before it's in the papers"
August 03, 2010
TO THE WORLD SCIENCE HOME PAGE
Light’s most exotic trick yet: so fast it goes backwards?
and World Science staff
In the past few years, scientists have found ways to make light go both faster and slower than its usual speed limit, but now researchers say they’ve gone one step further: pushing light into reverse.
As if to defy common sense, they say, the backward-moving pulse of light travels faster than light.
Confused? You’re not alone.
“I’ve had some of the world’s experts scratching their heads over this one,” says Robert Boyd of the University of Rochester in Rochester, N.Y., one of the researchers. “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.”
Einstein showed that nothing can travel faster than the speed of light in vacuum—300,000 kilometers (190,000 miles) per hour. If it goes any faster, than some observers could see things reach their destination before they left, violating a universal law of causality.
But researchers in recent years have found ways to make light go much slower than its normal speed, even to make it stop almost completely.
Boyd recently reported that he could slow down a pulse of light, or speed it up faster than its breakneck pace, using exotic techniques and materials. But he’s now taken what was once just a mathematical oddity—negative speed—and shown it working in the real world.
The findings are published in the May 12 issue of the research journal Science.
“It’s weird stuff,” says Boyd. “We sent a pulse through an optical fiber, and before its peak even entered the fiber, it was exiting the other end. Through experiments we were able to see that the pulse inside the fiber was actually moving backward, linking the input and output pulses.”
To understand how light’s speed can be manipulated, think of a funhouse mirror that makes you look fatter. As you first walk by the mirror, you look normal, but as you pass the curved portion in the center, your reflection stretches, with the far edge seeming to leap ahead of you momentarily.
In the same way, a pulse of light fired through special materials moves at normal speed until it hits the substance, where it is stretched out to reach and exit the material’s other side [See “fast light” animation].
Conversely, if the funhouse mirror were the kind that made you look skinny, your reflection would appear to suddenly squish together, with the leading edge of your reflection slowing as you passed the curved section. Similarly, a light pulse can be made to contract and slow inside a material, exiting the other side much later than it naturally would [See “slow light” animation].
To visualize Boyd’s reverse-traveling light pulse, replace the mirror with a TV and video camera. As you may have noticed when passing such a display in an electronics store window, as you walk past the camera, your on-screen image appears on the opposite side of the TV. It walks in the direction opposite to yours, and thus toward you. It passes you in the middle, and continues until it exits the other side of the screen.
A negative-speed pulse of light would act similarly [See “backward light” animation]. 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 it 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 beating light’s regular speed.
It’s as if you walked by the shop window, saw your image stepping toward you from the opposite edge of the TV screen, and that TV image of you created a clone at that far edge, walking in the same direction as you, several paces ahead.
So, wouldn’t Einstein shake a finger at all these strange goings-on? Not really, Boyd says, because his speed limit only applies to effects that carry some sort of information.
“In this case, as with all fast-light experiments, no information is truly moving faster than light,” said Boyd.
“The pulse of light is shaped like a hump with a peak and long leading and trailing edges. The leading edge carries with it all the information about the pulse and enters the fiber first. By the time the peak enters the fiber, 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 he’s working on ways to see what will happen if he can design a pulse without a leading edge. Einstein says the entire faster-than-light and reverse-light phenomena will disappear. Boyd is eager to put Einstein to the test.
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