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Science
by Pat Murphy & Paul Doherty

Bicycling at the Speed of Light

Under the right conditions, Paul Doherty (physicist, rock climber, and coauthor of this column) can bicycle at near-light speed.

Incredible, you say? Being a science fiction reader, you probably know that light crosses the vacuum of space at 300 million meters or 186,000 miles per second. You probably also know that Einstein's theory of special relativity indicates that the speed of light in a vacuum is a constant--light always travels at 186,000 miles per second. (As the favorite t-shirt of one Exploratorium staffer says: "186,000 miles per second: Not just a good idea. It's the law!")

So, as a knowledgeable science fiction reader you ask a sensible question: If Paul can pedal at near-light speed, why hasn't he won every bicycle race around? Well, if you reread our first sentence, you'll note that we mentioned that our statement was true only "under the right conditions." You see, under very specific conditions, Dr. Lene Hau, a researcher at the Rowland Institute, recently managed to slow light down to a speed of 17 meters per second, which is slightly faster than Paul can bicycle on the level.

What does Einstein have to say about that? And what does all this have to do with science fiction? We'll get to that soon. First, we need to consider the nature of light, and discuss the speed of light. Then we'll tell you how Dr. Hau managed to slow light down to Paul's bicycling speed and connect all this to Bob Shaw's classic story "Light of Other Days."




Make your own electromagnetic wave

Light is tricky stuff. Back in the 1870s, the equations of Scottish physicist James Clerk Maxwell indicated that light is an electromagnetic wave. (At least, that's one way to look at light. We'll save quantum mechanics for another column. In this one, we'll just talk about light as a wave.)

Back when Maxwell was working on his wave equations, physicists figured that light traveled through aether, an undetectable substance that permeated all matter and space. Pat admits a certain fondness for the theory of aether. It seems like the physicists' equivalent of ghosts. You can't see them, but you know that they are there. Paul describes the theory this way: Physicists knew that sound waves travelled faster through stiffer material. Light was so fast that they figured the aether had to be really stiff-yet this aether which was everywhere didn't seem to exert any drag forces on planets as they moved about in their orbits.

In the nineteenth century, physicists assumed that if light traveled as a wave, it had to be a wave in something. Later experiments by Michelson and Morley contradicted the predictions of the aether theory and it was abandoned. But that still left the problem: what exactly is light a wave in?

Before we tackle that question, here's an experiment that will help you get a feel for the nature of light. All you need are four pieces of scotch magic tape about as long as your fist is wide and a table or desk. (Since some tables can be marred by having tape stuck to them, we suggest you use a battered old one, like the desks at the Exploratorium, rather than your Aunt Minnie's favorite antique.) Stick two pieces of tape to the table, side by side. Stick two more pieces on top of them. Yank both top pieces off the bottom pieces.

Bring the two pieces of tape close together, and they'll repel each other, moving apart as you try to bring them together. Even though there is nothing connecting the two pieces of tape, they exert a force on each other at a distance. Magic? Nope, it's an electric force acting over a distance.

Tape, like most things, is made up of positively charged protons and negatively charged electrons (along with some neutrons). The tape starts out electrically neutral, with equal numbers of positive and negative charges. When you pull one piece of tape off another, each piece of tape gains or loses some charged particles and gains an electrical charge. Since you peeled both pieces of tape off the same surface, they have the same charge--either positive or negative, it doesn't really matter. Since similar (or, as physicists say) like charges repel each other, one piece of tape repels the other.

You can think of the interaction between the two pieces of tape in two parts: The charge on one tape creates an electric field which in turn exerts forces on the other tape. The electric field can cross empty space.

What does all this have to do with light? Well, when you wiggle one piece of tape, you're changing the electric field in the space surrounding that tape. The side-to-side changes in the electric field propagate from one tape to the other as a wave travelling at the speed of light. A wave in an electric field creates an accompanying wave in a magnetic field. The combination of changing electric and magnetic fields is called an electromagnetic wave. When you wiggle the tape, you make a wave that travels to the other tape and makes it wiggle. The electromagnetic wave you are creating by wiggling your tapes is an ELF. (No, not that kind of elf. This is a science column, after all.) ELF stands for Extremely Low Frequency radio wave.

Light is also an electromagnetic wave. To understand light, you need to remember that it starts with a moving electric charge. When you squint because sunlight gets in your eyes, you are responding to an electromagnetic wave that was created when an electric charge in the surface of the sun accelerated That wave travelled 93 million miles across the vacuum of space to reach your eye 8.3 minutes later. The light made electrons in chemicals that are in the rods and cones in your eye's retina wiggle--and that made you see light.

The electromagnetic wave that you make by moving your charged tape also has to travel. It may seem to you like the second tape moves the instant you move the first one, but there is a time delay. Since the wave propagates at the speed of light, it's a mighty short delay. The speed of light is one foot per nanosecond, so if the tapes were a foot apart, the delay would be a nanosecond, a billionth of a second.

The speed of light in a vacuum was first measured by Ole Roemer in 1676. While studying eclipses of the moons of Jupiter, Roemer noticed that the eclipses were seen 8 minutes earlier than average when the earth was on the side of its orbit nearest Jupiter. When the earth was on the far side of its orbit, eclipses were 8 minutes later than average. This disparity could be explained by assuming that light took 16 minutes to cross the diameter of the earth's orbit. Knowing the diameter of the earth's orbit gave Roemer the speed of light: in modern units, 300 million meters per second or 186,000 miles per second.




Slowing Light Down

That's how fast light travels in a vacuum. But what happens when light shines through something clear, like water or glass? In 1862 Jean Foucault measured the speed of light in glass and found out that it was slower than in vacuum. Light travels about 2/3 as fast through glass as it does through a vacuum.

When a light beam passes from one medium into another (from air into glass or glass into air) and slows down, it also bends. Light of different frequencies bends by different amounts, which is why prisms and raindrops separate white light into its component frequencies or colors. We're not going to dwell on the colorful aspects of bending light. For that, check out our column "Watch the Skies" back in December 1997. If you don't believe that light actually bends at a boundary, check out the experiment on page 00. Here, we're going to talk about why light slows down, rather than why it bends.

Here is a model of how light slows in a material. Step back to 1900 when Maxwell's equations were the last word on what light was, and when physicists knew that atoms contained electrons. Maxwell pictured light as a wave and he knew what light was a wave in. It was a wave in the electric field. (It is also a wave in the magnetic field but we can ignore that here.)

Electric fields exert forces on electric charges. So when an electric field wave passes by an atom, it exerts forces on the electrons in each atom. Now the electrons in glass are bound to their atoms. Yet they accelerate up and down under the electric forces from the passing light. Because they are bound to their atoms, these electrons lag a bit behind the electric field wave's ups and downs.

The moving electrons remove some energy from the light. The oscillating electrons then re-emit the light. But the re-emission is slightly delayed. After the light has gone far enough into the glass, almost all of the energy of the original light had been absorbed and re-emitted many times. The net effect of these continuous delays is the slowing of the light wave as it moves through the material.

Each atom has its own characteristic delay time. Light travels at a different speed through air, through different kinds of glass, through water. In between the atoms of all these materials, light travels at the same speed that it travels through a vacuum. But all those delays mean that its net travel time is longer.




Slowing Light Way Down

To slow light down to the speed of a bicycling physicist, Dr. Lene Hau worked with light of a specific frequency. Drive around almost any big city at night and you will see yellow street lights. These are sodium lights, in which electrically excited sodium metal vapor emits light in the yellow part of the spectrum.

The sodium atoms have a strong interaction for a narrow range of yellow light frequencies. If you shine yellow light into sodium vapor, the sodium atoms absorb the light. (They re-emit the light in another direction so it doesn't reach you.) So sodium vapor is opaque to yellow light.

Dr. Hau experimented with this. When she shone a yellow laser light at the sodium atoms, they absorbed that wavelength almost as well as a block of lead would have. However, Dr. Hau then shone another wavelength of laser light onto the sodium. This second laser beam made it so that the sodium could not absorb the original beam of yellow light, via a process known as laser-induced transparency. But the sodium atoms still interacted strongly with the yellow light, slowing it down without actually absorbing it! (In her experiment about 1/3 of the yellow photons made it through the sodium cloud.)

Dr. Hau then proceeded to cool the sodium atoms down to one of the coldest temperatures ever achieved in a laboratory anywhere- 50 nanokelvins, just 50 billionths of a degree above absolute zero! As the atoms cooled they slowed down.

All atoms obey Heisenberg's uncertainty principle. As the atoms slowed, their momentum became closer and closer to zero with less and less uncertainty. Heisenberg assures us that this means that their location in space then must become uncertain. The location became so uncertain that the sodium atoms began to overlap. All of the sodium atoms began to behave like one quantum mechanically coupled object. When changing their quantum state by absorbing energy, they all had to change their quantum state together. This quantum coupled system is known as a Bose-Einstein Condensate, or BEC, after Bose and Einstein who predicted it in 1924. (Okay--we are talking about quantum mechanics, but we aren't treating light quantum mechanically here; we're treating atoms. If you want to know more about BECs, we suggest you check out http://www.colorado.edu/physics/2000/bec/index.html.)

It was in this BEC of sodium atoms that Lene Hau managed to slow light. She fired in a pulse of light and timed how long it took to cross the cigar shaped region of the BEC. Using the first equation Paul ever learned in fourth grade-velocity equals distance divided by time-she calculated the speed of light through the condensate and found it to be slowed to 17 m/s.




The Science Fiction Connection

Lene Hau succeeded in slowing the speed of light to 17 meters per second. in 1998. As usual, a science fiction author was way ahead of the scientists.

Back in 1966, Bob Shaw was nominated for the Nebula Award for a short story titled "Light of Other Days." This story, like most good science fiction, uses a technological development to get at some basic emotional truths. The technological development is "slow glass," a material that slows light down. Light shining into a piece of slow glass that's 6 millimeters thick comes out the other side ten years later. A piece of slow glass in a window frame lets city dwellers replace their view of a dingy city with a beautiful natural scene that changes with the seasons. A troubled couple, considering the purchase of a pane of slow glass, discovers some things about love and loss and what really matters.

After reading Bob Shaw's short story, Paul immediately wanted to "do the numbers." Rounding off to show that he is indeed a physicist and not a mathematician, Paul found that the light in the slow glass traveled about 1/2 a millimeter a year. That means that the light travels about 5 x 10-4 m in 3 x 107 s or about 2 x 10-11 m/s. That's about one atomic diameter in 5 seconds! That's slow!

One difference between Bob Shaw's slow glass and Lene Hau's sodium BEC is that Bob Shaw's works across the entire visible spectrum and so recreates scenes in full color, while Dr. Hau's works only at one precise frequency. Other wavelengths or frequencies of light speed through the sodium at nearly the speed of light in a vacuum.

Dr. Hau hopes to be able to improve her result soon slowing light even more to a few centimeters per second. But she still has a long way to go to catch up to the vision of Bob Shaw.

Note: For more about Pat Murphy's and Paul Doherty's work, check out their web sites at: www.brazenhussies.net/murphy and www.exo.net/~pauld.

===THE END===




Disappearing Pennies

Water slows light down--and can bend it in the process.

You'll need: a penny, a shallow bowl, a pitcher of water, and a helper.

Put the penny in the bowl. Close one eye and stoop down or step back until the rim of the bowl just blocks your view of the penny.

Have your helper pour water into the bowl. Ask him or her to pour the water slowly so that the penny doesn't move. When the water is deep enough, the penny will reappear.

You see the penny because light bouncing off the coin gets into your eye and makes an image. The water bends the light that's reflecting from the penny so that it gets over the rim of the bowl into your eyes.

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