Researchers from the School of physics and astronomy of the University of Glasgow in the UK proved a half-century hypothesis, which implies that technologically advanced extraterrestrial civilizations have the potential to extract energy from rotating black holes. Article researchers published in the journal Nature Physics.

In 1969, the English physicist Roger Penrose suggested that the aliens can extract energy from a rotating black hole due to the fact that flying through the ergosphere a particle or a wave carry away the rotational energy of the black hole (this phenomenon became known as the Penrose process). Soviet physicist Yakov Zeldovich developed this idea and put forward the hypothesis that a rapidly rotating cylinder is able to amplify incident “twisted” electromagnetic waves (that is, having a certain orbital angular momentum), including quantum fluctuations in a vacuum. However, this effect has still not been tested experimentally, since the cylinder had to rotate with a frequency of at least a billion times per second.

In the new work, scientists first managed to watch the Zeldovich effect achieved by using acoustic waves with a frequency of 60 Hz. In the experiment, the researchers found 16 speakers in a circle and sent the sound to the side of the rotating disc made of noise absorbing foam. In this case the acoustic wave from one speaker behind the phase of the waves from the other speaker, which enabled to simulate the orbital angular momentum. Conditions, meeting the Zeldovich effect has been achieved when the disk rotates with a frequency of only 15-30 revolutions per second.

The results of the experiment confirmed that low frequency modes can increase to 30 percent, passing through the sound-absorbing layer disc. As soon as the disc rotation speed increases, the frequency of the sound waves decreases due to the Doppler effect, however, upon reaching a certain speed it comes back again to the previous value and volume (i.e. the amplitude) increases. This is due to the fact that the waves took away some of the energy of rotation of the disk.

The Penrose process takes place when the body has two parts, one of which falls beyond the event horizon. If two fragments have a certain speed, a particular position relative to each other and flying on the right path, then drop one of the fragments transmits the energy of the other part, greater than the energy which the body had originally. To the casual observer it looks like the body was divided into a part with positive energy and part of “negative energy” that the fall behind the horizon reduces the angular momentum of the black hole. The first fragment flies out of the ergosphere, “taking” the rotational energy of the black hole.