There could be a way to create black holes right here on Earth. If it works, says Michael Brooks, the holes will reveal some of the Universe's deepest secrets
One day soon, a courier will knock gingerly on the door of a physics lab and hand over a box labelled "Black Hole Kit: Handle With Care". Then he'll probably run. So perhaps the box should also bear the reassuring words of the device's inventor, Ulf Leonhardt: "The concerned reader should note that optical black holes are safe." No one, repeat, no one, is going to be sucked into oblivion.
Leonhardt and his colleague Paul Piwnicki at the Royal Institute of Technology in Sweden have devised a way to create black holes in the lab. Inside their machines, light will be sucked in, never to return.
These strange devices may uncover some even stranger natural laws. They might prove that the only thing able to escape from black holes, Hawking radiation, really exists. They might also provide an experimental stage for quantum gravity, the attempt to unite quantum mechanics with Einstein's general theory of relativity. This synthesis has evaded physicists for decades, and it could lead to still grander theories that unite all the forces of nature.
Real black holes suck in light using their ultrapowerful gravity. Any object with mass distorts space-time, creating a kind of depression in the cosmos, and something passing through this depression feels a pull towards the centre, rather like a golf ball passing over the edge of a hole. If it is going slowly, the ball falls into the hole; if it is going fast enough, it is only deflected. But black holes are so massive, and the depressions they create are so deep, that nothing goes fast enough to escape them. Even if you are moving at the speed of light, to stray beyond a frontier called the event horizon is a bad idea. You will be sucked into the hole.
Being the fastest thing in the Universe, light is the hardest thing for even a black hole to grab. So how can it be sucked in by a machine in the lab? Creating a real black hole would not only be incredibly difficult, it would be foolhardy-you don't want to be sucked into a device of your own making. Leonhardt and Piwnicki have come up with a simple answer: instead of trying to capture light when it's going full tilt, they plan to slow it down. A lot.
The speed of light is constant in a vacuum, but it changes when the light travels through another medium. In water, for instance, light goes at roughly three-quarters of its speed in empty space, slowing to 220 million kilometres per second. That's still pretty quick, but in the past year researchers have done much better: 8 metres per second in a vapour of rubidium atoms (New Scientist, 20 February 1999, p 10), and as slow as 50 centimetres per second-less than walking pace-in a special, ultracold state of matter known as a Bose-Einstein condensate.
And as Leonhardt and Piwnicki have shown (Physical Review Letters, vol 84, p 822), when a light-slowing medium moves, it can pull the light along with it. Light facing a fast enough headwind will go backwards. "If the velocity of light is low compared with the velocity of the medium, then the motion of the medium is overwhelming," Leonhardt says.
Blowing light around isn't quite the same as sucking it in. But Leonhardt and Piwnicki believe that if they created a swirling vortex in a medium like this, it would actually swallow light.
On Earth, the nearest things we have to black holes are vortices. Tornadoes, for example, can suck up trees, roofs and trucks. Their power comes from the low pressure in the centre of the vortex. And according to Leonhardt and Piwnicki's calculations, a vortex should apply the same sort of inward force to light.
If the vortex rotates much faster than the light can move, any ray that strays too close to its centre will get caught and dragged inexorably inwards (see Diagram). The light will eventually be absorbed by the gas. So just like a real black hole, the vortex has an event horizon beyond which escape is impossible.
"This is a really exciting idea," says Lene Hau of the Rowland Institute for Science in Massachusetts and of Harvard University. Hau led the team that first slowed light down to a pedestrian pace in a Bose-Einstein condensate. Within a couple of months, she hopes to slow light down to just a centimetre per second.
But condensates have a few drawbacks. To prevent light escaping, the material in a vortex must move much faster than the light within it. Even when light speed is just a centimetre per second, the vortex would only begin to suck in light when it moves at 2 metres per second. However, quantum vortices generally try to minimise their angular momentum by splitting into several slower vortices. So it would be difficult to create a single vortex that spins fast enough. "I'm already raising my eyebrows at two metres per second-that's quite a bit," Hau says. Setting the trap
What's more, if you spin a condensate rapidly, all the gas is squeezed out towards the edges, leaving a hollow core. With most gases, this "eye of the hurricane" would be wider than the event horizon, leaving no condensate at the centre to trap the light. Making black holes out of condensates begins to look difficult.
An ordinary gas might work better. "If you use a classical gas, you can accelerate it to large velocities and create classical vortices," Leonhardt says. A spinning bath of rubidium atoms kept at about 100 íC might just do the trick, he believes. Researchers have already managed to slow light to 8 metres per second in this kind of system. To trap light travelling at this speed, you need a vortex spinning at more than 300 metres per second. That's pretty fast, but not impossible to achieve.
Unfortunately, these high velocities present a problem of their own. Hau first has to prime her gas by firing in a laser beam of a precise frequency. That puts the atoms into a special quantum state that allows the gas to slow light of another frequency. But because of the Doppler shift caused by a rapidly spinning vortex, atoms will see the laser beam's frequency rise and fall as they move back and forth. "If you start to get large Doppler shifts, you might move outside the right bandwidth," says Hau. The the priming wouldn't work.
But Leonhardt isn't unduly concerned. If this does turn out to be a problem, he says, you could tune the laser so it worked just in the interesting region near the event horizon.
Even if all these problems can be overcome, Matt Visser, a physicist at Washington University, St Louis, thinks that Leonhardt and Piwnicki's prototype will need some tweaking before it is accepted by the relativity community as a proper black hole. He says that they need to increase its sucking power by making the medium flow towards the centre, as well as around it."They've got the basic structure right, and it's relatively straightforward to modify this model so you do get a proper black hole," Visser says.
Leonhardt disagrees with Visser's calculations, but admits that inward gas motion would help. In his simple spinning vortex, light approaching from one side of the hole-into the wind-is swallowed much more easily than light approaching on the other side, which gets whipped around faster by the swirling gas (see Diagram). It would be very hard to make such a vortex eat light on both sides.
The solution may be as simple as pulling the plug. If gas is pumped out of the centre of the vortex, the rest of it will move towards the middle, dragging the light with it. You would be sucking up light with a vacuum cleaner.
Whatever the eventual solution, Leonhardt believes that optical black holes are about five years of experimenting away. All those years of work will prove worthwhile if they help reveal the secrets of quantum gravity.
The two greatest theories of the 20th century are quantum mechanics, which describes how particles interact with each other via electromagnetic and nuclear forces, and Einstein's general relativity, which describes how space is bent by matter and energy, and how that produces the force of gravity. Blending the two theories to create a quantum theory of gravity has proved a mathematical nightmare.
Scientists need a theory of quantum gravity to describe the very beginning of the Universe, when matter was incredibly dense. But with the tenuous matter around us now, gravity is desperately weak on quantum scales. No one has devised a way to measure its effects.
Leonhardt points out that in an optical black hole, light experiences a kind of gravitational field-and a very strong one. In an optical black hole, the swirling gas tells space how to bend, at least as far as a beam of light is concerned. "This could be used for making predictions in quantum gravity," Leonhardt suggests.
Visser is also hopeful. "It gives you a realistic hope for experimentally testing at least part of the final theory of quantum gravity," he says. "That would be a vast improvement over the current situation."
There is already one famous predic-tion in this field just waiting to be tested. Back in 1974, Stephen Hawking showed that black holes shine. Accord-ing to quantum field theory, pairs of particles constantly pop into existence, recombine and disappear again. These "virtual" particles live on borrowed energy, and they can't exist for very long. But if the particle pair happens to be born just above the event horizon of a black hole, gravity can rip the pair apart. One of the particles falls into the hole while the other half gains some energy, allowing it to zoom off into the cosmos. Leonhardt compares the black hole to an amplifier, boosting vacuum noise into a real signal.
No one has been able to confirm Hawking's prediction. The black holes we know about are too big to produce a measurable effect. A big hole has gentler gravitational forces at its event horizon than a small one, so it produces less radiation. Microscopic black holes would be bright enough, and they might have been produced in the early Universe, but none has yet been seen.
An optical black hole should produce Hawking radiation of its own, as pairs of virtual photons are dragged apart by the flow. Leonhardt is now trying to work out whether it will be detectable or not. He says he is convinced that Hawking must be right, but it would be very satisfying to be able to prove it.
"It would be very significant," agrees Visser, a specialist in relativity, black holes and quantum gravity. "Unless we're lucky and find a microscopic black hole left over from the big bang, the only way we're going to be able to test Hawking radiation is through something like this."
This much could be done with an optical black hole made from classical gas. And if physicists succeed in making holes using condensates, another aspect of quantum gravity could be laid bare. In an optical black hole based on quantum flows, the effective curvature of space would be quantised too.
No one knows exactly what this quantisation will do to gravity. One way to measure it might be to see what Hawking radiation does to the vortices themselves-whether they dissipate, as real black holes are supposed to do. "We would be seeing something similar to the quantum structure of a gravitational object," says Leonhardt.
Leonhardt is confident that his optical black holes could provide a new way of tackling the most crucial questions in physics. His device might darken the lab, but our view of the Universe could become a lot clearer.
Michael Brooks is a science writer based in Sussex.
New Scientist issue: 18th March 2000
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