Light, which travels in a vacuum at
almost 300,000 kilometers per second (186,000 miles per second), takes
only 8 and 1/3 minutes to journey from the Sun to the Earth. Now
a team of physicists has managed to slow the speed down
by a factor of 20 million. Yes, that's correct, a 20-million-fold
reduction in the speed of light! The physicists have created
conditions under which light travels at only 17 meters per
second, which is about 38 miles per hour -- a speed that
cars approximately move through a city. At that rate, light
from the Sun would take almost 300 years to reach Earth. Let's
hope that these scientists do not put the brakes on
sunlight; otherwise, we may never see the Sun rise again.
Dr. Lene Vestergaard Hau, a physicist
at the Rowland
Institute and Harvard
University who is originally from Denmark, led a team that
included Dr. Steve Harris of Stanford University and two
Harvard graduate students Zachary Dutton and Cyrus Behroozi.
The Rowland Institute
for Science, where the extraordinary feat was performed, is
a private, nonprofit research facility
in Cambridge, Massachusetts endowed by Edwin Land, the
inventor of instant photography. Mr. Land might not be so
happy to find out that his money is being used to "mess around" with
the properties of light. What would become of photography
if light traveled at only 38 miles per hour? No more still
shots of athletes dunking basketballs and hitting
baseballs out of parks.
Tongue-and-check aside, this
is an incredible result, and Dr. Hau is equally
enthusiastic. When asked about her group's experiment, she
commented, "It's fascinating to see a pulse of light
almost come to a standstill."
Now what's going on here? Didn't
Einstein tell us that the speed of light was constant?
Are scientists trying
to play a joke on us? Did Einstein make a mistake? Can
we now move faster than the speed of light, and if so, then
when we outrun the light, do we grow younger? Can Dr. Hau's
apparatus be used as a fountain of youth? If an atomic bomb
is detonated in her apparatus, does it fizzle instead
of explode -- after all, the energy E released when some
mass m is destroyed is given by the
formula E=mc2 but with
the speed of light c being 20,000,000 times smaller will
only a little energy be generated?
Aficionados of Einstein's
special theory of relativity need not worry: The answers
to the questions in the last paragraph are no, no, no,
no and no! Albert did not make a mistake. Only in a vacuum
is the speed of light a constant independent of the
motions of the source and the observer. Dr. Hau's feat
is achieved by passing light through an unusual
system -- an ultracold gas of sodium atoms
bathed in laser light.
When light travels through
a transparent medium, such as glass or water, it moves
slightly slower than in a vacuum. This effect leads
to refraction, which is the bending of light as it
passes from one medium to another. Refraction is
important -- for example, without it, lenses and
eyeglasses would not be possible. The phenomenon is
also familiar to those who have seen sunrays pass through
a glass prism. The rays bend as they enter the glass. Since
the refraction effect is slightly different for
different colors, white light is separated into
its individual components thereby producing a prism's
rainbow. Ordinary rainbows are created in the atmosphere
when water droplets play the role of prisms.
The amount of
refraction, which actually varies from medium to
medium, is controlled by a number n called the medium's
index of refraction. When n is 1, no bending occurs. The
larger n is, the more light bends. It turns out
that the speed a wave in a medium is reduced by a
factor of n. For example, in water, which has the
refractive index of 1.33, light travels 25% slower
than in a vacuum, which has an index of refraction of 1.
Almost all substances
have indexes of refraction only somewhat larger
than one. So how did the Rowland Institute team
succeed in making light move so slowly? The answer
is by shining laser light of a particular color on
the ultracold sodium atoms. For such a quantum
system, an index of refraction varying rapidly
with color creates
a huge reduction in the speed of light.
of the index of refraction depends not
only on the medium but also on properties
of the laser light.
So Dr. Hau's team
achieved their success by using a special system
consisting of ultracold sodium atoms bathed in
laser light. When a second pulse of light (not to
be confused with the laser light, which continuously
shines on the atoms) is sent through the system,
the speed of the
pulse is dramatically reduced by a
factor 20 million. In effect, her
team has put the "brakes on light."
In the medium of sodium atoms, light
of a particular color
still travels at a speed of several
hundred thousand kilometers per second. This speed
is known as the phase velocity. However, light
of slightly different colors travel at different speeds. When a
collection of different colored light passes through the medium, the
speed of the unit, which is known as the group velocity, moves
at a much slower speed. This is because the different components
combine in an unusual way to cancel fast propagation, an effect
common to all wave phenomena known as interference.
A track star, who runs
the 100-yard dash in less than 10 seconds, would be
hard pressed to perform the same feat in a vat of
molasses. The same is true for light, but, instead
of molasses, a cold medium is employed. Now imagine
that you could switch a laser on or off to turn air
into molasses or molasses into air. Then the track
star's speed could be easily controlled. This is
what the members of Rowland Institute team have
short, these scientists have put the "deep freeze" on light.
How cold was the
medium? Very cold -- just a few
absolute zero temperature. Absolute zero is the
lowest possible conceivable temperature, a temperature at
which microscopic constituents such as atoms and
molecules stop their motions. It is the ultimate
cold. In effect, every iota of heat is squeezed out
of the system. Absolute zero is
approximately -273o in Celsius, it is
about -460o in
Fahrenheit, and it
is 0o in Kelvins by definition.
Dr. Hau's team found
that, at a microkelvin, the pulse of light
traveled through the cold, laser-bathed sodium
atoms at an astonishingly slow speed of about 50 meters
per second. But did the Rowland Institute physicists
stop here? You bet they didn't! They went on to cool
the gas to a temperature of
about 50 nanokelvins!2
At this incredibly low temperature, the sodium
atoms entered a new exotic state of matter called
a Bose-Einstein condensate. Constructed for
the first time in the laboratory only a few years
ago in 1995, Bose-Einstein condensates have opened
up a new exciting field
In this coldest of cold environments, the light
pulse sped through the condensate at
only 17 meters per second!
an enormously rapidly varying index of refraction, the laser
light had another important consequence for the
cloud of sodium atoms. Normally, the cloud is
opaque, meaning that light cannot travel through
it. Light is absorbed as it enters an opaque
medium. However, when the laser light was tuned
to a particular color, the gas became
clear, allowing about 25% of the pulse light to
pass. This effect is known as electromagnetically
induced transparency because laser light is used
to render the cloud transparent.
To achieve the
electromagnetically induced transparency, it is
important for the cloud to be very cold so as
to allow the laser light to interact
efficiently with the sodium atoms.
Like many scientists
who achieve an amazing result for the first time, Dr. Hau
is more excited about future prospects than what she
has already accomplished. "It is a breakthrough that
opens up so many possibilities," she said in a telephone
interview. "The system gives us a completely new
scientific tool that can probe Bose-Einstein condensates. Future
include nonlinear optics at
the single photon4 level."
may greatly benefit from electromagnetically
induced transparency in the areas of communications
and computers. Indeed, Dr. Hau is confident
that "the result will open up a whole new domain
in highly nonlinear optics."
Is this a major
scientific breakthrough? You bet it is! Dr. Harris, one
of the members of Dr. Hau's team, had previously
slowed down light approximately 100-fold using
the same method of electromagnetically induced
transparency. Another group of physicists had
used what-is-called self-induced transparency
to achieve about a 1000-fold reduction. Thus, the
Rowland Institute team smashed the previous world
record for slowing down light by an amazing four
orders of magnitude. The team hopes to
eventually "bring light to its knees" by slowing
it down to the "snail's pace" of a centimeter
Well, it's a fast snail.
The Rowland Institute
physicists are unlikely to get their names in the
Guinness Book of Records (although they should). But
Dr. Hau's team, whose work has been published
in a 1999 February issue of Nature, has astounded
scientists around the world and produced a feat
that will be considered one of great scientific
achievements of the decade of the 1990's.
For more on the Rowland Institute experiment, click here.
For a picture of Dr. Hau and her laboratory, click here.
Read the update: Physicists Stop Light in Its Tracks.
To the top of this file.
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