Faster thanitself

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

In the vacuum of space, light travels at a constant 186,171 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 it is possible to make light 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 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 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 fibre. 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 fibre’s unusual properties. ‘‘It’s really kind of showing the kind of manipulation of light we can do these days,’’ Hau said of Boyd’s experiment. 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.’’

(KENNETH CHANG)