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The Paradox of Reality


Light Travels Faster than the Speed of Light


Scientists, who don't understand it, are quick to point out what it doesn't
mean.

In a landmark experiment, scientists have broken the cosmic speed limit,
causing a light pulse to travel at many times the speed of light – so fast
that the peak of the pulse exited a specially prepared test chamber before it
even finished entering it.

That 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.

The new results are almost precisely the reverse of a celebrated experiment
reported last year, when Lene Hau, now at Harvard University, together with
Stanford physicist S.E. Harris and others, created an ultra-cold gas of
sodium atoms that reduced the speed of a light pulse to an amazing 38 miles
per hour – slow enough to be honked at on the Beltway.

The new work by Wang, A. Kuzmich and A. Dogariu is also very different from
other methods used recently to exceed the light-speed limit. Raymond Y. Chiao
of the University of California at Berkeley, Aephraim M. Steinberg of the
University of Toronto and others have shown that units of light, called
photons, that "tunnel" through a mirror or opaque barrier apparently do so at
about 1.7 times the speed of light.

However, physicist Jon Marangos of Imperial College in London writes in a
companion commentary that "the light pulses have always been distorted in the
process, so interpreting these experiments has been difficult."

The NEC results, experts emphasized, do not violate the fundamental law of
causality: Namely, that an effect cannot occur before its cause. Such an
irrational outcome is captured in a famous limerick:

There was a young lady named Bright,
Whose speed was far faster than light;
She set out one day,
In a relative way,
And returned home the previous night.

That is not the case in the NEC experiment, the researchers said, because all
the effects are explainable by traditional theories of wave behavior. The
initial pulse is plainly the cause of the reconstituted pulse, even if the
latter travels faster.

Although the pulse in the new experiment clearly exceeded the speed of light
in a vacuum, it did not convey any information – thus leaving intact the
belief of virtually all experts that no meaningful signal or energy can
exceed light speed.

The NEC experiment, said Steinberg of Toronto, "comes closer than any
previous work at violating the thing about energy," but "of course, it's
still not true." And although the leading edge of the pulse emerges from the
chamber in 1/300th the time that it would have taken in a vacuum, no
information was actually conveyed.

For genuine information transfer, he said, "you can't talk about just a
single pulse. You need two things that can be distinguished, even in
principle – two different states."

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."
The Washington Post, July 20, 2000 ©
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