Penrose process

The Penrose process is used to extract energy from a rotating black hole. This is not inside the event horizon but outside in its ergosphere in which there is no accretion but all objects become dragged by rotating spacetime.

A piece of matter entering the ergosphere is triggered to split into two parts. One piece escapes from the black hole with positive mass-energy, whilst the other falls past the event horizon into the black hole with negative mass-energy. The net effect is that more energy can be extracted than was originally provided, resulting in a slight decrease in the angular momentum of the black hole.

The efficiency for this process is about 20% for an uncharged black hole, with a theoretical maximum of 30% for a charged black hole. In comparison, accretion of material as it falls into a black hole entering the event horizon releases up to 40% of the energy of the material falling in; only an antimatter-matter collision is more efficient at 100%.

If the Penrose process is performed repeatedly, the black hole can eventually lose all of its angular momentum and becoming non-rotating.

The Blandford–Znajek process is an extension of the Penrose process used for the extraction of energy from spinning supermassive black holes. This is one of the mechanisms that naturally power quasars, or rapidly accreting supermassive black holes. It can also be used as an engine for a gamma-ray burst. This process is capable of producing the high-energy particles that are observed being emitted from quasars and other active galactic nuclei.