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Subject: "Gain Assisted Superluminal Light Propogation"...The Speed of Light Is Exceeded


Gain Assisted Superluminal Light Propogation


The Speed of Light Is Exceeded in Lab


overview of work by Dr. Lijun Wang


Scientists have apparently broken the universe's speed limit. For


generations, physicists believed there is nothing faster than light


moving through a vacuum - a speed of 186,000 miles per second. But in


an experiment in Princeton, N.J., physicists sent a pulse of laser


light through cesium vapor so quickly that it left the chamber before


it had even finished entering. The pulse traveled 310 times the


distance it would have covered if the chamber had contained a




This seems to contradict not only common sense, but also a bedrock


principle of Albert Einstein's theory of relativity, which sets the


speed of light in a vacuum, about 186,000 miles per second, as the


fastest that anything can go.


But the findings--the long-awaited first clear evidence of faster-


than-light motion--are "not at odds with Einstein," said Lijun Wang,


who with colleagues at the NEC Research Institute in Princeton, N.J.,


report their results in today's issue of the journal Nature.


"However," Wang said, "our experiment does show that the generally


held misconception that 'nothing can move faster than the speed of


light' is wrong." Nothing with mass can exceed the light-speed limit.


But physicists now believe that a pulse of light--which is a group of


massless individual waves--can.


To demonstrate that, the researchers created a carefully doctored


vapor of laser-irradiated atoms that twist, squeeze and ultimately


boost the speed of light waves in such abnormal ways that a pulse


shoots through the vapor in about 1/300th the time it would take the


pulse to go the same distance in a vacuum.


As a general rule, light travels more slowly in any medium more


dense than a vacuum (which, by definition, has no density at all).


For example, in water, light travels at about three-fourths its


vacuum speed; in glass, it's around two-thirds.


The ratio between the speed of light in a vacuum and its speed in a


material is called the refractive index. The index can be changed


slightly by altering the chemical or physical structure of the


medium. Ordinary glass has a refractive index around 1.5. But by


adding a bit of lead, it rises to 1.6. The slower speed, and greater


bending, of light waves accounts for the more sprightly sparkle of


lead crystal glass.


The NEC researchers achieved the opposite effect, creating a gaseous


medium that, when manipulated with lasers, exhibits a sudden and


precipitous drop in refractive index, Wang said, speeding up the


passage of a pulse of light. The team used a 2.5-inch-long chamber


filled with a vapor of cesium, a metallic element with a goldish


color. They then trained several laser beams on the atoms, putting


them in a stable but highly unnatural state.


In that condition, a pulse of light or "wave packet" (a cluster made


up of many separate interconnected waves of different frequencies) is


drastically reconfigured as it passes through the vapor. Some of the


component waves are stretched out, others compressed. Yet at the end


of the chamber, they recombine and reinforce one another to form


exactly the same shape as the original pulse, Wang said. "It's called




The key finding is that the reconstituted pulse re-forms before the


original intact pulse could have gotten there by simply traveling


though empty space. That is, the peak of the pulse is, in effect,


extended forward in time. As a result, detectors attached to the


beginning and end of the vapor chamber show that the peak of the


exiting pulse leaves the chamber about 62 billionths of a second


before the peak of the initial pulse finishes going in.


That is not the way things usually work. Ordinarily, when sunlight--


which, like the pulse in the experiment, is a combination of many


different frequencies--passes through a glass prism, the prism


disperses the white light's components.


This happens because each frequency moves at a different speed in


glass, smearing out the original light beam. Blue is slowed the most,


and thus deflected the farthest; red travels fastest and is bent the


least. That phenomenon produces the familiar rainbow spectrum.


But the NEC team's laser-zapped cesium vapor produces the opposite


outcome. It bends red more than blue in a process called "anomalous


dispersion," causing an unusual reshuffling of the relationships


among the various component light waves. That's what causes the


accelerated re-formation of the pulse, and hence the speed-up


In theory, the work might eventually lead to dramatic improvements


in optical transmission rates. "There's a lot of excitement in the


field now," said Steinberg. "People didn't get into this area for the


applications, but we all certainly hope that some applications can


come out of it. It's a gamble, and we just wait and see."


Visit the NEC Research Institute at http://www.neci.nj.nec.com/neci-





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