Ergosphere

The ergosphere is a region located outside a rotating black hole. Its name proposed by Remo Ruffini and John Archibald Wheeler during the Les Houches lectures in 1971, is derived from the Greek word ergon, which means “work”. It received this name because it is theoretically possible to extract energy and mass from this region. The ergosphere has an oblate spheroidal shape that touches the event horizon at the poles of a rotating black hole and extends to a greater radius at the equator. The equatorial (maximum) radius of an ergosphere corresponds to the Schwarzschild radius of a non-rotating black hole; the polar (minimum) radius can be as little as half the Schwarzschild radius (the radius of a non rotating black hole) in the case that black hole is rotating maximally (at higher rotation rates the black hole could not have formed).