All quiet on the quantum front

January 19, 2001

Heard the one about the lab with a black hole? Caroline Davis reports on the latest in wave physics.

Nothing can escape a black hole. Its gravitational pull is so strong that all matter and energy, including light, is swallowed for ever. They are not the kind of thing you would want on your desk. But two theoretical physicists in Scotland and Sweden hope to do just that.

Ulf Leonhardt, professor of theoretical physics at St Andrews University, and his student Paul Piwnicki, based at the Royal Institute of Technology in Stockholm, believe that they will have black holes on a laboratory benchtop within four years.

Fortunately, what the scientists envisage will have the most refined and slender of appetites. For Leonhardt and Piwnicki aim to create artificial sonic and optical black holes that suck in only sound or light of particular wavelengths. The project will mimic a critical property of astronomical black holes: travel beyond a critical point, known as the event horizon, and you can never get out.

Leonhardt was inspired by the work of Sir Michael Berry, Royal Society research professor at Bristol University. Berry was looking at waves in swirling fluids and found that a vortex, such as water going down a plughole, could produce the equivalent of a quantum phenomenon known as the Aharonov-Bohm effect. Leonhardt realised this idea could be used in the creation of an artificial black hole.

A helpful analogy is to imagine a salmon swimming in a rapid stream flowing towards a waterfall. As it approaches the waterfall, the fish reaches a point where the flow of the water is faster than it can swim, and from this point on - as if it had passed the waterfall's event horizon - it is doomed. Sound waves in fluids obey the same mathematical laws as light in gravitational fields. Leonhardt reasoned that when the flow of the fluid reaches supersonic levels, a sonic black hole is formed.

Fundamental to his plans is a recently discovered state of matter, a Bose-Einstein condensate. This is a tiny droplet of just a few million atoms, cooled almost to absolute zero (about -3C), in which all the atoms are in the same quantum state and the substance has zero viscosity. Its unique properties make it perfect for Leonhardt's scheme.

He plans to load a Bose-Einstein condensate of the metal rubidium into a ring, accelerate it and then force it through a constriction. At this point, its velocity will exceed the speed of sound and so, in theory, phonons (sound packets) will be unable to escape - thus creating a sonic black hole.

One problem is that the wave will be difficult to distinguish from the medium it travels in. Light waves are far more distinct and so would seem a more promising subject.

However, optical black holes would involve accelerating a fluid to the speed of light (300,000m/s in a vacuum as opposed to the speed of sound at 330m/s in dry air). While a new technique developed in the United States has been able to slow light to 50cm a second in an ultracold gas, Leonhardt calculates this would need to fall to 1cm a second to successfully make an optical black hole. The artificial black holes could be observed by the evaporation of the Bose-Einstein condensate, which Leonhardt believes would be the equivalent to the Hawking radiation emitted by astronomical black holes.

Leonhardt predicts that his artificial variety will model gravitational black holes in space. This will allow physicists to translate between microscopic quantum effects, observable on earth, and the macroscopic effects of general relativity. He believes this could eventually lead to the holy grail of a successful theory of quantum gravity.

However, Berry himself is more cautious: "All analogies in physics are helpful, but they have their limits. It has to capture the essence of real black holes. I think it is pushing it to say it will show the quantising of gravity." He is also concerned at the amount of technology needed to slow down light waves.

Leonhardt was due to present his work at the Royal Institution in London today. His theories will be voiced alongside those of Neil Turok, professor of mathematical physics at Cambridge University, who will talk about real black holes. This discussion may test how strong the analogy is.

The meeting could be crucial to the Engineering and Physical Sciences Research Council's decision on whether to fund the work. Despite his reservations, Berry believes that it would be wrong to dismiss Leonhardt's research. "Any work on waves is interesting," he said. "It is a new area of classical wave physics, as worthy as any other classical physics."

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