A 50-year theory that began as speculation on how an alien civilization could use a black hole power generation was experimentally verified for the first time in a Glasgow research laboratory.
Penrose predicted that the object would acquire negative energy in this unusual area of space. Release the object and split it in two so that one half falls into the black hole while the other is recovered, the recoil action would measure a loss of negative energy – in fact, the recovered half would get energy extracted from the rotation of the black hole . The scale of the engineering challenge that the process would require is so great, however, that Penrose suggested that only a very advanced, perhaps alien, civilization would be equal to the task.
Two years later, another physicist named Yakov Zel’dovich suggested that the theory could be tested with a more practical and grounded experiment. He proposed that the “twisted” light waves, hitting the surface of a rotating metal cylinder by turning at the right speed, would end up being reflected with additional energy extracted from the rotation of the cylinder thanks to an oddity of the rotational Doppler effect.
But Zel’Dovich’s idea has remained in the realm of theory only since 1971 because, for the experiment to work, its proposed metal cylinder would have to rotate at least a billion times per second – another insurmountable challenge for current limits of human engineering.
Now, researchers from the Glasgow UniversityThe School of Physics and Astronomy has finally found a way to experimentally demonstrate the effect that Penrose and Zel’dovich proposed by twisting sound instead of light – a much lower frequency source, and therefore much more practical to demonstrate in the laboratory.
In a new document published June 22, 2020, in Physics of nature, the team describes how they built a system that uses a small speaker ring to create a torsion of sound waves similar to the torsion of light waves proposed by Zel’dovich.
Those twisted sound waves were directed towards a rotating sound absorber made up of a foam disk. A series of microphones behind the disc collected sound from the speakers as it passed through the disc, constantly increasing the speed of its rotation.
What the team was trying to hear to know that Penrose and Zel’dovich’s theories were correct was a distinctive change in the frequency and amplitude of sound waves as they traveled across the disc, caused by that bizarre Doppler effect.
Marion Cromb, a PhD student at the University’s School of Physics and Astronomy, is the lead author of the document. Marion said: “The linear version of the Doppler effect is familiar to most people as the phenomenon that occurs when the tone of an ambulance siren seems to increase as it approaches the listener but drops as it moves away. It appears to rise because the sound waves reach the listener more frequently as the ambulance approaches, then less frequently as it passes.
“The rotational Doppler effect is similar, but the effect is limited to a circular space. Twisted sound waves change their tone when measured from the perspective of the rotating surface. If the surface rotates fast enough, the frequency of sound can do something very strange: it can go from a positive frequency to a negative one and, in doing so, steal some energy from the rotation of the surface. “
As the speed of the rotating disk increases during the researchers’ experiment, the tone of the sound from the speakers decreases until it becomes too low to be heard. Then, the tone goes up again until it reaches the previous tone, but louder, with an amplitude up to 30% greater than the original sound coming from the speakers.
Marion added: “What we listened to during our experiment was extraordinary. What is happening is that the frequency of the sound waves is moved to Doppler to zero as the spin speed increases. When the sound restarts, it is because the waves have been moved from a positive frequency to a negative frequency. Those negative frequency waves are able to take part of the energy from the spinning foam disk, becoming more noisy in the process, just as proposed by Zel’Dovich in 1971. “
Professor Daniele Faccio, also from the School of Physics and Astronomy of the University of Glasgow, is co-author of the document. Professor Faccio added: “We are thrilled to have been able to experimentally verify extremely strange physics half a century after the first proposal of the theory. It is strange to think that we were able to confirm a half-century old theory with cosmic origins here in our laboratory in western Scotland, but we think it will open many new avenues of scientific exploration. We look forward to seeing how we can study the effect on different sources such as electromagnetic waves in the near future. “
Reference: “Amplification of the waves from a rotating body” by Marion Cromb, Graham M. Gibson, Ermes Toninelli, Miles J. Padgett, Ewan M. Wright and Daniele Faccio, 22 June 2020, Physics of nature.
DOI: 10.1038 / s41567-020-0944-3
The research team article, entitled “Amplification of waves by a rotating body”, is published in Physics of nature. The research was supported by funding from the Research Council in Engineering and Physical Sciences (EPSRC) and the European Union’s Horizon 2020 program.