Light Revs Up in Reverse

May 24, 2006 — Light does something weird when you shift it into reverse – it moves faster than the speed of light.

Two research teams independently reported the counter-intuitive behavior, observed in two very different experiments, in the May 12 issue of Science.

"This is just so mind-boggling that it cries out for attention," said optics professor Robert Boyd of Rochester University in New York. He and his colleagues triggered the odd trick using optical fiber laced with the element erbium, the same kind of fiber used in telecommunications.

In their experiment, Boyd and his team split a burst of laser light, sending one beam through the optical fiber and allowing the other to travel without interference, for comparison. Oddly enough, the first beam’s peak exited the far end of the fiber way ahead of the peak of comparison pulse.

Odder still, the exiting peak escaped even before the orginal peak had entered the fiber. What had happened was that as soon as the leading edge of the original pulse entered the fiber, the fiber instantly cloned an identical pulse at the far end. That cloned peak exited before the rest of the orginal had been introduced.

At the same time, another cloned pulse fired backward through the fiber to cancel out the orginal. So ultimately, one pulse entered and one emerged, but with timing that appeared to violate light's natural speed limit.

To test if the pulse was traveling backward as it seemed, the team shortened the fiber a few inches at a time and repeated the experiment. By adding the experiments together in sequence, they watched the beam's backward progression.

The fiber itself, they concluded, instantly reconstructed the pulse at the far end, simultaneously sending another pulse backward.

In an independent experiment at the Universität Karlsruhe in Germany, researchers made a similar observation. There a team led by Gunnar Dolling sent a pulse of light through material whose unusual properties allow it to bend light in unexpected ways. Surprisingly, Dolling and colleagues witnessed the very same backward, faster-than-light behavior as the Rochester team.

"The propagation of (light) waves through dispersive media often leads to surprising or counterintuitive behavior," reported Dolling and colleagues. "Dispersion" is the ability of a material to separate light into different wavelengths, or colors.

These and other experiments with weird light are suddenly possible because of new light-dispersive materials, more sensitive technology to detect what light is doing, and because optics researchers are simply paying more attention to the strange things light does in odd materials.

"Up until several years ago people were not familiar with the concepts of slow and fast light," said Boyd.