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Beating the Limit

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7/21/00 4:05 p.m.
Beating the Limit
Speed freaks.

By John Farrell, writer and video producer in Boston

ne year after physicists slowed a pulse of light to go a measly 38 mph, scientists — in a landmark experiment described in this week's Nature — just jacked it up to beat the Einstein limit. In short, they made light go faster than light.

According to Einstein's special theory of relativity, nothing can go faster than the speed of light (in a vacuum). Press reports have already wondered whether there's a revolution brewing because of the new findings. Is relativity about to be undermined?

Not according to Lijun Wang, one of the scientists working on the experiment at the NEC Research Institute in Princeton, who told the Washington Post's Curt Suplee yesterday (7/20) that his experiment's results are "not at odds with Einstein." Why? Because nothing with mass can go faster than light (meaning you or me in a rocket ship — or, for that matter, a baseball or a blurb of electronic information). But physicists now are convinced that a pulse of light can.

How did they do it? Basically by firing laser beams through a collection of caesium atoms. According to the BBC News Online report, Wang and his team adjusted the frequency of their laser beams to match those of the energy levels in the caesium atoms, allowing them to create an "anomalous refractive index." Light's refractive index is normally the degree of slowing it undergoes when travelling through a medium like air or water (anything other than a vacuum). Wang's "anomalous refractive index" had the effect of increasing the light pulse's "group-velocity" to a speed faster than light. (The group velocity of a light pulse depends on the mixture of frequencies within the pulse and the medium through which it travels. It does not have to be the speed of the pulse itself.)

The effect, to say the least, was interesting. The light pulse seemed to leave the chamber of caesium atoms before it entered. It wasn't much faster than light normally is, but the result was definite. Now, it's important to point out that this manner of leap-frogging the speed-of-light limit and the law of cause and effect cannot be exploited to carry information. A famous equation for time in Einstein's theory shows that if velocity is input to exceed light, it yields a negative number, negative time, implying that time would go backward. (Go to a cool animated demonstration of special relativity with Flash.

Sending information faster than light, and therefore backward in time, is not physically possible. Otherwise, we'd all be trying to do what James Caviezel did in the movie Frequency when he sent messages back in time over a ham radio to Dennis Quaid in order to help Quaid escape his own impending death, and thereby altering history, etc. etc.

A lot of fun and a great deal of science fiction has been spawned entertaining this possibility. But it does drive certain people nuts, certain web-addicted anti-relativity types stuck to their laptops behind stacks and stacks of Diet Pepsi cans and Twinkie boxes in their parents' basements or at the back of post-office delivery depots where they're supposed to be tracking USPS shipments and instead are spamming other bolt heads on the newsgroups about how they've been able to disprove Einstein by using a couple of nerf balls and their mom's cooking timers.

It will be interesting to see in future experiments how far scientists can extend the faster-than-light jump. But rest assured: It doesn't threaten the course of world history. (Yet.)

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