If the Universe has different civilizations, each with their own characteristics, they should however share a common point:the need for energy. According to the technological scale established by Russian physicist Nikolai Kardachev, a Type III civilization is capable of harnessing all the energy of its host galaxy, including its central supermassive black hole. And it is through this that we might be able to detect the presence of an advanced extraterrestrial civilization. Indeed, the energetic exploitation of a black hole would certainly leave traces detectable from Earth.
This energy-harvesting technology could leave a signature just outside the event horizon of a spinning black hole — the limit beyond which a black hole's gravity becomes too strong for matter and energy can escape. And the process could explain some plasma flares that astrophysicists often detect near black holes.
Indeed, rotating black holes could provide an almost limitless source of energy for a technologically advanced civilization. But before that, astrophysicist Luca Comisso of Columbia University in New York, explains that the preliminary step is to determine what the energy artificially extracted from a black hole could look like for distant observers. This would allow Earthlings to potentially detect distant extraterrestrial civilizations.
The idea of harnessing the energy of a black hole, proposed by Comisso and his colleagues, is not a first. The most famous study was carried out in 1969 by the famous physicist Roger Penrose, who won the Nobel Prize in Physics in 2020 for his work on black holes. He proposed a mechanism known as the Penrose process, in which a particle splits in two right next to a black hole rotating at near the speed of light.
Part of the particle then falls through the ergosphere, a chaotic region of rotating spacetime just outside the black hole's event horizon, before falling into the black hole itself. According to calculations, objects falling into this ergosphere can have negative energy, which is not possible anywhere else in the Universe.
And because adding a particle with negative energy to a black hole is equivalent to extracting energy from it, aliens could actually harness the energy of the black hole by capturing the part of the particle that escaped the intense gravity of the black hole. Whereas in his original study, Penrose only considered a single particle that splits into two, the recent study considers the astronomically sized plasmas generated in the accretion disk around a black hole — the massive disk and very hot material that orbits most black holes.
Because plasmas contain a large number of particles, they could produce significant amounts of energy. In theory, black holes also “evaporate” over time by emitting Hawking radiation — a phenomenon involving quantum mechanics, proposed by physicist Stephen Hawking, but this process is too weak to detect, yet .
Physicists suggest that the plasmas to extract energy from a spinning black hole are created by magnetic reconnection events — where intense magnetic field lines tangle, snap and reconnect — just outside the event horizon. Magnetic reconnections are commonly seen on the surface of stars like the Sun, where they release huge amounts of energy in the form of a plasma flare, which travels in diametrically opposite directions.
As plasma flares created on the surface of stars fall back into the star or are ejected into space, the ergosphere of a rotating black hole would imply that a jet of plasma could acquire negative energy, while that the corresponding jet would escape by gaining additional energy from the black hole. By defining the precise characteristics of a technological capture of this energetic plasma, astrophysicists on Earth could then identify a potential extraterrestrial technosignature of energy extraction.
