possessesangular momentum. In particular, it rotates about one of its axes of symmetry.
All currently known celestial objects, includingplanets,stars (Sun),galaxies, and black holes, spin about one of their axes.[1][2][3]
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There are four known, exact, black hole solutions to theEinstein field equations, which describe gravity ingeneral 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 theno-hair theorem, that (except for quantum fluctuations) stable black holes can be completely described at any moment in time by these 11 numbers:
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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 astrophysical black holes are expected to have non-zero angular momentum, due to their formation via collapse of rotating stellar objects, but effectively zero charge, since any net charge will quickly attract the opposite charge and neutralize. For this reason the term "astrophysical" black hole is usually reserved for the Kerr black hole.[5]
Rotating black holes are formed in thegravitational collapse of a massive spinningstar 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 starsrotate and realistic collisions have non-zero angular momentum, it is expected that all black holes in nature are rotating black holes.[1][2] 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 inThe Astrophysical Journal. A black hole in the Milky Way,GRS 1915+105, may rotate 1,150 times per second,[6] approaching the theoretical upper limit.
The formation of a rotating black hole by acollapsar is thought to be observed as the emission ofgamma ray bursts.
A rotating black hole can produce large amounts of energy at the expense of its rotational energy.[7][8] This can happen through thePenrose process inside the black hole'sergosphere, in the volume outside its event horizon.[9] In some cases of energy extraction, a rotating black hole may gradually reduce 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.[10]
A rotating black hole is a solution ofEinstein's field equation. There are two known exact solutions, theKerr metric and theKerr–Newman metric, which are believed to be representative of all rotating black hole solutions, in the exterior region.
In the vicinity of a black hole, space curves so much that light rays are deflected, and very nearby light can be deflected so much that ittravels several times around the black hole. Hence, when we observe a distant background galaxy (or some other celestial body), we may be lucky to see the same image of the galaxy multiple times, albeit more and more distorted.[11] A complete mathematical description for how light bends around the equatorial plane of a Kerr black hole was published in 2021.[12]
In 2022, it was mathematically demonstrated that the equilibrium found byRoy Kerr in 1963 wasstable and thus black holes—which were the solution to Einstein's equation of 1915—were stable.[13]
Rotating black holes have two temperature states they can exist in: heating (losing energy) and cooling.[14]
Kerr black holes are featured extensively in the 2009visual novelSteins;Gate (alsoTV /manga), for their possibilities intime travelling.[15] These are, however, magnified greatly for the purpose of story telling.Kerr black holes are also key to the "Swan Song" project byJoe Davis.[16][17] They are also a key element in the 2014 filmInterstellar.
