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Light Exceeds Its Own Speed Limit, or Does It?

News/Current Events News
Source: New York Times
Published: May 30, 2000 Author: JAMES GLANZ
Posted on 05/30/2000 09:13:11 PDT by H.R. Gross

Light Exceeds Its Own Speed Limit, or Does It?

By JAMES GLANZ
 

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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.

    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.


    1 Posted on 05/30/2000 09:13:11 PDT by H.R. Gross
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    To: H.R. Gross

    There are tricksters and jokesters in the physics community. The important question to ask is: "can this fast wave transmit information at 300 times the speed of light?".

    2 Posted on 05/30/2000 09:19:43 PDT by Fitzcarraldo
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    bump

    3 Posted on 05/30/2000 09:21:25 PDT by Fitzcarraldo
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    To: H.R. Gross

    Huh? And I thought legalese was incomprehensible. :)

    4 Posted on 05/30/2000 09:32:03 PDT by martin_fierro
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    To: H.R. Gross

    "the main part of the pulse exits the far side of the chamber even before it enters at the near side."

    What?

    Can we use this to avoid speed traps?

    This sounds like a bad case of Montezumas' revenge.

    5 Posted on 05/30/2000 09:40:12 PDT by laotzu
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    To: H.R. Gross

    Like the "photonic bandgap" results of several years ago, this effect exploits the difference between the group velocity and the phase velocity of a light pulse. The same discussion about the apparently anti-causal behavior ensued back then.

    Sounds like a nice bit of work. And actually, this wasn't as bad a bit of science reporting as is usual.

    6 Posted on 05/30/2000 10:25:28 PDT by Physicist (sterner@sterner.hep.upenn.edu)
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    To: H.R. Gross

    the speed of light thing was a premise only.

    7 Posted on 05/30/2000 10:30:11 PDT by RightWhale
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    To: Physicist

    Physicist, I'm no physicist, but this sounds like a job for Super String. It is possible that the light pulse already exists at the other end in potential?

    8 Posted on 05/30/2000 10:34:56 PDT by JohnYankeeCmpsr
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    To: H.R. Gross

    Forget all this rocket surgeon mumbo-jumbo and answer the important question. If it's dark out and I turn on my headlights, will I be able to see the exit for Jupiter?

    9 Posted on 05/30/2000 10:37:48 PDT by Hatteras
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    To: Physicist

    What is a "a low-intensity precursor "? Is it part of the wave, part of the packet, or what? Do they exist apart from waves or packets? What causes them? Do they similarly exist for all electromagnetic radiation? Is it a wave or a particle or both? Do precursors have precursors? Huh?

    Scientifically illiterate Freeregards,

    10 Posted on 05/30/2000 10:45:49 PDT by Buckhead
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    To: Fitzcarraldo

    I read this article this morning and was amazed. Just this weekend I had did some reading on tachyons. Tachyons are theoretical subatomic particles with real energy but imaginary mass and traveling faster than the speed of light.

    In my web search I fiund many New Age sites and science fiction sites tied into them and promoting everything from healing powers to whatever.

    At one science web site I found the following limerick:

    There was a young lady named Bright,

    Whose speed was far faster than light,

    She went out one day, in a relative way,

    And returned the previous night.

    11 Posted on 05/30/2000 10:46:10 PDT by worldviewer tom
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    To: H.R. Gross

    Somehow, I am sure, this proves the earth is 6000 years old.

    12 Posted on 05/30/2000 10:52:19 PDT by Eddeche
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    To: H.R. Gross

    Mr. Data, set course for Starbase 12, warp factor 5. Engage.

    13 Posted on 05/30/2000 10:59:18 PDT by ABG(anybody but Gore)
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    To: ABG(anybody but Gore)

    Mr. Data, set course for Starbase 12, warp factor 5. Engage.

    Captain, a curious thing has just occurred.

    I was just about to enter the coordinates into the astral navigator as per your orders, but then the display suddenly changed to show that we're already there.

    A check of all of the other instruments confirms it, Captain: we're already there. I cannot explain this.

    14 Posted on 05/30/2000 11:34:03 PDT by CubicleGuy
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    To: H.R. Gross

    Bump for later!

    15 Posted on 05/30/2000 11:51:45 PDT by jazzraptor
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    To: laotzu

    "This sounds like a bad case of Montezumas' revenge."

    I think the reporter misunderstood what he was told.

    16 Posted on 05/30/2000 12:02:36 PDT by Michael Rivero
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    To: H.R. Gross

    It is all really quite simple -

    I saw the beam of light tomorrow,
    I will see the beam of light...yesterday?

    17 Posted on 05/30/2000 12:24:17 PDT by Michael_S
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    To: worldviewer tom

    [ There was a young lady named Bright, Whose speed was far faster than light, She went out one day, in a relative way, And returned the previous night.]

    lol

    18 Posted on 05/30/2000 12:59:39 PDT by hosepipe
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    To: Michael Rivero

    I think the reporter misunderstood what he was told.

    I don't think the reporter is capable of understanding the subject.

    "a transparent chamber filled with specially prepared cesium gas"

    Why didn't he just call it a black box like the rest of 'um?

    19 Posted on 05/30/2000 13:16:36 PDT by I dial 357
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    To: H.R. Gross

    what this really means is simply that "scientist" once again made an assumption and called it fact. Obviously light can travel faster than before thought. The top speed, however they acomplished it, is still the top speed. It cannot be there before the light is turned on, it only accelerates the light once it enters the gas. This is like saying that if I drive at 70mph and you drive at 50mph then I am in effect time traveling since I arrive there before you do. Ridiculous.

    20 Posted on 05/30/2000 14:57:36 PDT by Semper Liberty (jm7910@bellsouth.net)
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    To: Semper Liberty

    The only problem is though: if you go 70 mph in THAT particular car, then you will generate a massive black hole that will obliterate our galaxy. Slow down, dude!! :)

    21 Posted on 05/31/2000 13:23:53 PDT by tatterdemalion (galumpher@hotmail.com)
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    To: Eddeche

    Somehow, I am sure, this proves the earth is 6000 years old.

    Depends - from which point of view in the Universe?

    Remember, the core of Einstein's Theory of Relativity still holds. Therefore, if you observe the Earth from the center of the Universe (say, at the start of the Big Bang), the Earth is quite likely 6,000 years old (a rough estimate of course). If you observe the Earth from the vantage point of the Earth however, it is probably billions of years old. Einstein showed this even before the latest interesting development.

    Just don't tell that to the fanatics who feel that Biblical text must be considered incompatible with science at all costs.

    22 Posted on 05/31/2000 13:39:45 PDT by Levine2001
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