Glasgow scientists prove theory proposing how aliens could use black holes for energy
Scientists at the University of Glasgow have proven a 50-year-old theory proposing how an alien civilisation could use a black hole to generate energy.
The theory was first proposed in 1969 by British physicist Roger Penrose who suggested that black holes could be used to generate energy by a sufficiently advanced alien civilisation.
To do this, the aliens would lower an object into the black hole’s ergosphere, the very outer layer of its event horizon where nothing can return from, where objects acquire negative energy.
Mr Penrose proposed that by splitting an object here in two, one half of it would fall into the black hole while the other half would be recoiled back out.
The recoil would cost the object some of its negative energy – but removing negative energy equals positive energy, effectively taken from the black hole’s rotation.
Of course the engineering challenge to perform this feat on a black hole was then, and is now, well beyond humanity’s ability – so Mr Penrose suggested a sufficiently advanced alien civilisation could perhaps manage it.
Two years after Mr Penrose’s suggestion another scientist, this time Soviet physicist Yakov Zel’dovich, suggested testing the theory on Earth using “twisted” light waves hitting the surface of a rotating metal cylinder.
But even this proposal was beyond the reach of scientists; the metal cylinder would need to be rotating at least a billion times a second.
Fortunately researchers from the University of Glasgow’s school of physics and astronomy have found a way to successfully demonstrate this effect by using sound waves instead of light waves.
In a new paper published in the journal Nature Physics the team explain how they used a ring of speakers to create a twist in sound waves similar to the twist in light waves proposed by Mr Zel’dovich.
Marion Cromb, a PhD student at the university and the paper’s lead author, explained how the experiment worked – measuring the sound waves using the doppler effect.
“The linear version of the doppler effect is familiar to most people as the phenomenon that occurs as the pitch of an ambulance siren appears to rise as it approaches the listener but drops as it heads away.
“It appears to rise because the sound waves are reaching the listener more frequently as the ambulance nears, then less frequently as it passes.
“The rotational doppler effect is similar, but the effect is confined to a circular space. The twisted sound waves change their pitch when measured from the point of view of the rotating surface.
“If the surface rotates fast enough then the sound frequency 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.”
During the course of the experiment the speed of the spinning disc increases, causing the pitch of the sound to drop before rising again – and become even louder than the original sound from the speakers.
Ms Cromb added: “What we heard during our experiment was extraordinary. What’s happening is that the frequency of the sound waves is being doppler-shifted to zero as the spin speed increases.
“When the sound starts back up again, it’s because the waves have been shifted from a positive frequency to a negative frequency.
“Those negative-frequency waves are capable of taking some of the energy from the spinning foam disc, becoming louder in the process – just as Zel’dovich proposed in 1971.”
Professor Daniele Faccio, also at Glasgow and a co-author on the paper, added: “We’re thrilled to have been able to experimentally verify some extremely odd physics a half-century after the theory was first proposed.
“It’s strange to think that we’ve been able to confirm a half-century-old theory with cosmic origins here in our lab in the west of Scotland, but we think it will open up a lot of new avenues of scientific exploration.
“We’re keen to see how we can investigate the effect on different sources such as electromagnetic waves in the near future.”
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