Light—So Fast It Goes Backward

A Rochester scientist demonstrates a mathematical oddity. By Jonathan Sherwood
 LIGHT TIME: “It’s weird stuff,” says Boyd of his recent work to demonstrate the mathematical oddity of “negative speed.” With Boyd are graduate students George Gehring and Aaron Schweinsberg.

In the past few years, scientists have found ways to make light go both faster and slower than its usual speed limit, but Robert Boyd has gone a step further.

The Rochester professor has pushed light so far forward that it goes into reverse.

And, as if to defy common sense, the backward-moving pulse of light travels faster than, well, light.

Confused? You’re not alone.

“I’ve had some of the world’s experts scratching their heads over this one,” says Boyd, the M. Parker Givens Professor of Optics. “Theory predicted that we could send light backward, but nobody knew if the theory would hold up or even if it could be observed in laboratory conditions.”

In results published this spring in the journal Science, Boyd and his research team showed how to take what was once a mathematical oddity—negative speed—and demonstrated it working in the real world.

“It’s weird stuff,” says Boyd.

 Light propagating backwards

In the experiment, Boyd, along with Rochester graduate students George Gehring and Aaron Schweinsberg, and undergraduates Christopher Barsi of Manhattan College and Natalie Kostinski of the University of Michigan, sent a burst of laser light through an optical fiber that had been laced with the element erbium. As the pulse exited the laser, it was split in two. One pulse went into the erbium fiber and the second traveled along undisturbed as a reference. 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.

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

In previous studies, Boyd 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.

To understand how light’s speed can be manipulated, an analogy is to 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 for a moment.

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 exit the material’s other side.

Similarly, a light pulse can be made to contract and slow inside a material, exiting the other side much later than it naturally would.

To visualize Boyd’s reverse-traveling light pulse, replace the mirror with a big-screen 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 far side of the TV. It walks toward you, passes you in the middle, and continues moving in the opposite direction until it exits the other side of the screen. [more]

 How Does Light Go Backward? As the initial pulse of light approaches the glass, a new pulse forms at the far end. The new pulse splits in two, one traveling backward in the glass, the other already exiting. The backward pulse meets and cancels out the initial pulse. Only the final pulse remains.

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 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 traveling faster than the regular speed of light.

To use the TV analogy again—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 this? After all, the results seem to violate Einstein’s tenet that nothing can travel faster than the speed of 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,” says 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 is 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.