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Impressive New Tricks of Light, All Within the Laws of Physics
By KENNETH CHANG
Published: May 16, 2006

To make light go backward, hold up a mirror. Light bounces off the mirror and goes back.

Robert W. Boyd, a professor of optics at the University of Rochester, however, did not choose this easy, straightforward technique.

Instead, in the latest example of logic-defying tricks that physicists can now perform with light, Dr. Boyd and his colleagues demonstrated an optical fiber — a glass strand that transmits pulses of light — with a couple of odd characteristics:

¶A pulse of light shot into the fiber departs before it enters.

¶Within the fiber, the pulse travels backward — and faster than the speed of light.

Perhaps most amazingly, Dr. Boyd's results do not violate any law of physics. The effect is indeed predicted by the equations describing the propagation of waves.

"This is a good example of something which is very counterintuitive that the laws of nature permit," Dr. Boyd said.

An article describing the experiment appears in the current issue of the journal Science.

In the vacuum of space, light travels at a constant 186,282 miles per second. When it passes through a transparent material like glass or water, it slows slightly, in effect bouncing off atoms as it moves.

In 1999, physicists led by Lene Vestergaard Hau of Harvard slowed the speed of light to a leisurely 38 miles per hour by shining it into an exotic ultracooled material known as Bose-Einstein condensate.

Two years later, Dr. Hau's group, as well as a second team of scientists at the Harvard-Smithsonian Center for Astrophysics, were able to bring light to a standstill — and then release it with its original properties intact.

In other experiments, scientists have shown that it is possible to make light at least appear as if it is traveling faster than the speed of light. Physicists hope to harness such manipulations of light to speed up optical communications.

For Dr. Boyd's trick, the scientists used an optical fiber of glass with small amounts of the metal erbium, which acts as an amplifier. In the experiment, a pulse of laser light was fired into the fiber. Even before the peak of the pulse entered the fiber, another pulse appeared, seemingly out of nowhere, at the far end of the fiber.

This new pulse then split in two. One, a twin of the original pulse, moved forward, while the other moved backward through the fiber.

The backward pulse, which traveled faster than the speed of light, and the original pulse met at the front end of the fiber, where they canceled each other.

Even though one pulse momentarily became three, the experiment did not violate the law mandating conservation of energy because the amplifying effect of the erbium added a temporary surge of energy.

At first glance, the experiment appears to flout the usual speed limit on the transmission of signals as the original pulse jumped to the forward-moving pulse on the other side of the fiber.

However, the pulses were in a shape known as Gaussian, which is, in principle, infinite in width, though in practice not quite that wide.

Thus, the outgoing pulse was actually just part of the original pulse that was reshaped by the fiber's unusual properties.

"It's really kind of showing the kind of manipulation of light we can do these days," Dr. Hau said of Dr. Boyd's experiment.

Dr. Boyd said this effect might find some application in speeding up optical communications, but it is, for now, mostly just an impressive trick of physics. "I find it neat," he said. "I find it nifty."

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