Rotating Black Holes

Physicists Verify Half-Century-Old Theory about

Physicists from the University of Glasgow and the University of Arizona have experimentally verified a half-century-old theory that began as speculation about how an advanced alien civilization could use a rotating black hole to generate energy.

An artist’s impression of a rotating black hole. Image credit: Sci-News.com.

In 1969, British mathematical physicist Roger Penrose proposed a method to extract the energy of a rotating black hole, now known as Penrose superradiance.

He suggested that a very advanced, perhaps alien, civilization might one day be able to extract energy from a rotating black hole by lowering and then releasing a mass from a structure that is co-rotating with the black hole.

In 1971, Belarusian theoretical physicist Yakov Zel’dovich translated this idea of rotational superradiance from a rotating black hole to that of a rotating absorber such as a metallic cylinder.

He proposed that ’twisted’ light waves, hitting the surface of a rotating cylinder turning at just the right speed, would end up being reflected with additional energy extracted from the cylinder’s rotation thanks to a quirk of the rotational Doppler effect.

But Zel’dovich’s idea has remained solely in the realm of theory since 1971 because, for the experiment to work, a proposed metallic cylinder would need to rotate at least a billion times a second.

Schematic outline of the team’s experiment: 16 loudspeakers are arranged in a ring (diameter – 0.47 m) to create an OAM acoustic field, channeled by acoustic waveguides to a smaller area (diameter – 0.19 m) and incident on a rotating disk of sound-absorbing foam (S); the absorbing disk also carries two closely spaced (2 cm distance) microphones (M); the microphones transmit their data via Bluetooth for live data acquisition while in rotation; insets indicate the various configurations used in the experiments for the rotating disk and absorbing foam: left inset, the supporting disk with microphones and absorber are co-rotating; center left inset, the absorber is detached and remains static, while microphones rotate; center right inset, the absorber is placed in front of only one of the two microphones; right inset, the absorber is completely removed, and microphones rotate. Image credit: Cromb et al, doi: 10.1038/s41567-020-0944-3.

Now, Professor Daniele Faccio, Professor Miles Padgett and their colleagues have finally created a system which uses small ring of speakers to create a twist in the sound waves analogous to the twist in the light waves proposed by Zel’dovich.

Those twisted sound waves were directed towards a rotating sound absorber made from a foam disk.

A set of microphones behind the disk picked up the sound from the speakers as it passed through the disk, which steadily increased the speed of its spin.

What the authors were looking to hear in order to know that Penrose and Zel’dovich’s theories were correct was a distinctive change in the frequency and amplitude of the sound waves as they traveled through the disk, caused by that quirk of 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,” said first author Marion Cromb, a Ph.D. student in the School of Physics and Astronomy at the University of Glasgow.

“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.”

As the speed of the spinning disk increases during the experiment, the pitch of the sound from the speakers drops until it becomes too low to hear.

Then, the pitch rises back up again until it reaches its previous pitch — but louder, with amplitude of up to 30% greater than the original sound coming from the speakers.

“What we heard during our experiment was extraordinary,” Cromb said.

“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 disk, becoming louder in the process — just as Zel’dovich proposed in 1971.”

“We’re thrilled to have been able to experimentally verify some extremely odd physics a half-century after the theory was first proposed,” Professor Faccio said.

The team’s work was published in the journal Nature Physics.

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M. Cromb et al. Amplification of waves from a rotating body. Nat. Phys, published online June 22, 2020; doi: 10.1038/s41567-020-0944-3

This article is based on text provided by the University of Glasgow.