Light's most exotic trick yet -- so fast it goes backwards?

By Aussiegirl

More and more it seems like we're living in Through the Looking Glass -- here are Alice and the Red Queen running as fast as they can:

`Now! Now!' cried the Queen. `Faster! Faster!' And they went so fast that at last they seemed to skim through the air, hardly touching the ground with their feet, till suddenly, just as Alice was getting quite exhausted, they stopped, and she found herself sitting on the ground, breathless and giddy.

The Queen propped her up against a tree, and said kindly, `You may rest a little now.'

Alice looked round her in great surprise. `Why, I do believe we've been under this tree the whole time! Everything's just as it was!'

`Of course it is,' said the Queen, `what would you have it?'

`Well, in our country,' said Alice, still panting a little, `you'd generally get to somewhere else -- if you ran very fast for a long time, as we've been doing.'

`A slow sort of country!' said the Queen. `Now, here, you see, it takes all the running you can do, to keep in the same place.

If you want to get somewhere else, you must run at least twice as fast as that!'

(By the way, be sure to click on the three animations that illustrate the text: "fast light", "slow light", and "backward light"!)

Light�s most exotic trick yet: so fast it goes backwards?

Light’s most exotic trick yet: so fast it goes backwards?

May 12, 2006
Courtesy University of Rochester
and World Science staff

In the past few years, physicists have found ways to make light go both faster and slower than its usual speed limit. Now researchers say they’ve gone a 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,” said Robert Boyd of the University of Rochester in Rochester, N.Y., one of the researchers. “It’s weird stuff.”

“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 determined that nothing can be accelerated to a speed greater than that of light in a vacuum. That’s about 300,000 kilometers (190,000 miles) per second.

If something broke that limit, then some observers could see it reach its destination before it left, violating a universal law of causality.

But physicists in recent years have reported finding tricks to slow light to a near-standstill, or even speed it up in apparent violation of Einstein’s rule.

Now, Boyd said, he’s 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.

Boyd and colleagues sent bursts of laser light through an optical fiber laced with the element erbium. An optical fiber is a thin, transparent tube that transmits light by letting it bounce along its interior.

“The pulse of light is shaped like a hump with a peak,” Boyd explained. “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.”

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. The far edge seems to leap ahead of you momentarily.

In the same way, a pulse of light fired through a special material may move 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 type 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 later than it naturally would (See “slow light” animation).

To visualize the backward-moving light pulse reported by Boyd, 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. The image 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 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.

Wouldn’t Einstein shake a finger at all these strange goings-on? Not necessarily, Boyd said, because Einstein’s speed limit applies only 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 hump-like pulse has long leading and trailing edges, Boyd explained. “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 should disappear. Boyd is eager to put Einstein to the test.