Physicists create 'black hole for sound'

An artificial black hole that traps sound instead of light has been created in an attempt to detect theoretical Hawking radiation. The radiation, proposed by physicist Stephen Hawking more than 30 years ago, causes black holes to evaporate over time.

Astrophysical black holes are created when matter becomes so dense that it collapses to a point called a singularity. The black hole's gravity is so great that nothing – not even light – can escape from a boundary around it called an event horizon.

But physicists have also been developing 'black holes' for sound. They do this by coaxing a material to move faster than the speed of sound in that medium, so that sound waves travelling within it cannot keep up, like fish swimming in a fast-moving stream. The sound is effectively trapped in the stream-like event horizon.

The team cooled 100,000 or so charged rubidium atoms to a few billionths of a degree above absolute zero and trapped them with a magnetic field. Using a laser, the researchers then created a well of electric potential that attracted the atoms and caused them to zip across the well faster than the speed of sound in the material.

This setup created a supersonic flow that lasted for some 8 milliseconds, fleetingly forming an acoustic black hole capable of trapping sound.

The implications of such work could be profound, as it could lead to the first detection of Hawking radiation.

Quantum mechanics says that pairs of particles can spontaneously appear out of empty space. These pairs, which consist of a particle and its antiparticle, should exist for a fleeting moment before they annihilate each other and disappear.

But in the 1970s, Hawking proposed that if the pair was created near the edge of a black hole, one particle might fall in before it is destroyed, leaving its partner stranded outside the event horizon. To observers, this particle would appear as radiation. In acoustic black holes, Hawking radiation would take the form of particle-like packets of vibrational energy called phonons.




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