Though we've long known about them, we still don't understand black holes completely, and the little we do know is full of quirks and head-scratchers. If there's one thing you probably do know about black holes, it's that nothing can escape them. However, a peculiar theorized phenomenon known as the Penrose Process suggests a way to extract energy from a rotating black hole. Now, researchers have found a way to show in the lab that this is possible.
Obviously, we cannot make a black hole in the lab, but researchers at the University of Glasgow used the next best thing: sounds waves. The findings, reported in Nature Physics, shows that it is perfectly possible to extract energy from a rotating system.
Roger Penrose first proposed how it was possible to use a black hole as a generator in 1969. The approach has a piece of matter or an object placed in a particular orbit not in the event horizon of the black hole, but within the rotating space-time region that surrounds it: the ergosphere.
Penrose realized that it was possible to steal some of the rotational energy by separating the matter in two and arranging for one piece to fall into the black hole on a specific trajectory, with the other piece heading out from the black hole. The piece that escapes will have more energy than it had when it was originally put in. Two years later, Yakov Zel’dovich showed that you don’t need to have actual black holes to prove that: a rotating cylinder reflecting light can demonstrate a similar effect. There is one issue though, the cylinder has to rotate a billion times per second to make it work.
Almost fifty years on, the technology is still not there to test this with light, but researchers have found a way to test this phenomenon with sounds. They created a twisted soundwave with a ring of speakers and made it interact with a rotating disk. Past a certain velocity of the disk, the waves' twist is inverted and it steals some of the energy of the disk, becoming louder. The phenomenon is based on the more familiar Doppler shift, observed when the source of waves is moving relative to an observer.
“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,” lead author Marion Cromb explained in a statement.
“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.”
The researchers say that having been able to confirm a half-century-old theory, they are keen to explore in the future how they can investigate the effect on different sources, like electromagnetic waves.
We are not advanced enough to actually get energy out a rotating black hole yet, but this tech could have some fun amplification applications.