Jan 19. 2021
Scientists have described a new way to extract energy from black holes. It can be carried away by plasma particles accelerated as a result of magnetic reconnection. This is not the first mechanism for extracting energy from black holes – the possibility of this process was proved by Roger Penrose in 1969, and methods of its implementation were later proposed by Stephen Hawking, Roger Blanford and Roman Znayek. The article was published in the journal Physical Review D.
The theory of relativity predicts that rotating black holes can be used as energy sources. In 1969 Roger Penrose described a process for doing this. There is an ergosphere around the rotating black holes – the region that precedes the event horizon. All bodies in the ergosphere rotate together with the black hole. According to the Penrose mechanism, if a particle is divided into two fragments in the ergosphere, one of them will fall beyond the event horizon, and the other, on the contrary, will be thrown beyond the static limit into the region where the bodies no longer rotate around black hole. In this case, the energy of the ejected fragment will be greater than the energy of the original particle. This is because a debris that has fallen into a black hole has negative energy. By absorbing it, the hole’s rotation speed is reduced, and some of the rotational energy is transferred to the escaping particle. The Penrose process indicates the possibility of obtaining energy from a black hole, but it cannot be called a good practical method. For its implementation, it is necessary that two newborn particles have a speed exceeding half the speed of light. The expected frequency of such events is so rare that it will not allow obtaining significant amounts of energy, so scientists are actively looking for other mechanisms. For example, Stephen Hawking showed that black holes can release energy through radiation. Another way to extract energy is the Blanford-Znaek process, based on electromagnetic interaction. Luca Comisso of Columbia University and Felipe A. Asenjo of Adolfo Ibáñez University described in their article another alternative to the Penrose process. the holes are surrounded by hot plasma, the particles of which have a magnetic field. The basis of a new mechanism for obtaining energy from rotating black holes is the reconnection of magnetic field lines inside the ergosphere. In this case, the black hole should be in an external magnetic field, have a large spin (a ~ 1) and the surrounding plasma with strong magnetization. For example, black holes formed as a result of long and short gamma-ray bursts and supermassive black holes in active galactic nuclei possess the necessary properties. Magnetic reconnection accelerates some of the plasma in the direction of the hole’s rotation. The other part accelerates in the opposite direction and falls beyond the event horizon. The release of energy, as in the Penrose mechanism, occurs if the absorbed plasma has negative energy, and the accelerated one “escapes” from the ergosphere. The difference is that the formation of particles with negative energy requires dissipation of the energy of the magnetic field. In the process described by Penrose, only the inertia of the particles plays a role.
As scientists say, the efficiency of the described process is 150 percent. This means that the process allows you to get one and a half times more energy than you need to spend on its implementation. Achieving an efficiency of more than 100 percent is possible because the plasma particles released from the ergosphere carry away the energy of the black hole. The discovery of a new mechanism for extracting energy from black holes will allow astronomers to better estimate their rotational momentum and understand how they radiate energy. The discovery is still far from practical application: it is necessary to figure out how to fly to the black hole and place something in its ergosphere without falling beyond the event horizon.
We have already written about the effect of magnetic reconnection on plasma, this process is the cause of solar flares. Luca Comisso and Felipe Asenjo believe that magnetic reconnection may also be the cause of X-ray bursts from black holes. They were detected in 2019.
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