Radiation trickling from sonic black hole

Black holes aren’t completely black – or so says Stephen Hawking. The famed theoretical physicist, in the 1970s, postulated that even though a black hole’s immense gravitational pull is so strong it drags in light, a little trickle of radiation manages to escape, meaning it will eventually shrink and disappear.

Some 40 years later and no one’s seen this Hawking radiation in real life. But Jeff Steinhauer, a physicist at Technion-Israel Institute of Technology, reports spotting a version of it in the lab – dribbling from a black hole of sorts he created himself.

He published his observations in the journal Nature Physics.

Hawking radiation, its namesake stated, arises because a black hole’s event horizon – where not even light can escape – separates quantum-entangled particles that appear from energy fluctuations.

Normally, such entangled particles, such as photons, almost immediately annihilate each other. But what if one should find itself on one side of the event horizon with its mate on the other before they can mutually destruct?

One photon, it’s thought, would disappear into the black hole and the other emitted into space as Hawking radiation. Over time, this slow leak would cause the black hole to shrink and eventually disappear altogether.

Unsurprisingly, the photons bouncing out of the black hole would be few and far between – a signal so weak it would be next to impossible to detect in real life.

So in the 1980s, Canadian physicist William Unruh suggested scientists build and observe a black hole analogue in the lab, but where sound replaces light. Instead of photons, packets of light, use phonons – packets of sound.

And this is what Steinhauer did. His artificial sonic black hole consisted of a Bose-Einstein condensate – a dilute gas of rubidium atoms, cooled to temperatures only a smidge above absolute zero or -273.15 C, and trapped in a tube.

At such low temperatures, the atoms barely move and the speed of sound brakes to just half a millimetre per second – a tiny fraction of the speed of sound at sea level, 340 metres per second.

Steinhauer fashioned an acoustic event horizon by nudging the atoms with a laser until some were moving at roughly one millimetre per second – faster than the speed of sound in the tube. This meant any phonons on the supersonic side of his acoustic event horizon would be trapped there.

The energy fluctuations that give rise to entangled photons in space did so in Steinhauer’s Bose-Einstein condensate too, but as entangled phonons. And as they appeared and were split up by the acoustic event horizon, he took pictures.

He repeated the experiment 4,600 times. After over six days of continuous measurement, he saw phonons on one side of the event horizon were matched with a phonon equally spaced on the other side, but with the opposite energy – the paired, entangled radiation like Hawking’s prediction.

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