Speed of Light No Longer the Limit

July 19, 2000 -- Scientists have apparently broken the universe’s speed limit.

For generations, physicists believed there is nothing fasterthan light moving through a vacuum — a speed of 186,000 miles persecond.

But in an experiment in Princeton, N.J., physicists sent a pulseof laser light through cesium vapor so quickly that it left thechamber before it had even finished entering.

The pulse traveled 310 times the distance it would have coveredif the chamber had contained a vacuum.

Researchers say it is the most convincing demonstration yet thatthe speed of light — supposedly an ironclad rule of nature — can bepushed beyond known boundaries, at least under certain laboratorycircumstances.

“This effect cannot be used to send information back in time,”said Lijun Wang, a researcher with the private NEC Institute.“However, our experiment does show that the generally heldmisconception that ‘nothing can travel faster than the speed oflight’ is wrong.”

The results were published in Thursday’s issue of the journalNature.

The achievement has no practical application right now, butexperiments like this have generated considerable excitement in thesmall international community of theoretical and opticalphysicists.

Previously Thought Impossible“This is a breakthrough in the sense that people have thoughtthat was impossible,” said Raymond Chiao, a physicist at theUniversity of California at Berkeley who was not involved in thework. Chiao has performed similar experiments using electricfields.

In the latest experiment, researchers at NEC developed a devicethat fired a laser pulse into a glass chamber filled with a vaporof cesium atoms. The researchers say the device is sort of a lightamplifier that can push the pulse ahead.

Previously, experiments have been done in which light alsoappeared to achieve such so-called superluminal speeds, but thelight was distorted, raising doubts as to whether scientists hadreally accomplished such a feat.

The laser pulse in the NEC experiment exits the chamber withalmost exactly the same shape, but with less intensity, Wang said.

The pulse may look like a straight beam but actually behaveslike waves of light particles. The light can leave the chamberbefore it has finished entering because the cesium atoms tradeenergy with the leading edge of the waves as they pass through.

This produces an almost identical light pulse that exits thechamber and travels about 60 feet before the main part of the laserpulse finishes entering the chamber, Wang said.

Wang said the effect is possible only because light has no mass;the same thing cannot be done with physical objects.

The Princeton experiment and others like it test the limits ofthe theory of relativity that Albert Einstein developed nearly acentury ago.

According to the special theory of relativity, the speed ofparticles of light in a vacuum, such as outer space, is the onlyabsolute measurement in the universe. The speed of everything else— rockets or inchworms — is relative to the observer, Einstein andothers explained.

Only in the LabIn everyday circumstances, an object cannot travel faster thanlight.

The Princeton experiment and others change these circumstancesby using devices such as the cesium chamber rather than a vacuum.

Ultimately, the work may contribute to the development of fastercomputers that carry information in light particles.

Not everyone is convinced the NEC scientists did what theyclaim.

Aephraim Steinberg, a physicist at the University of Toronto,said the light particles coming out of the cesium chamber may nothave been the same ones that entered, so he questions whether thespeed of light was broken.

Still, the work is important, he said: “The interesting thingis how did they manage to produce light that looks exactly likesomething that didn’t get there yet?”