A Pulse of Light Breaks the Ultimate Speed Limit
The textbooks all say the same thing: Nothing can move faster than light shooting through a vacuum. The textbooks, it seems, could be wrong.
A new experiment published in today’s issue of the journal Nature has found an exception to the rule, showing that a pulse of light can apparently break the ultimate speed limit of 186,282 miles per second. A speeding light pulse can bend the laws of common sense--but not relativity--by exiting a chamber before it enters it, the experiment indicates.
If you’re mystified, you’re not alone. Even the lead physicist on the experiment, Lijun Wang of the NEC Research Institute in Princeton, N.J., said many of his colleagues are still puzzling about the true nature of light and over the new findings, which have been widely discussed among physicists. “There are lots of questions to be answered,” he said.
For nearly a century, physicists have believed that anything moving faster than the speed of light in a vacuum would violate the most basic notions of Einstein’s theory of relativity. Because of relativity’s relationship between speed and time, moving faster than light would, it was thought, be like taking a sneak peek into the future. That would crumble the deeply held notion of causality, that the cause of something always precedes its effect.
Now those assumptions may need to be reexamined. “People have said nothing can go faster than light. Nothing is too strong a word,” said Raymond Y. Chiao, a professor of physics at Berkeley who first predicted that light pulses could surpass the speed of light.
“The broadest message is, never trust your textbook,” said Aephraim Steinberg, a physicist at the University of Toronto who has conducted similar experiments.
The physicists are clear on one point--nothing in the new experiment actually contradicts Einstein’s theory. Ultra-fast light pulses are possible--and explainable in the world as we know it--because light, unlike matter, has no mass and can be viewed as a series of particles and as waves.
“Einstein’s theory of relativity still holds,” Wang said.
The new experiment does not mean that ordinary objects somehow can go backward or forward through time, said Chiao, an expert in the field of quantum optics, the study of the quantum nature of light. “No, you can’t kill your ancestors,” he said.
While some more fanciful minds see a future of warp speed and time travel arising from such experiments, scientists say those ideas remain fantasies. The experiments do offer hope, however, that information may one day travel across computer systems and the Internet much faster than is now possible. Currently, because of electrical effects, information travels across computer circuits at a relative crawl, moving across a tiny chip in the same time that light jumps a foot or more.
It’s agreed by most physicists that information carried by a light pulse still has a speed limit. That’s because meaningful information consists of complex signals that could not take the same “shortcut” across space as a simple light pulse.
“Unfortunately, energy and information still can’t break any speed limits,” said Steinberg.
Light pulses, however, apparently can.
Here are the details: A pulse of light is actually an ensemble of waves of different frequencies. The pulse, like a wave in the ocean, has a peak, but also a leading edge that precedes it.
Passed through very particular materials that physicists can create in their labs, light can begin to act very strangely. Physicists at Harvard and Berkeley have shown in the last year that they can slow light--in one case down to the speed of an Olympic runner. While that is a feat, experiments to accelerate light beyond light speed are considered much more difficult.
In this case, the experimenters used a clever and highly artificial creation--a 2.4-inch glass cell filled with cesium vapor. They shot the cell with two laser pulses that raised each of the cesium atoms in the vapor to a specific altered energy level. A precisely tuned third laser soaked up the excess energy and shot out of the cell at a speed faster than light.
According to Wang and several other physicists, the reason the light behaves the way it does is that when the leading edge of that third laser pulse begins to enter the chamber, it carries with it all the information needed to reconstruct the entire light wave. That allows the cesium atoms in the chamber to spit out a light beam before the entering pulse has fully reached them.
As a result, the time needed for the pulse to move through the cell is about one three-hundredth the time it would normally take light to travel the length of the cell in a vacuum. That transit time was incredibly short: two-tenths of a billionth of a second.
The experiment was conducted by Wang, Alexander Kuzmich and Arthur Dogariu, all physicists at the NEC Institute, a basic research arm of computing giant NEC.
The latest experiment is one in a series of tests in optics research showing that light pulses can be superluminal, or move faster than the speed of light, if they are sent through special materials.
Data from the current experiment is “very nice and very clear. The effect is very dramatic,” said Steinberg, who had conducted similar research in 1993. Earlier descriptions of superluminal pulses of light date back to 1970 and include work by Stephen Chu, a Nobel-winning physicist at Stanford.
But those early experiments did not attract much attention or many converts and lay dormant for several years, Steinberg said. Some of the early experiments were not as acceptable to physicists because the light waves that were accelerated were absorbed or distorted by the equipment used in the experiments.
At least some physicists, however, remain skeptical of the experiment, which has been the topic of private mutterings among some academics who disagree with the conclusions. Several physicists who are critical of the work declined to be interviewed about their reservations. Others who are critical said they did not know the experiment in enough detail to comment publicly.
But many who have read the actual report are impressed. “This is a very clean experiment,” said Chiao. “I hope it will convince everyone that [light] pulses can travel faster than light.”
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Breaking the Speed Limit
Scientists have succeeded in pushing light pulses to go faster than the speed of light. Objects cannot move at such speeds. The feat is possible only with light because it has no mass and acts as a wave.
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Light pulses consist of waves of different frequencies. Scientists shoot a light pulse toward a glass chamber containing cesium vapor that has been primed by laser beams.
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All information about the incoming light pulse is contained in the leading edge of its waves.
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This information is all the cesium atoms need to replicate the pulse and send it out the other side.
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By the time it would normally take light to cross the 2.4-inch chamber, the exiting pulse already has traveled 60 feet.
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Source: NEC Institute
