Physics & Math

STEPHEN HAWKING should be pleased. The first signs of an effect the British physicist predicted more than 30 years ago - known as Hawking radiation - have finally materialised from the simulated edge of a black hole.

Quantum mechanics tells us that entangled pairs made up of a particle and its antiparticle can spontaneously pop out of otherwise empty space, exist for a fleeting moment, and then annihilate each other and disappear. In the 1970s, Hawking predicted that if such a pair were created near a black hole's event horizon, one of its members might fall into the black hole before it could be annihilated. The partner left stranded outside the event horizon would appear to an observer to have been radiated from the black hole.

It's tough to see what's going on around real black holes, so physicists hoping to test the prediction have been trying to create artificial event horizons in the lab (New Scientist, 16 February, p 15). One promising way to mimic a black hole is to use a supercooled substance known as a Bose-Einstein condensate (BEC). If one region of the BEC is manipulated to move faster than the speed of sound, then sound waves travelling through the rest of the substance would not be able to keep up, effectively becoming trapped behind an event horizon. Hawking radiation should show at this boundary as the production of particle-like packets of vibrational energy called phonons.

Now Iacopo Carusotto at the University of Trento in Italy and his colleagues claim to have seen just that in a computer simulation of a BEC. Their model shows that phonons do appear at the event horizon - and that one member of the pair falls into the "black hole" while the other remains outside as predicted. Both phonon partners created identical density patterns in the surrounding BEC, confirming that they were entangled.

Until now, researchers studying the way BECs should behave have used approximate calculations based on the equations used to analyse real black holes. Carusotto's team, by contrast, did not assume any similarities to black holes. "In this way, our observations can be considered as [the] first independent proof of the existence of Hawking radiation," the authors write in a preprint of their paper, which has been submitted to a major journal (www.arxiv.org/abs/0803.0507).

Ralf Schützhold, an authority on artificial black holes at Dresden University of Technology in Germany, points out that the group will have to create the effect in a real BEC before they can claim to have observed Hawking radiation. He believes their simulation will help experimenters spot the signature of such an event. "Until now, we did not know a good way to measure entangled phonons, but the density signature that the group has found will help."

Quantum World - Learn more about a weird world in our comprehensive special report.

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