Messages Fly No Faster Than Light

Faster-than-light tricks are old hat; in the past few years, scientists have performed "superluminal" experiments using weird quantum effects, special mirrors, and cells full of vapor. What would be really exciting--and a violation of Einstein's theory of relativity--is if someone could use these techniques to send a message faster than the speed of light. Now three American scientists have made the most well-controlled trial of sending faster-than-light messages and failed, implying that there are no loopholes in Einstein's speed limit.

The most impressive type of superluminal experiment uses a chamber of gas with a property known as "anomalous dispersion." When a pulse of light--which is made up of a bunch of superimposed waves--shines through such a chamber, the gas shifts some of the waves in such a way that the pulse appears to travel faster than the speed of light (ScienceNOW, 21 July 2000).

In the 16 October issue of Nature, Daniel Gauthier of Duke University in Durham, North Carolina, and colleagues describe experiments in which they transmitted light pulses through just such a chamber of potassium vapor. At first glance, it looks as if a light pulse travels through about 27 nanoseconds faster than the speed of light would allow. But when the scientists tried to transmit information with that speeded-up light--by tweaking the amplitude of the pulse to represent a 1 or a 0--the modification came through the chamber slower than the speed of light even as the pulse itself came through faster. "Even though you're receiving photons faster, you're not gaining information from those photons," says Gauthier. "It's consistent with special relativity."

Aephraim Steinberg, a physicist at the University of Toronto, calls the experiment "beautiful." But although mainstream scientists agree that it's not possible to use superluminal signals to transmit information, there are subtle loopholes in any experimental setup that would allow the possibility of superluminal information transfer. "I don't think you can ever completely convince the hardliners," says Steinberg. However, he adds, "they've done about as good a job as you could imagine doing."

Related sites
Gauthier's site
Steinberg's site