## Scientists Put the Deep Freeze on Light and Slow It Down to 38 Miles Per Hour!

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. The variation 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 accomplished. In short, these scientists have put the "deep freeze" on light.
How cold was the medium? Very cold -- just a few microkelvins1 above 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 cryogenics.3 In this coldest of cold environments, the light pulse sped through the condensate at only 17 meters per second!
Besides generating 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 practical applications include nonlinear optics at the single photon4 level."
Someday, humanity 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 per second.5 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.

Read the update: Physicists Stop Light in Its Tracks.

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This report was prepared by the staff of Jupiter Scientific, the publishers of The Bible According to Einstein and an organization devoted to the promotion of science through books, the internet and other means of communication.

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