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Light So Fast It Actually Goes Backwards

Boyd's team collected data showing light traveling 'backwards,' seemingly faster than light speed. Image credit: Univ. of Rochester
by Staff Writers
Rochester NY (SPX) May 12, 2006
In the past few years, scientists have found ways to make light go both faster and slower than its usual speed, but now researchers have gone one step further: pushing light into reverse. As if to defy common sense, the backward-moving pulse of light travels faster than light.

"I've had some of the world's experts scratching their heads over this one," said lead author Robert Boyd of the University of Rochester. "Theory predicted that we could send light backwards, but nobody knew if the theory would hold up or even if it could be observed in laboratory conditions."

Boyd recently showed how he can slow down a pulse of light to slower than an airplane, or speed it up faster than its breakneck pace, using exotic techniques and materials. Now, his team has taken what was once just a mathematical oddity - negative speed - and shown it working in the real world.

"It's weird stuff," Boyd said.

Reporting in the May 12 issue of Science, the researchers said they sent a burst of laser light through an optical fiber that had been laced with the element erbium. As the pulse exited the laser, they split it in two. One pulse went into the erbium fiber and the second traveled along undisturbed as a reference.

They found the peak of the pulse emerged from the other end of the fiber before the peak entered the front of the fiber, and well ahead of the peak of the reference pulse.

To find out if the pulse was truly traveling backward within the fiber, the team cut back the fiber every few inches and re-measured the pulse peaks when they exited each pared-back section. By arranging that data and playing it back in a time sequence, they were able to depict, for the first time, the pulse of light was moving backward within the fiber.

Boyd describes the reverse-traveling light pulse as the equivalent to a person's image captured by a video camera and played on a big-screen TV. When a person passes such a display in a store window, as he or she walks past the camera, the on-screen image appears on the far side of the TV. It walks toward the subject, passes in the middle, and continues moving in the opposite direction until it exits the other side of the screen.

A negative-speed pulse of light acts much the same way: As the pulse enters the material, a second pulse appears on the far end of the fiber and flows backward. The reversed pulse not only propagates backward, but also releases a forward pulse out the far end of the fiber.

In this way, the pulse that enters the front of the fiber appears out the end almost instantly, apparently traveling faster than the regular speed of light.

Using the TV analogy, it is as though a person walked by the shop window, saw his or her image approaching from the opposite edge of the TV screen, and that TV image created a person's clone at that far edge, walking in the same direction, but several paces ahead.

"I know this all sounds weird, but this is the way the world works," Boyd said.

The problem with the results is they seem to violate Einstein's tenet that nothing can travel faster than light � not even light.

"Einstein said information can't travel faster than light, and in this case, as with all fast-light experiments, no information is truly moving faster than light," Boyd explained. "The pulse of light is shaped like a hump with a peak and long leading and trailing edges.�

He said the leading edge of the pulse carries all the information about the pulse and enters the fiber first. �By the time the peak enters the fiber,� he continued, �the leading edge is already well ahead, exiting. From the information in that leading edge, the fiber essentially reconstructs the pulse at the far end, sending one version out the fiber, and another backward toward the beginning of the fiber."

Boyd said his team already is working on ways to see what will happen if they can design a pulse without a leading edge. According to Einstein, the entire faster-than-light and reverse-light phenomena will disappear. Boyd said he is eager to put Einstein to the test.

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EADS Astrium To Build Gaia Satellite
Toulouse, France (SPX) May 12, 2006
ESA has awarded EADS Astrium the contract to develop and build its Gaia satellite. Due to be launched in 2011, its mission is to construct the largest and most precise map ever of the Milky Way.

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