"I've had some of the world's experts scratching their heads over this one," says Robert 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."
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. But he's now taken what was once just a mathematical oddity and shown it working in the real world.
"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."
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 into 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 re-measure 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.
Does the experiment point to potential weakness in Einstein's sacred tenet that nothing can travel faster than the speed of light? Not really, Boyd explains. "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 already 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.
"I know this all sounds weird, but this is the way the world works," adds Boyd.
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Researchers reveal light's quirky side
Above, in the first frame, the initial pulse of light approaches the glass, and a new pulse forms at the far end. In the second frame, the new pulse splits into two, one traveling backward in the glass, the other already exiting. Next, the backward pulse meets and cancels out the initial pulse, and, at the end, only the final pulse remains.
n the past few years, scientists have found ways to make light go both faster and slower than its usual speed limit, but now University researchers have published a paper in the journal Science illustrating one of light's logic-defying properties known as negative speed.
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