"Light can't go faster than its speed"
Have you guys seen this? Maybe the statement above ain't true. Found this in today's SF Chronicle (from NY Times, though). Kind of interesting.
Physics Approaching Speed of Light
Mind-bending new studies appear to break Einstein's rules
James Glanz, New York Times Tuesday, May 30, 2000
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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 to freshman textbooks and conversation at sophisticated wine bars; if anything could, Einstein's theory of relativity would crumble, and theoretical physics would fall into disarray.
Two new experiments have demonstrated how flexible or misleading that comfortable wisdom can be in the right circumstances. Using a combination of atomic and electromagnetic effects, researchers have produced light beams in the laboratory that appear to travel much faster than the normal speed of light. Einstein's theory survives, physicists say, but the results of the experiments, they agree, are mind-bending.
In the most striking of the new experiments, a pulse of light that passes through a transparent chamber filled with specially prepared cesium gas appears to be 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 chamber even before it enters.
It is as if someone looking through a window at 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, the physicists explain, because the effect could never be used to signal back in time to change the past and, in the example, avert the accident.
A paper on the experiment, by Lijun Wang of the NEC Research Institute in Princeton, N.J., has been submitted to the journal Nature and is undergoing peer review. It is only the most spectacular example of work by a wide range of researchers who have recently produced superluminal speeds of propagation in various materials, in hopes of finding a chink in Einstein's armor and of 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 at Berkeley, who, like a number of physicists in the close-knit community of optics research, is knowledgeable about Wang's work.
LOOKING INTO THE FUTURE?
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, 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.
INTERNATIONAL RESEARCH
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 Wang's experiment is normally used to amplify waves of laser light, not speed them up, said Aephraim 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 through the chamber soaks up some of that energy and gets amplified when it passes through the atoms.
But the amplification occurs only if the second beam is tuned to a certain precise wavelength, Steinberg said. By cleverly choosing a slightly different wavelength, 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,'' Steinberg said.
Wang's experiment uses another property of light signals. 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.
WHEN LIGHT IS LIKE THE BAY BRIDGE
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 the backward light waves can, according to the laws of quantum mechanics, 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.
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