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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

 

vacuum.

 

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

 

re-phasing."

 

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-

 

website/index-page.html

 

 

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