Damien Broderick wrote:
>
> At 02:29 PM 30/05/00 -0400, Brian Atkins <brian@posthuman.com>
> wrote:
>
> >http://www.nytimes.com/library/national/science/053000sci-physics-light.html
>
> This is not accessible to anyone outside the States - that is, we can't
> register.
>
> So what's the story about?
>
> Damien
>From the link above:
May 30, 2000
Light Exceeds Its Own Speed Limit, or
Does It?
By JAMES GLANZ
he 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.
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.
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.
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.
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.
"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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
"The effect is really quite dramatic," Dr. Steinberg said.
"For a first
demonstration, I think this is beautiful."
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.
"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."
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.
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.
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.
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.
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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