Simulated black hole experiment backs Hawking prediction

BBC News
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A lab experiment has lent support to one of Stephen Hawking’s most important predictions about black holes.

Prof Jeff Steinhauer simulated a black hole in a super-cooled state of matter called a Bose-Einstein condensate.

In the journal Nature Physics, he describes having observed the equivalent of a phenomenon called Hawking radiation – predicted to be released by black holes.

Prof Hawking first argued for its existence in 1974.

“Classical” physics dictates that the gravity of a black holes is so strong that nothing, not even light, can escape. So Hawking’s idea relies on quantum mechanics – the realm of physics which takes hold at very small scales.

These quantum effects allow black holes to radiate particles in a process which, over vast stretches of time, would ultimately cause the black hole to evaporate.

But the amount of radiation emitted is small, so the phenomenon has never actually been observed in an astrophysical black hole.

Prof Steinhauer, from the Technion – Israel Institute of Technology in Haifa, uncovered evidence that particles were spontaneously escaping his replica black hole.

Furthermore, these were “entangled” (or linked) with partner particles being pulled into the hole – a key signature of Hawking radiation.

The Bose-Einstein condensate used in the experiment is created when matter, in this case a cloud of rubidium atoms inside a tube, is cooled to near the temperature known as absolute zero, -273C.

In this environment, sound travels at just half a millimetre per second. By speeding up the atoms partway along the tube, to faster than that speed, Prof Steinhauer created a sort of “event horizon” for sound waves.

It was packets of sound waves, called “phonons”, that played the part of entangled particles on the fringe of a black hole.

The findings do not help answer one of the trickiest puzzles about black hole physics: the Information Paradox.

One of the implications of Hawking’s theory is that physical information – for example, about properties of a sub-atomic particle – is destroyed when black holes emit Hawking radiation.

But this violates one of the rules of quantum theory.

Toby Wiseman, a theoretical physicist at Imperial College London, told BBC News: “Analogues are very interesting from an experimental and technological point of view. But I don’t think we’re ever going to learn anything about actual black holes [from these simulations]. What it is doing is confirming the ideas of Hawking, but in this analogue setting.”

Dr Wiseman, who was not involved with the research, compared idea of the Bose-Einstein condensate simulation to water in a bathtub.

“It relies on the fact that there’s a precise mathematical analogue between the physics of particles near black holes and ripples in flowing fluids… It’s an elegant idea that goes back some way.

“If you pull the plug in a bath, you create a flow down the plug, and the ripples on the the water get dragged down the plughole. The flow gets quicker as it gets toward the plughole and if you have a system where the flow is going faster than the speed of the ripples, when those ripples flow past some point near the plughole, they can never come back out.”

Dr Wiseman said this point was equivalent to the event horizon – the point of no return for matter being drawn in by the gravity of a black hole.

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