Penrose process
The Penrose process is theorised by Sir Roger Penrose as a means whereby energy can be extracted from a rotating black hole. The process takes advantage of the ergosphere – a region of spacetime around the black hole dragged by its rotation faster than the speed of light, meaning that from the point of view of an outside observer any matter inside is forced to move in the direction of the rotation of the black hole.
In the process, a working body falls into the ergosphere. At its lowest point the body fires a propellant backwards; however, to a faraway observer both seem to continue to move forward due to frame-dragging. The propellant, being slowed, falls to the event horizon of the black hole. The remains of the body, being sped up, fly away with an excess of energy.
The maximum amount of energy gain possible for a single particle decay via the original Penrose process is 20.7% of its mass in the case of an uncharged black hole. The energy is taken from the rotation of the black hole, so there is a limit on how much energy one can extract by Penrose process and similar strategies.
Details of the ergosphere
The outer surface of the ergosphere is the surface at which light that moves in the direction opposite to the rotation of the black hole remains at a fixed angular coordinate, according to an external observer. Since massive particles necessarily travel slower than light, massive particles will necessarily move along with the black hole's rotation. The inner boundary of the ergosphere is the event horizon, the spatial perimeter beyond which light cannot escape.Inside the ergosphere even light cannot keep up with the rotation of the black hole, as the trajectories of stationary objects become space-like, rather than time-like, or light-like. Mathematically, the component of the metric changes its sign inside the ergosphere. That allows matter to have negative energy inside of the ergosphere as long as it moves counter the black hole's rotation fast enough. The Penrose mechanism exploits that by diving into the ergosphere, dumping an object that was given negative energy, and returning with more energy than before.
In this way, rotational energy is extracted from the black hole, resulting in the black hole being spun down to a lower rotational speed. The maximum amount of energy is extracted if the black hole is rotating at the maximal rate, the object just grazes the event horizon and decays into forwards and backwards moving packets of light.
In an adjunct process, a black hole can be spun up by sending in particles that do not split up, but instead give their entire angular momentum to the black hole. However, this is not a reverse of the Penrose process, as both increase the entropy of the black hole by throwing material into it.
In popular culture
's 2005 short story "The Clockwork Atom Bomb" involves the use of the Penrose Process for military applications and power generation.A major plot point of the 2014 science fiction film Interstellar is the use of the Penrose process to slingshot the spacecraft Endurance towards Edmund's planet flying through the ergosphere of fictional black hole Gargantua.
In the narrative of 2024 cooperative horde shooter game Helldivers 2, the Penrose process was utilized to decrease the velocity of a singularity.