An illustration of what a black hole with an accretion disk © Getty Images

Researchers verify ‘extremely odd’ black hole physics

A team at the University of Glasgow set out to validate British physicist Roger Penrose’s 1969 work.

A team of researchers claim to have verified a decades-old theory that energy can be created by black holes.

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Scientists at the University of Glasgow’s School of Physics and Astronomy set out to validate Roger Penrose’s 1969 work. They used sound waves in an attempt to endorse the “extremely odd physics a half-century after the theory was first proposed”.

British physicist Penrose theorised that energy could be created by dropping objects such as a rocket into a black hole and splitting the object in two. One half escapes from the black hole, while the other falls past the point of no return (the ‘event horizon’).

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The half that escapes steals a small amount of rotational energy from the black hole, meaning it ends up with slightly more energy overall.

He theorised the energy could be stored and used to power entire worlds but Penrose said the engineering challenge posed by the experiment could only be carried out by an advanced, possibly alien, civilisation.

In 1971, physicist Yakov Zeldovich suggested the theory could be tested with twisted light waves projected onto a spinning surface. To test that, the spinning surface would need to rotate at at least a billion times a second – a feat still not possible due to the limitations of engineering.

Now, the Glasgow researchers have finally found a way to experimentally demonstrate the effect using sound waves, which require a much slower rotating surface.

Using a ring of speakers, the researchers sent a rotating wave of sound towards a spinning foam disk. Attached to the back of the disk were two microphones, which found that as the sound waves passed through the disk, the sounds was amplified by up to 30 per cent, known as a rotational Doppler effect.

Marion Cromb, the paper’s lead author, said: “The linear version of the Doppler effect is familiar to most people. The phenomenon 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 researcher added: “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 experiment, researchers found that as the sound wave hits the spinning disk, the pitch dropped until it became too low to hear. Then, when passing through, the pitch was raised again and the sound it was amplified by up to 30 per cent compared to the original sound.

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Professor Daniele Faccio, co-author on the paper, said: “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.

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“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.”

Reader Q&A: Why does light travel faster than sound?

Asked by: Toby Graham, Shrewsbury

According to Einstein’s Special Relativity, the speed of light has a unique status: it’s a fundamental feature of our Universe, representing the maximum speed at which information can travel from place to place. As such, nothing can match the 300,000km/s achieved by light travelling through a vacuum – least of all sound, which being waves of compression and expansion in a substance doesn’t even exist in a vacuum.

That said, light can be slowed down by being passed through transparent materials – by around 33 per cent in the case of glass. Even so, it still zooms through glass around 50,000 times faster than sound waves.

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