In a Glasgow research lab, an old hypothesis that was first proposed as conjecture about how an alien civilization may harness a black hole to produce energy has been empirically confirmed for the first time.
The black hole’s ergosphere is the outer layer of its event horizon, where an item would have to move faster than the speed of light in order to remain still. British scientist Roger Penrose proposed that energy may be produced by lowering an object into it.
In this peculiar region of space, Penrose projected that the item would have a negative energy. The recoil action would measure a loss of negative energy if the object were dropped and divided in half, with one half falling into the black hole and the other being recovered. In reality, the recovered part would receive energy from the black hole’s rotation. However, due to the size of the engineering difficulty involved, Penrose hypothesized that only an extremely evolved civilization—possibly an alien one—would be capable of completing the procedure.
Two years later, Yakov Zeldovich, a different physicist, proposed a more realistic, earthbound experiment to test the idea. He claimed that a peculiarity of the rotational Doppler effect would cause “twisted” light waves, striking the surface of a revolving metal cylinder turning at just the proper speed, to end up being reflected with additional energy extracted from the cylinder’s rotation.
Zel’dovich’s concept, however, has remained purely theoretical since 1971 since, in order for the experiment to function, his suggested metal cylinder would need to rotate at least a billion times per second—another impossibility for the present capabilities of human engineering.
Now, scientists from the School of Physics and Astronomy at the University of Glasgow have discovered a way to experimentally demonstrate the effect that Penrose and Zel’dovich proposed by twisting sound rather than light—a source with a much lower frequency, making it much more feasible to demonstrate in the lab.
The team describes how they created a system that uses a small ring of speakers to create a twist in the sound waves similar to the twist in the light waves predicted by Zel’dovich in a new publication published today in Nature Physics.
The revolving foam disc sound absorber was the target of the twisted sound waves. The sound from the speakers was picked up by a series of microphones behind the disc, which gradually accelerated the spin rate.
The team was hoping to detect a significant variation in the frequency and amplitude of the sound waves as they passed through the disc, brought on by the peculiarity of the Doppler effect, in order to confirm the validity of Penrose and Zel’dovich’s hypotheses.
The paper’s lead author is Marion Cromb, a Ph.D. candidate at the university’s School of Physics and Astronomy. The linear Doppler effect, according to Marion, is the phenomenon where the pitch of an ambulance siren appears to rise as it gets closer to the listener but reduces as it moves away. It seems to climb because the frequency of sound waves hitting the listener increases as the ambulance draws closer and decreases as it moves past.
The rotational Doppler effect is comparable, but it only affects a circular area. When viewed from the perspective of the revolving surface, the twisted sound waves exhibit a shift in pitch. The sound frequency can do a very unusual thing if the surface rotates quickly enough—it can change from a positive frequency to a negative frequency and absorb part of the surface’s rotational energy in the process.
The pitch of the sound coming from the speakers in the experiment by the researchers decreases as the speed of the spinning disc rises until it is too low to hear. The pitch then rises once more until it reaches its prior pitch, but now it is louder and has an amplitude that is up to 30% more than the sound that first arrived.
“What we heard during our experiment was extraordinary,” Marion continued. As the spin speed rises, the sound waves’ frequency is Doppler-shifted until it equals zero. The reason the sound picks back up is that the waves have changed from a positive to a negative frequency. Just as Zel’dovich suggested in 1971, the negative-frequency waves are capable of absorbing some of the energy from the rotating foam disc and growing louder as a result.
The paper’s co-authors include Professor Daniele Faccio, a member of the University of Glasgow’s School of Physics and Astronomy. A half-century after the theory’s inception, Prof. Faccio continued, “We’re happy to have been able to experimentally prove some extremely unusual physics. It seems unusual to think that our lab in the west of Scotland has been able to prove a fifty-year-old idea with cosmic origins, but we believe it will lead to many fresh directions for scientific inquiry. We’re eager to explore how, in the near future, we can investigate the impact on various sources like electromagnetic waves.
