Summary: Scientists have long believed nature has a speed limit. It's the speed of light -- 186,000 miles (299,300 kilometers) per second. And the principle that nothing can go faster would mean most science-fiction tales of interstellar travel are impossible. At that speed, it would take us many generations to reach even the closest galaxies. Now, however, physicists are coming closer to finding out how, in some situations, light may actually travel faster than that.
By Jack Lucentini
Scientists have long believed nature has a speed limit.
It's the speed of light -- 186,000 miles (299,300 kilometers) per second. And the principle that nothing can go faster would mean most science-fiction tales of interstellar travel are impossible. At that speed, it would take us many generations to reach even the closest galaxies.
Now, however, physicists are coming closer to finding out how, in some situations, light may actually travel faster than that. But there are doubts as to whether the discoveries will have any practical use. An experiment by physicists in Florence, Italy provided what some call the clearest demonstration to date that light can indeed break its own speed limit.
"It's very counterintuitive, but under certain circumstances you can have light travel in a vacuum in a mode which is faster than light," said Raymond Chiao, a physics professor at the University of California at Berkeley.
Light adores a vacuum
The study, published in the May 22 issue of the journal Physical Review Letters, was the first to show light apparently behaving this way in a vacuum -- that is, empty space -- experts said.
Several previous experiments had involved shooting light beams through a thin piece of material. The light waves, while crossing through the material, were slightly distorted in a way that forced them to arrive ahead of schedule.
The new experiment, led by Anedio Ranfagni of the Italian National Research Council, involved no such barriers. It involved shining light beams at a mirror, curved like the inside of a bowl. The mirror shot the beams back toward an instrument that measured the speed of the rays. The beam at the "axis" of the set-up -- that is, coming from the center of the mirror -- was clocked going at between about 5 and 7 percent above light speed.
However, the authors said, this effect only works over relatively short distances, like the 1 meter (a bit over a yard) covered in their set-up. Most physicists, however, say that while a beam may travel faster than light, such a beam cannot carry a signal, that is, information. A signal is a sudden change in the character of light waves within the beam -- for example, a change in the wavelength, the length of the wave.
That's a key point for two reasons. First, if the light beam cannot contain a signal, it has no special practical use. Second, it rescues our common-sense understanding of causes and effects. That's because tried-and-tested physical equations show that if signals could outpace light speed, then causality would be reversed -- the results of an event would happen before the event itself.
In the study, the authors themselves don't claim the beams they created contain a signal.
Nevertheless, the effects seen in the experiment are "promising candidates" for future research, the authors said. They added that the work "strongly simplifies the problem" of creating faster-than-light phenomena, by eliminating some of the complexities of previous experiments. Chiao, who led some of the previous research, claimed there is a limitation to both his own work and that of the Italian physicists, which lends a quasi-illusory character to all their results.
If you shine a searchlight into space and spin it very quickly, the beam's far reaches will slice through space at faster-than-light speed. But there is no actual object moving at that speed.
The situation is somewhat similar for faster-than-light experiments to date, Chiao said. The beams contain a mathematical entity, called modes -- the patterns by which the waves hang together -- that outpace light. But there is no physical entity that does so.
"In one sense, it's accurate," he said. "In another, it's very misleading."