# Putting the Brakes on Light

## Light travels 186,000 miles per second in a vacuum; in Lene Hau’s lab, it ambles at 38 miles an hour

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So I'm readin' the paper and see where this Danish woman has slowed light down to 38 miles per hour. It takes a bit for that to sink in. But yes, that's per hour, and that is 38 miles. Right there in print. Now we have all known all our lives that in a vacuum light travels 186,000 miles per second and not all that much slower — for our purposes — when it is moving through air, say, or water. Light from the sun takes only eight minutes to reach us, some 93,000,000 miles away. If light dawdled along at 38 miles per hour, it would take approximately 275 years for a photon to move from the sun to the earth. Horses can run faster than 38 miles per hour, for goodness' sake. If you'll allow a little poetic license about light sources and whatnot, this woman has figured out how to catch a sunbeam.

A teacher at Harvard University, Lene Vestergaard Hau also works at the Rowland Institute for Science, a research center founded by the inventor of Polaroid photography, Edwin H. Land. She took one of the most esoteric technological accomplishments of recent times and, if you'll pardon the pun, shed new light on its usefulness. Hau works with matter in a state so nonintuitive it can only be called weird. When you cool the right stuff down to a temperature of a few hundred billionths of a degree above absolute zero, strange things happen. The motion that animates every atom (in the macro world, we feel that motion as heat) nearly stops. According to the Heisenberg uncertainty principle, once we begin to know the momentum of an atom very accurately (extremely close to zero in this case) we cannot know where it is. This is the world of quantum mechanics, where classic physics no longer applies. The space in which each atom can be found spreads out farther and farther, overlapping with all the others. The atoms lose their identities, flowing instead into an amorphous cloud known as a Bose-Einstein condensate. (It is named for Satyendra Nath Bose, an Indian physicist, and Albert Einstein, who predicted the condensate's existence a full 70 years before technology could get us even close to the necessary temperatures.)

The condensate is fascinating in its own right. The atoms, now one big atom, act in unison. They are analogous to the photons in a laser, all lined up the same way, producing what physicists call a coherent beam of light. On the quantum mechanical level, atoms have dual personalities, just like photons of light. For some purposes, it works best to think of them as particles, as we lay folk normally do. For others, however, it works better to consider them as waves. In the condensate, the waves of each atom have coalesced into one big wave. This wave can be so big, in fact, that it can be seen by the human eye.

Hau aimed a laser beam through just such a cloud of nearly motionless sodium atoms. (The cloud was just 1/125 inch long.) This first beam, known as the coupling beam, induces quantum changes in the atoms such that the cloud becomes transparent, but with an extremely rapid change in the refractive index. Refraction is what happens to light as it passes from one medium (such as air) into another (glass, for example). We've all seen that a pole partly in water appears to be bent at the surface, even though we know it isn't. We see refraction at work every time we watch a clear sunset. The sun appears to be still above the horizon when in fact it has already set, because the earth's atmosphere refracts the light coming from it. In Hau's laboratory, the nonlinear refractive index of the sodium cloud is about a hundred trillion times higher than that of glass in optical fiber. The cloud will now do to light what molasses up to her shoulders would do to a runner.

A second laser beam, the probe pulse, goes through this transparent-but-oh-so-refractive cloud at a right angle to the first. It is the light in this beam that is slowed to a crawl. Hau is not resting on her laurels. She hopes to use her apparatus to reduce the speed of light even further, to as little as 120 feet per hour.

One of the legends of the pre-Jackie Robinson Negro Leagues was James "Cool Papa" Bell, so fast that he often stole two bases on one pitch. The equally legendary pitcher Satchel Paige said that Bell was so quick he could turn off the light and "be in bed before the room was dark." With the help of Hau, I could do it myself, even though normally it would be the miracle of the ages.

As it happens, I have no immediate plans for using light traveling at 38 miles per hour. The idea that someone has been able to slow down light to such an extent, however, has knocked down a whole wall in my mind. The speed of light is one of the most fundamental numbers we know: to the best of our knowledge, nothing that is at rest or traveling at any speed less than that of light can ever be made to move at the speed of light, much less faster. Not only does the mass of the object approaching the speed of light increase toward infinity, the object becomes shorter and shorter in the direction of travel until it loses the dimension of length. And yes, as far as our unidentified object is concerned, time stops. Now light does travel more slowly in air than it does in a vacuum, and still slower in water, a mere 139,500 miles a second. But not one of my aging neurons thought for a minute that light could be slowed to the approximate velocity of a galloping horse or, if Hau reaches her goal, a snail.

Crazy images flicker through my head. Suppose we learn to slow sound at will. And suppose someday, somewhere, I blurt out something stupid. I could jump out of my chair and step in front of my words as they mosey across the room, and say: "Ignore what you're about to hear. It's all a mistake." And because I am closer to the hearer, that person hears my "Ignore" before hearing the original gaffe, even though I uttered the gaffe first. It's almost, almost, a tiny, trivial bit of time reversal.

Let's go for broke here. Death and taxes have nothing on gravity. It is with us all the time and everywhere. (As I age, gravity seems to be pulling me into the ground with increasing vigor. It can't be true, can it?) Physicists speak of the gravitational constant. If you must know, the gravitational constant, known as Big G, is 6.67 x 10-8 cm3/g sec2. Little g is the strength of gravity at the earth's surface. Now hush. The point is that it's constant, at least until now. Today we have new ways of measuring the longest distances in the universe, using exploding stars as our ruler ticks. We are finding that instead of being slowed by the aggregate gravitational pull of all the galaxies, stars and dark matter behind them, the galaxies farthest away appear to be accelerating away from us.

Well, my fantasy is obvious. Phenomena that we believe exist only at astronomical scales can surprise us. Black holes objectify "big" and "strong." The small ones are the remains of massive stars that collapsed in on themselves. The big ones are the equivalent of millions of stars, sitting at the centers of galaxies like so many insatiable spiders. And yet Stephen (A Brief History of Time) Hawking once imagined black holes so small that were they floating by us in the park, even residing inside our homes, they might go undetected. So suppose antigravity can also exist on a small, let's say human, scale. How strong might it be? Oh, just enough for you or me to float up into the sky at will. Before everyone falls out of their chairs laughing, let's wait to see what the scientists come up with to explain the large-scale antigravity force. Let's see if they can duplicate the force in a laboratory, making little shuffleboard pucks on a tabletop rebound from each other in the same way that the north poles of two magnets brought together will repel each other. And let's all remember that electricity was once no more than a laboratory curiosity.