Physicists
Slow Speed of Light
By William J. Cromie
Gazette Staff
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Light, which normally travels the 240,000 miles from the Moon to Earth in
less than two seconds, has been slowed to the speed of a minivan in rush-hour
traffic -- 38 miles an hour.
An entirely new state of matter, first observed four years ago, has made
this possible. When atoms become packed super-closely together at super-low
temperatures and super-high vacuum, they lose their identity as individual
particles and act like a single super- atom with characteristics similar to a
laser.
Such an exotic medium can be engineered to slow a light beam 20 million-fold
from 186,282 miles a second to a pokey 38 miles an hour.
"In this odd state of matter, light takes on a more human dimension;
you can almost touch it," says Lene Hau, a
Hau led a team of scientists who did this
experiment at the Rowland Institute for Science, a private, nonprofit research
facility in
In the future, slowing light could have a number of practical consequences,
including the potential to send data, sound, and pictures in less space and
with less power.
But that's not why Hau, a research scientist at
both Harvard and the Rowland Institute, originally set out to do the
experiments. "We did them because we are curious about this new state of
matter," she says. "We wanted to understand it, to discover all the
things that can be done with it."
It took Hau and three colleagues several years to
make a container of the new matter. Then followed a series of
27-hour-long trial runs to get all the parts and parameters working together.
"So many things have to go right," Hau comments. "But the results finally exceeded
our expectations. It's fascinating to see a beam of light almost come to a
standstill."
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Members of Hau's team included Harvard graduate
students Zachary Dutton and Cyrus Behroozi. Steve
Harris from
Making a Super-atomic Cloud
The idea of this new kind of matter was first proposed in 1924 by
This so-called Bose-Einstein condensate was not actually made until 1995,
because the right technological pot to cook it up in did not exist. Vacuums hundreds of trillions of times lower than the pressure of
air at Earth's surface, and temperatures almost a billion times colder that
that in interstellar space, are needed to produce the condensate.
Temperatures must be lowered to within a few billionths of a degree of absolute
zero (minus 459.7 degrees F), where atoms have the least possible energy and
all but cease to move around.
Hau and her group started with a beam of sodium
atoms injected into a vacuum chamber and moving at speeds of more than a
thousand miles an hour. These hot atoms have an orange glow, like sodium
highway and street lights.
Laser beams moving at the normal speed of light collide with the atoms. As
the atoms absorb particles of light (photons), they slow down. The laser light
also orders their random movement so they move in only one direction.
When the atoms are slowed to a modest 100 miles an hour or so, the
experimenters load the atoms into what they call "optical molasses,"
a web of more laser beams. Each time an atom collides with a photon it is
knocked back in the direction from which it came, further slowing it down, or
cooling it.
The atoms are now densely packed in a cigar-shaped clump kept floating free
of the walls of their container by powerful magnetic fields.
"It's nifty to look into the chamber and see the clump of cold atoms
floating there," Hau remarks.
In the final stage, known as "evaporative cooling," atoms still
too hot or energetic are kicked out of the magnetic field.
The stage is now set for slowing light. One laser is shot across the width
of the cloud of condensate. This controls the speed of a second pulsed laser
beam shot along the length of the cloud. The first laser sets up a "quantum interference" such that the moving
light beams of the second laser interfere with each other. When everything is
set up just right, the light can be slowed by a factor of 20 million.
The process is described in detail in the Feb. 18 issue of the scientific
journal Nature. (Warning: Don't try this at home.)
Relativity and the Internet
Slowing light this way doesn't violate any principle of physics. Einstein's
theory of relativity places an upper, but not lower, limit on the speed of
light.
According to relativity theory, an astronaut traveling at close to the speed
of light will not get old as fast as those she leaves behind on Earth. But
driving at 38 miles an hour, as everyone knows, will not affect anyone's rate
of aging.
"However, slowing light can certainly help our understanding of the
bizarre state of matter of a Bose-Einstein condensate," Hau points out.
And a system that changes light speed by a factor of 20 million might be
used to improve communication. It can be used to greatly reduce noise, which
allows all types of information to be transmitted more efficiently.
But what about the cost and exotic equipment needed for such improvements?
"Technologies that push past old limits are always expensive and
impractical to begin with; then they become cheaper and more m
What will she do next?
Hau sweeps her hand over a roomful of equipment
and explains how things are already being set up to slow light speed even more,
to one centimeter (less than a half-inch) a second. That's a leisurely 120 feet
an hour.
Hau will give a lecture on her
experiments at
