Science

Black holes: can they help us detect potential extraterrestrial civilizations?

If the Universe has different civilizations, each with their own characteristics, they should however share one thing in common: the need for energy. According to the technological scale established by Russian physicist Nikolai Kardachev, a Type III civilization is capable of harnessing all of the energy of its host galaxy, including its central supermassive black hole. And it is thanks to this that we might be able to detect the presence of an advanced extraterrestrial civilization. Indeed, the energy exploitation of a black hole would certainly leave traces detectable from the Earth.

This energy-mining technology could leave a signature just outside the event horizon of a rotating black hole – the limit beyond which a black hole’s gravity becomes too great for matter and energy can escape. And the process could explain some plasma eruptions that astrophysicists often detect around black holes.

Indeed, the rotation of black holes could constitute an almost unlimited 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 might look like to distant observers. This would allow Terrans to potentially detect distant extraterrestrial civilizations.

Harnessing the energy of a black hole

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 renowned 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 half right next to a black hole spinning at a speed close to the speed of light.

In the case of rotating black holes like Kerr’s black holes, the rotational motion of the ergosphere drives space-time with it. This is the Lense-Thirring effect. This area could be used to harness energy from a black hole. © nrumiano

Part of the particle then falls through the ergosphere, a chaotic region of rotating space-time 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 effectively harness the energy of the black hole by capturing the part of the particle that escaped the black hole. intense gravity of the black hole. While in his original study, Penrose only considered a single particle that splits in two, the recent study considers the astronomically-sized plasmas generated in the accretion disc around a black hole – the massive disc and very hot material that orbits most black holes.

Plasma and magnetic reconnections

Because plasmas contain a large number of particles, they could produce substantial 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 be detected at this time. .

black hole energy exploitation diagram
Diagram of the mechanism for extracting energy from a rotating black hole by magnetic reconnection in the ergosphere of the black hole. A configuration with antiparallel magnetic field lines adjacent to the equatorial plane is favored by the Lense-Thirring effect of the rapidly rotating black hole (views (a) and (b) represent the southern and equatorial views, respectively). Magnetic reconnection in the plasma which rotates in the equatorial plane extracts energy from the black hole if the decelerated plasma which is absorbed by the black hole has negative energy, while the accelerated plasma with a component in the same direction as the rotation from the black hole escapes to infinity. The outer limit (static limit) of the ergosphere is indicated by the short dotted lines in both views. In view (b), the long, solid dotted lines indicate the magnetic field lines below and above the equatorial plane. © Luca Comisso and Felipe A. Asenjo

Physicists suggest that the plasmas to extract energy from a spinning black hole are created by magnetic reconnection events – where strong magnetic field lines become tangled, broken, and reconnected – just outside of 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 plasma eruptions, which travel in diametrically opposite directions.

As the plasma eruptions 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 plasma jet 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.

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