A 50-year-old theory that started out as speculation about how an extraterrestrial civilization could use a black hole generating energy has been tested experimentally for the first time in a Glasgow research laboratory.
In 1969, British physicist Roger Penrose suggested that energy could be generated by lowering an object into the black hole ergosphere – the outer layer of the black hole event horizon, where an object should move faster that the speed of light to stay still.
Penrose predicted that the object would acquire negative energy in this unusual area of space. By dropping the object and dividing it in half 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 gain of energy extracted from the rotation of the black hole. The scale of the engineering challenge that the process would require, however, is so great that Penrose suggested that a very advanced, perhaps alien, civilization would be up to the task.
Two years later, another physicist named Yakov Zel’dovich suggested that the theory could be tested with more practical, earth-related experience. He proposed that the “ twisted ” light waves hitting the surface of a rotating metal cylinder rotating at the right speed would eventually be reflected with additional energy extracted from the rotation of the cylinder through an oddity of the effect. Rotational doppler.
But Zel’dovich’s idea has only remained in the realm of theory since 1971 because, for the experiment to work, its proposed metal cylinder would have to spin at least a billion times per second – another insurmountable challenge for current limits of human engineering.
Now, researchers from the University of GlasgowThe school of physics and astronomy 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 article published on June 22, 2020, in Nature physics, the team describes how they built a system that uses a small ring of speakers to create a twist in the sound waves analogous to the twist in the light waves proposed by Zel’dovich.
These twisted sound waves were directed to a rotary sound absorber made from a foam disc. A set of microphones behind the disc picked up the sound from the speakers as it passed through the disc, which regularly increased the speed of rotation.
What the team was trying to hear in order to know that the theories of Penrose and Zel’dovich were correct was a distinctive change in the frequency and amplitude of the sound waves during their journey through the disc, caused by this quirk of the doppler effect.
Marion Cromb, doctoral student at the University’s School of Physics and Astronomy, is the main author of the article. Marion said, “The linear version of the Doppler effect is familiar to most people because the phenomenon that occurs when the tone of an ambulance siren seems to increase as it approaches the listener, but decreases as it moves away. It seems to increase because the sound waves reach the listener more frequently when approaching the ambulance, then less frequently when it passes.
“The rotational Doppler effect is similar, but the effect is limited to a circular space. Twisted sound waves change height when measured from the point of view of the rotating surface. If the surface rotates fast enough, the frequency of the sound can do something very strange – it can go from a positive frequency to a negative frequency and, in doing so, steal energy from the rotation of the surface. “
As the speed of the spinning disc increases during the researchers’ experiment, the pitch of the speakers decreases until it becomes too low to be heard. Then the pitch rises again until it reaches its previous height – but louder, with an amplitude up to 30% greater than the original sound from the speakers.
Marion added: “What we heard during our experience was extraordinary. What happens is that the frequency of the sound waves is shifted Doppler to zero when the spin speed increases. When the sound starts again, it is because the waves have passed from a positive frequency to a negative frequency. These negative frequency waves are able to take some of the energy from the spinning foam disc, becoming noisier in the process – just as Zel’dovich proposed in 1971. “
Professor Daniele Faccio, also of the School of Physics and Astronomy at the University of Glasgow, is co-author of the article. Professor Faccio added: “We are delighted to have been able to experimentally verify an extremely strange physics half a century after the first proposition of the theory. It is strange to think that we have been able to confirm a theory of half a century of cosmic origin here in our laboratory in the west of Scotland, but we believe that this will open up many new avenues of scientific exploration. We are looking forward to seeing how we can study the effect on different sources such as electromagnetic waves in the near future. “
Reference: “Amplification of waves from a rotation body” by Marion Cromb, Graham M. Gibson, Ermes Toninelli, Miles J. Padgett, Ewan M. Wright and Daniele Faccio, June 22, 2020, Nature physics.
DOI: 10.1038 / s41567-020-0944-3
The article of the research team, entitled “Amplification of the waves of a rotating body”, is published in Nature physics. The research was funded by the Engineering and Physical Sciences Research Council (EPSRC) and the European Union’s Horizon 2020 program.