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F R E N D Z of martian
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This looks really interesting but I'm too busy/tired to get into it. If
anyone can be bothered to read it and post a summary, I'd really appreciate
it. This physics stuff is hard work ... :-)
----- Original Message -----
From: Rohit Khare <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Wednesday, May 31, 2000 4:56 AM
Subject: GeeK: FWD: Light Exceeds Its Own Speed Limit, or Does It?
> [SERIOUSLY -- the "easily unnerved" should NOT read this piece. PG-13
> physics, in deed.... Rohit]
>
> May 30, 2000
>
http://www.nytimes.com/library/national/science/053000sci-physics-light.html
>
> Light Exceeds Its Own Speed Limit, or Does It?
>
> By JAMES GLANZ
>
> 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.
>
> 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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