What process extracts energy from a rotating black hole?

The Penrose process extracts energy from a rotating black hole. Proposed by Roger Penrose in 1969, it exploits the ergosphere, the region outside a rotating black hole's event horizon where spacetime is dragged around by the hole's rotation.

An object entering the ergosphere can split into two parts. One part falls into the black hole with negative energy relative to a distant observer, while the other escapes with more energy than the original object carried. The extra energy comes from the black hole's rotational energy, causing it to spin down.

The process is associated with a Kerr black hole, not a non-rotating Schwarzschild black hole. It is also different from Hawking radiation, a quantum effect that causes black-hole mass loss, and from the Blandford–Znajek process, a magnetic-field-driven mechanism that can extract rotational energy astrophysically.

For an idealized maximally rotating, uncharged black hole, a single classical particle-splitting event can increase the escaping particle's energy by about 20.7% of its original mass-energy. Across repeated extraction, no more than about 29% of the black hole's original mass-energy can be removed through rotational energy.