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Clarifying Einstein’s rules of the road

“Faster-than-light” transmissions contradict no laws of physics, just the misconceptions people have about them, researchers say.
Albert Einstein, shown in a playful mood in 1951, focused on light's velocity in his theory of special relativity.
Albert Einstein, shown in a playful mood in 1951, focused on light's velocity in his theory of special relativity.

Can anything break the cosmic speed limit of 186,000 miles per second? For weeks, scientific circles have been buzzing about an experiment that pulsed light through a special chamber so fast that it left the apparatus before it fully entered it. Now the research has been released at last, and the experimenters say their findings contradict no laws of physics — just the misconceptions people have about them.

Einsteins’s theory of special relativity set the tone in 1905 with the claim that an object cannot be accelerated to beyond the speed of light in a vacuum: 186,000 miles per second, or 300,000 kilometers per second.

Since then, the light-speed threshold has served as the inspiration for such science-fiction standbys as warp drives and backward time travel — reinforcing the idea that going faster than 186,000 miles a second was pure fantasy.

However, in recent years there has been increasing evidence that 186,000 miles a second is not necessarily the limit for light transmission, depending on your definitions. The clearest evidence so far comes from an experiment conducted at the NEC Research Institute in New Jersey and described in Thursday’s issue of the journal Nature.

The experiment highlights the loopholes in relativity theory — caveats that usually don’t figure in popular discussions of what Einstein proposed. First of all, it’s important to remember that this business about 186,000 miles per second applies only to light in a vacuum, and that the speed limit applies only to objects that have mass. Since light waves are massless, physicists say that light can travel as fast as … well, as fast as light.

It’s well known that light can travel slower than 186,000 miles a second, depending on the medium it’s passing through. That medium can be as common as air, water or glass. Or it can be more exotic. For example, researchers at the Rowland Institute for Science and Harvard University say they’ve been able to slow light down to 1 mph, by passing it through a chamber containing supercooled atoms.

Speeding up a light wave
Lijun Wang, Alexander Kuzmich and Arthur Dogariu used a different kind of exotic medium in their experiment: The researchers used lasers to “pump” cesium atoms, contained in a 6-centimeter (2.4-inch) chamber, to an excited state that doesn’t occur naturally.

Then they passed a smooth light pulse, lasting about three-millionths of a second, through the chamber. The atoms in the cesium gas were in just the right state to shift the pattern of peaks and troughs in the many wavelengths that made up the light pulse.

Because of this shift, the pulse popped up on the other side of the chamber far sooner than it should have, based on the speed of light in a vacuum. In fact, the time difference — 62-billionths of a second — meant that the peak of the pulse appeared on the far side of the chamber before it entered the near side of the chamber. That’s an instance of what’s known as “negative delay” or “negative velocity,” a phenomenon that seems paradoxical. It was almost as if the light wave could figure out, on the basis of the very beginning of the pulse, how to reconstruct the full peak on the other side.

This isn’t the first time researchers have made light waves travel “faster than light,” but it’s the most clear-cut case. Other experiments, including research from Italy that was published in May, involved more manipulation of the light wave in transit.

Word of the NEC experiment leaked out weeks ago, but the researchers couldn’t discuss their findings publicly because of Nature’s embargo. Now they are free to declare that the results mesh quite well with what’s known about electromagnetism and quantum mechanics.

“Our experiment is not at odds with Einstein’s special relativity,” they said in a statement. “The experiment can be well explained using existing physics theories that are consistent with relativity. In fact, the experiment was designed based on calculations using existing physics theories.”

What it means
All this might make it sound as if the NEC researchers found a way to send a message at speeds faster than 186,000 miles a second — which could theoretically open the way for a sort of time travel.

But the researchers contend that is not the case. Their experiment dealt with smooth changes in a pulse, and “a smooth function cannot carry information,” Dogariu said. Sending information — for example, the flashes of a laser semaphore — would require sharper variations in frequency that could not be processed in the type of finely tuned atom chamber used by the NEC researchers.

This would fit the view that a cosmic speed limit may apply to the transmission of information rather than light per se — a concept that has been the subject of debate with regard to another spooky and widely misunderstood phenomenon known as quantum teleportation.

Such fine points were lost in most advance reports about the experiment. In fact, the NEC scientists have been spending much of their time trying to correct erroneous claims about their research.

“We are in the middle of a battle to save Einstein’s relativity theory from bad press interpretations of our experiment,” Dogariu said in an e-mail message.

Another physicist familiar with the experiment, Aephraim Steinberg of the University of Toronto, agreed that it was easy to misinterpret what the NEC researchers had done.

“The researchers have been very careful in the point of view that they’ve taken,” he told “Certainly in these experiments, there is something that’s faster than light or even a negative speed, which is very dramatic. But then we have to back up and interpret what’s going on.

“It’s very different from what we might want in our science-fiction dreams,” he said.

So if the NEC experiment doesn’t violate the laws of physics or show us the path to warp speed, does that mean it’s just a glorified parlor trick? Not at all, the researchers say. On one level, the experiment and others like it are shedding new light — so to speak — on some dark corners of relativity theory. Moreover, the techniques are likely to find practical application, perhaps as a way of speeding up information transfer within circuitry to 186,000 miles per second, known to physicists as “c.”

“Using this effect, one might be able to increase information transfer speed up to ‘c,’” the researchers said. “In present-day technology, information is transmitted at speeds far slower than ‘c’ in most cases, such as through the Internet and inside a computer.”