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An old theory of 50 years ago, started as a speculation on how a supposed alien civilization could use a black hole to extract energy , has just been verified experimentally in a research laboratory of Glasgow. The study just published in Nature Physics.

The original idea was from Roger Penrose, the british physicist who taught us, along with Stephen Hawking, much of what we know about black holes. In 1969, Penrose suggested that it would be possible to generate large amounts of energy by placing an object in the “ergosfera” of a black hole, the outermost layer of its event horizon. Clear that in order to be able to remain there, the object in question would have to move at relativistic speeds, impossible to achieve with our technology.

A “reactor” of negative energy

however Penrose predicted that, if someone overcame an obstacle, in that area which is so unusual of the space that the object would acquire negative energy. To retrieve a portion of that energy, would you put the object in the ergosfera and divide it in two, so that one half was acquired by the black hole and the other, thanks to the recoil action, could escape him. In this way, the half recovered to win energy extracted from the own rotation of the black hole. Unfortunately, and in terms of the engineering, the challenge was so huge that Penrose suggested that only an alien civilization extremely advanced would be able to to carry out the task.

Two years later, another physicist named Jacob Zel’dovich suggested that, after all, the theory of Penrose could be tested with an experiment more practical, and “earth”. In this way, he proposed that light waves “twisted” that hit the surface of a metal cylinder that is spinning at the correct speed, would end up being reflected with an energy “extra”, obtained from the own rotation of the cylinder by virtue of a peculiarity of the Doppler effect rotational. That is to say, that would do the same as the hypothetical object placed at the edge of the black hole.

The experiment seemed simple, but it is not. In fact, from the moment that he formulated his idea in 1971, the experiment of Zel’dovich has remained in the realm of speculation theoretical, since for the thing to work the metal cylinder proposed would need to rotate by at least a thousand million times a second, another challenge insurmountable, and outside the limits of the current human engineering.

The solution: change light by sound

Now, however, researchers of the Faculty of Physics and Astronomy of the University of Glasgow have found, finally, a way to prove experimentally the effect proposed by Penrose and later by Zel’dovich.

The “secret” was to twist sound instead of light , a source of much lower frequency and, therefore, much more practical and affordable for you to make a demonstration laboratory.

in his article In Nature Physics, the scientists described how they managed to build a system that uses a small ring of speakers to create a shift in the sound waves analogous to the spin of the light waves proposed by Zel’dovich.

The Doppler effect

These sound waves twisted were directed toward a rotating object that is able to absorb sound, in this case a disc of foam. A set of microphones placed behind caught the sound of the speakers as we passed through the disk, increasing constantly, its speed of rotation.

To know that the theories of Penrose and Zel’dovich were correct, the team of researchers was to capture a distinctive change in the frequency and amplitude of the sound waves as they traveled through the disk, changes caused by the Doppler effect.

Marion Cromb, lead author of the article, explains that “the linear version of the Doppler effect is familiar to most people, since the phenomenon occurs when the pitch of a siren of an ambulance seems to increase as it approaches the listener and lower when the ambulance is moving away”. The increase is due to the fact that, when approaching us, the speed of the ambulance is in addition to the of sound waves that emits its siren, so that its frequency is higher (and we hear a tone each time more acute). On the contrary, when the ambulance moves away, its speed makes the sound to have that “stretch” more to reach us. In other words, its frequency decreases (and we hear a tone every time more severe).

The Doppler effect rotational -continues Cromb – is similar to , but only limited to a circular space. The sound waves twisted change of tone, when measured from the point of view of the rotating surface. And if that surface rotates fast enough, then the frequency of the sound can go from positive to negative and, by doing so, steal some of the energy of the rotating surface”.

Waves of frequency negative

as the speed of the rotating disc was increased during the experiment, the pitch of the sound emitted by the speakers slowed to become too low to be heard. Later, the tone turned to climb up to reach the previous level, but more high, c on an amplitude of up to 30% higher than that of the original sound of the speakers. It had worked the “miracle.”

“What we listened to during our experiment -ensures Cromb – was extraordinary. What was happening is that the frequency of the sound waves would go down to zero as the rotation speed increased. And when the sound began again, it was because the sound waves had gone from a often positive to a negative one. These waves of frequency negative they were able to extract energy from the rotating disk of foam , becoming stronger in the process, as proposed by Zel’dovich in 1971″.

Daniele Faccio, co-author of the research, says: “we are delighted to have been able to verify experimentally a physical extremely strange half a century after the theory was first proposed. It is extraordinary to think that we have been able to confirm here, our laboratory in the west of Scotland, a theory of cosmic half a century old, but we believe that will open up many new avenues of scientific exploration. We are anxious to see how we can investigate, in the near future, the effect on different sources, such as electromagnetic waves”.

Penrose, therefore, was right. Now I just need to find out if, in fact, there is somewhere an alien civilization sufficiently advanced like to take advantage of this effect and extract energy without limits to the very edge of a black hole…