From: Ian Field (field_ian@hotmail.com)
Date: Tue May 30 2000 - 21:54:49 MDT
If anyone else needs more structure (because, like me, your brain is too
chaotic to make order out of random CR/LFs), here's a link to the story:
http://enablia.dynip.com/public/
M. Lorrey Pasted:
|
| 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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