Excerpted from the New York Times, (C)05/30/2000 for those without a free NYT account:
"...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.
...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.
-more- @NYT
http://search1.nytimes.com/search/daily/bin/fastweb?getdoc+site+site+45617+0+wAAA+superluminal