The Penrose Process

The Penrose process is a remarkable theoretical method for extracting energy from a rotating black hole, proposed by Roger Penrose in 1969. It shows that black holes are not purely destructive: under the right conditions, energy can be mined from them.

How it works

A spinning black hole drags the spacetime around it into rotation, creating a region just outside the event horizon called the ergosphere, where nothing can stay still. Penrose imagined sending an object into the ergosphere and splitting it in two. If one piece is sent on a carefully chosen path so that it falls into the black hole carrying what is, in effect, negative energy, the other piece can fly back out with more energy than the original object had to begin with. The extra energy is drawn from the black hole's rotation, which slows very slightly as a result.

The connection to Hawking

The Penrose process dovetails with Hawking's own work. There is a strict limit on how much energy can be extracted this way: you can drain a black hole's rotational energy, but the area of its horizon can never decrease, exactly as Hawking's area theorem in black hole thermodynamics requires. The energy comes from spin, not from the black hole's irreducible mass.

The process also has a wave version called superradiance, and it foreshadows the deeper idea that black holes have usable energy and obey thermodynamic-style accounting, the line of thinking that ultimately led Hawking to Hawking radiation. It is another example of how Penrose's geometric insights and Hawking's physics fed into each other.