Three years ago, researchers created a light pulse that appeared to defy nature's fundamental speed limit—it traveled faster than the speed of light in a vacuum. If it were possible to transmit information at such speeds, Einstein's theory of relativity would be in tatters, and the principle of causality—the idea that cause must always come before effect—would go out the window. With a faster-than-light telephone, you could place a call back in time and tell your parents not to conceive you, for example. Now physicists (and everyone vexed by time-travel paradoxes) can breathe a sigh of relief. A recent series of experiments by experimental physicist Dan Gauthier of Duke University confirm that the earlier result was a kind of illusion; information cannot outrun light's fastest pace.
The ruckus began in 2000, when physicist Lijun Wang of the NEC Research Institute in Princeton, New Jersey, and his colleagues beamed a pulse of light through a chamber filled with a cloud of cesium atoms and recorded how long it took for the light to emerge from the other side. In apparent disregard for Einstein's physics, the light pulse exited the chamber before the researchers saw it enter. When the peak of the light pulse entered the chamber, the different waves that made up the pulse split apart, each changing in both wavelength and frequency. As the waves exited the chamber, they recombined to form a peak identical to the one that Wang saw enter the chamber a split second later. The waves behaved as though they'd been stretched and hurled forward in time, with the gas in the chamber acting like a slingshot. But the waves had not really broken any rules—only their shape had changed. And yet, because at least part of the waves had traveled faster than the speed of light, Wang claimed that light's speed limit wasn't immutable after all.
His claims would have come as no surprise to Einstein, were he alive today. In the early part of the 20th century, Einstein worried that experiments might someday be developed to challenge the limit of the speed of light. Concerned about the paradoxes that might arise if things could travel so fast that cause and effect might be reversed, he and his cronies came up with the revised theory of special relativity, which states that no mass, pulse of information, or energy can travel faster than the speed of light. But nobody was really sure how this revised theory would affect the speed limit of a simple wave.
Wang had not claimed to have transmitted information faster than light. In fact, physicists had never clocked the maximum speed of an information-carrying beam of light. Nevertheless, many popular news stories described Wang's work as a challenge to Einstein, and many physicists also had a hard time understanding how a beam of light could escape a test chamber before it entered. "We were intrigued by the results and wondered if we could figure out how to measure the speed of information," Gauthier says.
Gauthier and his student Michael Stenner, along with Mark Neifeld of the University of Arizona, devised an experiment much like Wang's, using light pulses moving through a gas of potassium atoms. As expected, the light pulses appeared to move at faster-than-light velocities. Gauthier's real goal was clocking how fast information could travel to a given location, so he and his colleagues imprinted a simple signal on the pulse—two discontinuities that could represent the one and zero of a binary code—and watched to see when the signals came out of the chamber. Whereas Wang observed the wave peak, Gauthier focused on the wave front, the first photon of the imprinted signal on the pulse, reasoning that if the wave front did not travel faster than the speed of light, then no information within the pulse could, either. "You can have the peak of the pulse traveling faster, so it catches up," Gauthier explains. "But you can't make the pulse go faster than that very first moment."
The experiments, published in the October 16 issue of Nature, revealed that the first photon of the changed pulse inched up to the maximum speed of light but did not surpass it, even though subsequent peaks within the pulse gained on the wave front at faster-than-light speeds. The elaborate series of tests all boiled down to a simple conclusion: As usual, Einstein had been right all along.