A 50-year-old theoretical process that would like to exploit the rotational energy of a black hole has been confirmed experimentally. Using components similar to those needed, physicists demonstrated the reasonableness of the Penrose mechanism.
This is not always the case, often many theoretical works remain in a drawer or in an old issue of a scientific journal. Sometimes, however, they can be used to create new technologies, or as in this case, to exploit the properties of most extreme objects in the Universe.
I black holes they are the result of the death of gigantic stars; after the supernova phase, the nucleus is no longer able to sustain gravitational attraction and collapses into a singularity, a point of infinite density.
The singularity lies within an “event horizon”, an area where the escape velocity is greater than the speed of light. Just outside the event horizon, space-time is dragged along with the rotation of the black hole, whose speed it was measured for the first time a short time ago. This is where the Penrose trial. In 1969, the mathematician Roger Penrose proposed to extract energy from the ergosphere, the area where the drag of space-time is strongest.
If an object, divisible in two, were left in the ergosphere, one part would fly towards the event horizon and the other would be thrown out of the ergosphere. According to Penrose calculations, the second part would come out with 21% more energy than the initial one.
Now, we can’t go near a black hole and test this hypothesis. In 1971 the physicist Yakov Zel’dovich proposed a more practical experiment: the black hole is replaced by a rotating metal cylinder on which rays of light are sent. If the rotation speed is high enough, the light is reflected with additional energy, absorbed by the rotation of the cylinder by Doppler effect.
There is a small problem with Zel’dovich’s proposal: the “right rotation speed” is a billion rotations per second, making it not much more practical than a black hole. A Glasgow team has thought of solving the problem replacing light with sound waves. The apparatus consists of a speaker ring while the “virtual black hole” is a rotating absorbent material, the speed of which increases when hit by sound waves. Microphones are placed beyond the absorbent disc to detect the passage of sound.
The proof of the functioning of the Penrose trial it is an increase in the amplitude of the sound wave and a change in its frequency. “If the surface rotates fast enough, a strange thing happens at the frequency of the wave: it can go from a positive value to a negative value and in doing this it steals energy from the disk, ” explains Marion Cromb of the University of Glasgow.
The results were excellent, the experiment managed to increase the frequency of a wave by making it go from audible to inaudible, confirming Zel’dovich’s proposal and indirectly the Penrose trial.
The researchers plan to extend their technique to light rays, it would be a giant step forward in the prospect of developing an engine that harnesses the energy of a black hole.
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