Breaking speed barrier, light travels backwards

KENNETH CHANG | Agencies | May 17, 2006, 09.59 PM IST
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, Boyd and his colleagues demonstrated an optical fibre-a glass strand that transmits pulses of light-with a couple of odd characteristics:

A pulse of light shot into the fibre departs before it enters. Within the fibre, the pulse travels backward - and faster than the speed of light.

Perhaps most amazingly, 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," Boyd said.

In the vacuum of space, light travels at a 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, 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 travelling faster than the speed of light. Physicists hope to harness such manipulations of light to speed up optical communications.

For Boyd's trick, the scientists used an optical fibre 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 fibre.

Even before the peak of the pulse entered the fibre, another pulse appeared, seemingly out of nowhere, at the far end of the fibre. This new pulse then split in two. One, a twin of the original pulse, moved forward, while the other moved backward through the fibre.

The backward pulse, which travelled faster than the speed of light, and the original pulse met at the front end of the fibre, where they cancelled each other.

NYT News Service

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