DIY black holes could supply the missing link
Jan 24, 2001
The gulf between relativity and quantum mechanics - which has intrigued physicists for decades - has proved remarkably difficult to close. But now physicists believe they may be able to bridge the gap by studying tiny 'black holes' created in the laboratory. Around 70 physicists gathered at The Royal Institution in London last week to debate what these new techniques may reveal about astrophysical black holes and the structure of the universe itself.
When a certain type of star collapses into itself, the gravitational force is so strong that even light waves cannot escape - hence the expression 'black hole'. In the 1980s, William Unruh of the University of British Columbia in Canada realised that sound waves in fluids can behave similarly to light waves in a gravitational field. Moreover, if a fluid travels faster than the light or sound waves within it, it should be possible to create an artificial black hole inside the fluid. The point at which the fluid speed overtakes the wave speed is equivalent to the event horizon of a true black hole - the 'point of no return' for energy and matter.
Ulf Leonhardt and Paul Piwnicki at the University of St Andrews in the UK plan to make artificial black holes using both light and sound waves. An 'optical' black hole may be created when laser light is 'slowed down' in a vapour of very cold atoms. Neil Turok of the University of Cambridge likens an optical black hole to a collection of mirrors: "The light will only be totally absorbed if you shine your torch in the right direction", he told PhysicsWeb. 'Sonic' black holes, on the other hand, would exploit the unusual properties of a fluid known as a Bose-Einstein condensate. Peter Zoller's team at the University of Innsbruck in Austria has devised a method to forcing the condensate through a nozzle to make it flow faster than the speed of sound. "Sonic black holes would be easier to create and will be more analogous to astrophysical black holes because the waves are completely trapped", says Turok.
In the 1970s, Stephen Hawking of Cambridge University predicted that quantum effects at the event horizon of a black hole should release radiation into space. Astrophysicists have so far failed to observe this 'Hawking radiation' against the noise of the cosmic background radiation - and therefore to relate the large-scale structure of the Universe to the subatomic world. But it is likely that artificial black holes will exhibit a similar effect and this is a major goal of the new research.
But Turok cautions that we should not read too much into the analogy between artificial and real black holes because it is not perfect - for one thing artificial black holes do not involve gravity and distortions of space and time. "Nevertheless, this is a very exciting field", he says, "and I'm sure something important will come out of it - even if it isn't a true black hole".