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 the black hole in 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 if the black hole is rotating maximally.

Within the ergosphere, spacetime is dragged along in the direction of the rotation of the black hole at a speed that varies with distance from the event horizon, becoming zero at the outer surface of the ergosphere. This process is known as the Lense-Thirring effect or frame-dragging. Because of this dragging effect, objects within the ergosphere could not be stationary with respect to the rest of the universe unless they were to travel faster than the speed of light with respect to local spacetime. A suspended plumb, held stationary outside the ergosphere, will experience infinite/diverging radial pull as it approaches the static limit. At some point it will start to fall, resulting in gravitomagnetically induced spinward motion. Another result of this dragging of space is the existence of negative energies within the ergosphere.