Black hole recipe: Slow light, swirl atoms

Physicists may soon create artificial black holes in the laboratory, analogous to the ones expected to lurk in distant space. A new study by a pair of theorists in Sweden describes how swirling clouds of atoms could slug down all nearby light, making them as black as their astronomical cousins.

Computer-generated plot shows paths of light rays sucked into an optical black hole. Leonhardt and Piwnicki/Physical Review A

Called optical black holes, these eddies could provide an extraordinary test-bench for the theory of general relativity, which gave rise to the concept of gravitational black holes, the researchers say. Ulf Leonhardt and Paul Piwnicki of the Royal Institute of Technology in Stockholm find that the same mathematics describes both the terrible tug of an astronomical black hole on light and the gentle corralling of rays by an atom vortex.

“We were quite surprised that it worked that well,” Piwnicki says. “We’re still working on it to understand it more deeply,” he adds. The researchers report their findings in the Jan. 31 Physical Review Letters and the December 1999 Physical Review A.

The laboratory analogy goes only so far, however. Black holes out in space are massive remnants of collapsed stars that pull in not just light but everything else in their vicinity. By contrast, the proposed atomic whirlpools would have too little gravity to swallow any matter.

Tiny tornadoes within wispy clouds of gas, they would snag photons through their remarkable ability to slow light pulses. The proposed mechanism by which such a vortex would capture light rests on principles discovered in the 1800s. Many substances, such as water or glass, retard light as it passes through them. Consequently, a fluid flow can drag light along with it.

Leonhardt and Piwnicki show that, theoretically, an eddy can trap a beam if it circulates faster than the speed of light in the liquid, just as fish can be trapped in a whirlpool that’s rotating faster than they can swim. So far, however, no material slows down light enough for a vortex’s velocity to exceed the radiation’s reduced pace.

That may change soon. Last year, Lene V. Hau, now of Harvard University, and her colleagues used a stationary, laser-manipulated atom cloud to limit light to an astoundingly sluggish 17 meters per second—roughly bicycle speed (SN: 3/27/99, p. 207).

To make an optical black hole work, Leonhardt and Piwnicki estimate that light would need to crawl along still more slowly, at a mere 1 centimeter per second.

“We’re actually aiming for that, and I think it will be possible,” Hau says. Her experiments have already achieved a slowdown to 50 cm/s, she told Science News. She calls the Stockholm proposal “a very exciting idea” but cautions that aspects of the proposed vortex might prevent the black-hole effect. “There are things one must look into,” she says.

The Stockholm researchers discovered that German physicist Walter Gordon, back in the 1920s, found the same mathematical equivalence between light in a moving fluid and in a gravitational field but didn’t investigate specific patterns of flow, Piwnicki says.

Modeling swirling flows in their new study, he and Leonhardt found that photons passing near the fast-spinning optical black hole, but outside a critical radius, follow a bent course.

Those straying close spiral inexorably into the center.

Pursuing similar studies, theorist James Anglin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his coworkers are exploring links between sound waves in a moving fluid and light waves in a gravitational field. Atom clouds may also trap acoustic vibrations, creating sonic or so-called dumb, black holes, Anglin says.

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