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Light's Most Exotic Trick Yet: So Fast it Goes ...
Posted on: Monday
May 22, 2006
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.'
Robert Boyd, professor of optics
(PHOTO CREDIT: University of Rochester)
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
'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
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'
'I know this all sounds weird, but this is the
way the world works,' says Boyd.
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