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- The speed at which light travels through a vacuum, about
186,000 miles per second, is enshrined in physics lore as a universal speed
limit. Nothing can travel faster than that speed, according freshman textbooks
and conversation at sophisticated wine bars; Einstein's theory of relativity
would crumble, theoretical physics would fall into disarray, if anything
could.
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- Two new experiments have demonstrated how wrong that
comfortable wisdom is. Einstein's theory survives, physicists say, but
the results of the experiments are so mind-bending and weird that the easily
unnerved are advised--in all seriousness--not to read beyond this point.
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- In the most striking of the new experiments a pulse of
light that enters a transparent chamber filled with specially prepared
cesium gas is pushed to speeds of 300 times the normal speed of light.
That is so fast that, under these peculiar circumstances, the main part
of the pulse exits the far side of the chamber even before it enters at
the near side.
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- It is as if someone looking through a window from home
were to see a man slip and fall on a patch of ice while crossing the street
well before witnesses on the sidewalk saw the mishap occur--a preview of
the future. But Einstein's theory, and at least a shred of common sense,
seem to survive because the effect could never be used to signal back in
time to change the past--avert the accident, in the example.
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- A paper on the experiment, by Lijun Wang of the NEC Research
Institute in Princeton, N.J., has been submitted to Nature and is currently
undergoing peer review. It is only the most spectacular example of work
by a wide range of researchers recently who have produced superluminal
speeds of propagation in various materials, in hopes of finding a chink
in Einstein's armor and using the effect in practical applications like
speeding up electrical circuits.
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- "It looks like a beautiful experiment," said
Raymond Chiao, a professor of physics at the University of California in
Berkeley, who, like a number of physicists in the close-knit community
of optics research, is knowledgeable about Dr. Wang's work.
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- Dr. Chiao, whose own research laid some of the groundwork
for the experiment, added that "there's been a lot of controversy"
over whether the finding means that actual information--like the news of
an impending accident--could be sent faster than c, the velocity of light.
But he said that he and most other physicists agreed that it could not.
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- Though declining to provide details of his paper because
it is under review, Dr. Wang said: "Our light pulses can indeed be
made to travel faster than c. This is a special property of light itself,
which is different from a familiar object like a brick," since light
is a wave with no mass. A brick could not travel so fast without creating
truly big problems for physics, not to mention humanity as a whole.
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- A paper on the second new experiment, by Daniela Mugnai,
Anedio Ranfagni and Rocco Ruggeri of the Italian National Research Council,
described what appeared to be slightly faster-than-c propagation of microwaves
through ordinary air, and was published in the May 22 issue of Physical
Review Letters.
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- The kind of chamber in Dr. Wang's experiment is normally
used to amplify waves of laser light, not speed them up, said Aephraim
M. Steinberg, a physicist at the University of Toronto. In the usual arrangement,
one beam of light is shone on the chamber, exciting the cesium atoms, and
then a second beam passing thorugh the chamber soaks up some of that energy
and gets amplified when it passes through them.
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- But the amplification occurs only if the second beam
is tuned to a certain precise wavelength, Dr. Steinberg said. By cleverly
choosing a slightly different wavelength, Dr. Wang induced the cesium to
speed up a light pulse without distorting it in any way. "If you look
at the total pulse that comes out, it doesn't actually get amplified,"
Dr. Steinberg said.
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- There is a further twist in the experiment, since only
a particularly strange type of wave can propagate through the cesium. Waves
Light signals, consisting of packets of waves, actually have two important
speeds: the speed of the individual peaks and troughs of the light waves
themselves, and the speed of the pulse or packet into which they are bunched.
A pulse may contain billions or trillions of tiny peaks and troughs. In
air the two speeds are the same, but in the excited cesium they are not
only different, but the pulses and the waves of which they are composed
can travel in opposite directions, like a pocket of congestion on a highway,
which can propagate back from a toll booth as rush hour begins, even as
all the cars are still moving forward.
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- These so-called backward modes are not new in themselves,
having been routinely measured in other media like plasmas, or ionized
gases. But in the cesium experiment, the outcome is particularly strange
because backward light waves can, in effect, borrow energy from the excited
cesium atoms before giving it back a short time later. The overall result
is an outgoing wave exactly the same in shape and intensity as the incoming
wave; the outgoing wave just leaves early, before the peak of the incoming
wave even arrives.
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- As most physicists interpret the experiment, it is a
low-intensity precursor (sometimes called a tail, even when it comes first)
of the incoming wave that clues the cesium chamber to the imminent arrival
of a pulse. In a process whose details are poorly understood, but whose
effect in Dr. Wang's experiment is striking, the cesium chamber reconstructs
the entire pulse solely from information contained in the shape and size
of the tail, and spits the pulse out early.
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- If the side of the chamber facing the incoming wave is
called the near side, and the other the far side, the sequence of events
is something like the following. The incoming wave, its tail extending
ahead of it, approaches the chamber. Before the incoming wave's peak gets
to the near side of the chamber, a complete pulse is emitted from the far
side, along with a backward wave inside the chamber that moves from the
far to the near side.
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- The backward wave, traveling at 300 times c, arrives
at the near side of the chamber just in time to meet the incoming wave.
The peaks of one wave overlap the troughs of the other, so they cancel
each other out and nothing remains. What has really happened is that the
incoming wave has "paid back" the cesium atoms that lent energy
on the other side of the chamber.
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- Someone who looked only at the beginning and end of the
experiment would see only a pulse of light that somehow jumped forward
in time by moving faster than c.
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- "The effect is really quite dramatic," Dr.
Steinberg said. "For a first demonstration, I think this is beautiful."
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- In Dr. Wang's experiment, the outgoing pulse had already
traveled about 60 feet from the chamber before the incoming pulse had reached
the chamber's near side. That distance corresponds to 60 billionths of
a second of light travel time. But it really wouldn't allow anyone to send
information faster than c, said Peter W. Milonni, a physicist at Los Alamos
National Laboratory. While the peak of the pulse does get pushed forward
by that amount, an early "nose" or faint precursor of the pulse
has probably given a hint to the cesium of the pulse to come.
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- "The information is already there in the leading
edge of the pulse," Dr. Milonni said. "You can get the impression
of sending information superluminally even though you're not sending information."
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- The cesium chamberhas reconstructed the entire pulse
shape, using only the shape of the precursor. So for most physicists, no
fundamental principles have been smashed in the new work.
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- Not all physicists agree that the question has been settled,
though. "This problem is still open," said Dr. Ranfagni of the
Italian group, which used an ingenious set of reflecting optics to create
microwave pulses that seemed to travel as much as 25% faster than c over
short distances.
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- At least one physicist, Dr. Guenter Nimtz [[umlaut over
u]] of the University of Cologne, holds the opinion that a number of experiments,
including those of the Italian group, have in fact sent information superluminally.
But not even Dr. Nimtz believes that this trick would allow one to reach
back in time. He says, in essence, that the time it takes to read any incoming
information would fritter away any temporal advantage, making it impossible
to signal back and change events in the past.
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- However those debates end, however, Dr. Steinberg said
that techniques closely related to Dr. Wang's might someday be used to
speed up signals that normally get slowed down by passing through all sorts
of ordinary materials in circuits. A miniaturized version of Dr. Wang's
setup "is exactly the kind of system you'd want for that application,
Dr. Steinberg said.
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- Sadly for those who would like to see a computer chip
without a speed limit, the trick would help the signals travel closer to
the speed of light, but not beyond it, he said.
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