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vendredi 23 mai 2003

According to this Economist's column about relativity, researchers from Duke University in North Carolina have moved parts of light waves faster than the speed of light. Intrigued? Read on.

Light normally travels fastest in a vacuum. But on May 22nd Dr Gauthier revealed to a meeting of the optics division of the American Physical Society, in Boulder, Colorado, that this is not always the case. Parts of a light wave can be accelerated beyond this speed by passing the wave through a vial of energetic potassium atoms.
On the face of it, this contradicts Einstein's special theory of relativity. So, rather than saying as Einstein did that nothing can be faster than the speed of light in a vacuum (abbreviated as c), physicists now prefer a more precise version: information cannot travel faster than c.

How did they perform this experiment?

Dr Gauthier began his experiment with a vial of gaseous potassium and a laser. He used the laser to excite the potassium atoms in a particular way, illuminating the vial with two slightly different frequencies of light.
Dr Gauthier's results were so clear-cut that portions of the laser pulse appeared to leave the vial before they entered it.

So is Eintein's theory of relativity wrong? Not at all.

What Dr Gauthier was manipulating is actually something called the "group velocity" of the pulse. Loosely speaking, this is the speed at which the pulse's crest travels. When he looked closely at the leading edge of the pulse, he found that it travelled at the plain old speed of light, c.
That is where the rewriting of Einstein to refer to information comes in. Because the pulse is smooth, the appearance of the "superluminal" crest can be inferred mathematically from the shape of the pulse's leading edge. That means no additional information is conveyed by the arrival of the crest itself. In other words, any information encoded in the crest is not travelling faster than light -- and the (modified) version of relativity still holds water.

Those of you interested by the subject can read the abstract of this research paper, "Information velocity in `fast light' optical pulse propagation."

Source: The Economist, May 22, 2003

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