Light's Most Exotic Trick Yet: So Fast it Goes ... Backwards?   
Physics : May 11, 2006
Robert Boyd, professor of optics (PHOTO CREDIT: University of Rochester)

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 at the
University of Rochester have published a paper today in Science on how
they've gone one step further: pushing light into reverse. As if to defy
common sense, 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, the M. Parker Givens Professor of Optics at the
University of Rochester. "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."
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—negative speed—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."
So, wouldn't Einstein shake a finger at all these strange goings-on?
After all, this seems to violate Einstein's sacred 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 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.
So How Does Light Go Backwards?
Boyd, along with Rochester graduate students George M. 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.

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
(the reference walker) 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 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 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.

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 [See "backward
light" animation].

"I know this all sounds weird, but this is the way the world works,"
says Boyd.

Source: University of Rochester, by Jonathan Sherwood