Ergosphere

The ergosphere is the region of a rotating (Kerr) black hole that lies between the event horizon and the static limit — the surface where the time-translation symmetry of the spacetime becomes spacelike. Within it, frame dragging is so strong that no observer can remain static with respect to the distant stars; every object is forced to co-rotate with the hole. Crucially, however, the event horizon lies further in, so an object in the ergosphere can still escape back to infinity. The name comes from the Greek ergon, 'work', because energy can be extracted from it.

The outer boundary of the ergosphere, the static limit, is given by r_E(θ) = M + √(M² − a² cos²θ) in geometrized units, where a is the spin per unit mass. It coincides with the event horizon at the poles and bulges outward to r = 2M at the equator, independent of spin — so the ergosphere is a crescent-shaped shell, thickest at the equator and vanishing at the poles. A non-rotating Schwarzschild black hole has no ergosphere at all; the region appears the instant the hole acquires angular momentum.

Because the conserved energy-at-infinity of a particle can be negative inside the ergosphere, the region makes a black hole into an energy source. Splitting a body so that one fragment carries negative energy into the horizon lets the other escape with more energy than entered — the Penrose process — at the cost of the hole's spin. The astrophysical analogue acting on magnetic fields, the Blandford–Znajek mechanism, is the leading model for powering the relativistic jets of quasars and active galactic nuclei.

Implicit in Roy Kerr's 1963 metric; named and physically characterized by Remo Ruffini and John Archibald Wheeler in 1971, following Roger Penrose's 1969 demonstration that energy could be extracted from it.