Experiment confirms 50-year-old theory describing how an alien civilization could exploit a black hole

In a Glasgow research lab, a 50-year-old idea that started out as conjecture about how an extraterrestrial society may harness a black hole to generate energy has been empirically proven for the first time.

Roger Penrose, a British scientist, proposed in 1969 that energy may be produced by lowering an item into the ergosphere, the outermost region of the black hole's event horizon, where an object would have to travel faster than light to stay still.

Penrose anticipated that in this peculiar region of space, the item would pick up negative energy. The recoil action would measure a loss of negative energy if the object was dropped and divided in half, with one half falling into the black hole and the other being retrieved. In other words, the recovered half would acquire energy from the black hole's spin. Penrose proposed that only a highly developed, maybe extraterrestrial civilization might be capable of completing the procedure due to the immense engineering difficulty it would entail.

Two years later, Yakov Zel'dovich, a different scientist, proposed that a more realistic, on-Earth experiment might test the idea. He postulated that a peculiarity of the rotational doppler effect would cause "twisted" light waves to be reflected with extra energy taken from the cylinder's rotation when they struck the surface of a revolving metal cylinder spinning at precisely the proper speed.

Zel'dovich's experiment would need a metal cylinder to revolve at least a billion times per second, which is beyond the existing capabilities of human engineering. As a result, Zel'dovich's notion has remained only theoretical since 1971.

The effect that Penrose and Zel'dovich proposed has now finally been experimentally demonstrated by researchers from the University of Glasgow's School of Physics and Astronomy by twisting sound instead of light, a much lower frequency source and therefore much more practical to demonstrate in the lab.

The team describes how they created a system that employs a tiny ring of speakers to generate a twist in the sound waves that is similar to the twist in the light waves that Zel'dovich envisioned in a new publication that was published today in Nature Physics.

The revolving foam disc sound absorber was the target of the bent sound waves. The sound from the speakers was picked up by a series of microphones behind the disc, which caused the disc to spin faster and faster as it went past.

The researchers needed to detect a noticeable shift in the frequency and amplitude of the sound waves as they passed through the disc, which was brought on by that peculiarity of the doppler effect, to confirm that Penrose and Zel'dovich's hypotheses were accurate.

The principal author of the article is Marion Cromb, a Ph.D. candidate in the University's School of Physics and Astronomy. According to Marion, "The majority of people are familiar with the linear form of the doppler effect, which is the phenomena where the pitch of an ambulance siren appears to raise as it approaches the listener and drop as it moves away. It seems to climb because as the ambulance gets closer, the listener hears the sound waves more often, and as it passes, the listener hears them less often.

"The effect is limited to a circular region, but the rotating doppler effect is comparable. When seen from the perspective of the revolving surface, the twisted sound waves exhibit a shift in pitch. The sound frequency can perform an extremely odd thing if the surface spins quickly enough: it can go from a positive to a negative frequency and take some energy from the surface's rotation in the process."

The pitch of the sound coming from the speakers lowers until it is inaudible during the researchers' experiment as the speed of the rotating disc rises. After after, the pitch increases once again until it reaches its initial pitch, but this time it sounds louder and has an amplitude that can be up to 30% higher than what was initially audible from the speakers.

"What we heard during our experiment was extraordinary," Marion said. What is actually occurring is that when the spin speed rises, the sound waves' frequency is doppler-shifted to zero. The reason the sound comes back is that the waves have changed from having a positive frequency to a negative frequency. As Zel'dovich suggested in 1971, the negative-frequency waves are able to absorb part of the energy from the rotating foam disc and increase in volume as a result."

Co-author of the article is Professor Daniele Faccio of the University of Glasgow's School of Physics and Astronomy. A half-century after the hypothesis was initially put forward, Prof. Faccio continued, "We're thrilled to have been able to experimentally verify some extremely odd physics." It's surreal to consider that in our lab in the west of Scotland, we have been able to prove the cosmic roots of a fifty-year-old idea, but we believe this will pave the way for many new scientific investigations. In the near future, we're interested in learning how we might look at the impact on other sources, such electromagnetic waves."

"Amplification of waves from a rotating body," the study team's work, is published in Nature Physics.