The textbooks say that information can’t travel faster than light, but researchers at Rochester University have uncovered tantalizing evidence that this may not be the case. To put Einstein to the test, they’re planning new experiments based on their work to date that shows that light can travel faster than the speed of light if it’s traveling backwards. If it sounds confusing, don’t worry, you’re not alone.
“I’ve had some of the world’s experts scratching their heads over this one,” says Robert Boyd, Professor of Optics at the University of Rochester.
To understand Boyd’s reverse-traveling light pulse, think of a big-screen TV and video camera. When passing such a display in a 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. Boyd’s negative-speed pulse of light acts much the same way. As the pulse enters the material, a second pulse appears at 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. In our store-front analogy, 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.
In the Rochester experiment, detailed in Science, the scientists 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 that the pulse of light was moving backward within the fiber.
“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.”
Does this violate one of physics central creeds – 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’s team are already 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 is eager to put Einstein to the test.
Check out an animation of BFTL from Rochester.