"It's not the things we don't know that causes the trouble, it's the things we know that ain't so." -- Artemus Ward

In 1966, the Northern Irish writer Bob Shaw published a story, "Light of Other Days." In it he conceived the idea of "slow glass." This material is transparent, just like ordinary glass, so you can make windows out of it. However, light takes years, rather than a small fraction of a nanosecond, to traverse a pane of slow glass. You take a sheet of it out to a place where the scenery is spectacular, and leave it there for a long time. Then you take it home, install it in your living room, and for the next few years you will seem to be looking out onto the Rocky Mountains, or Antarctica, or the Serengeti Plain.

In writing about this story a few years ago, I praised it highly, because Bob Shaw used his idea of slow glass not just as a gimmick, but as the basis of a heartbreaking tale. Then I added, "Slow glass is not scientifically possible, because the speed of light in any material varies inversely with its refractive index. If you could ever make anything with the huge refractive index needed for slow glass, incident light would all converge to a single direction, and it could never be unscrambled to produce an image."

I felt very comfortable in making that statement, which I will call Mistake Number One.

I did not add that in the real world, refractive indices are never greater than about 2.5, so in, for example, a diamond windowpane the speed of light is still more than a hundred thousand kilometers a second. Vacuum, by definition, has a refractive index of unity. Since no refractive index is less than that, the speed of light in vacuum is as high as it can get. That's what is meant by the statement, accepted by almost all scientists, "Nothing in the universe can travel faster than light." To this should really be added the words, "in a vacuum."

Much more recently, in fact within the past year, in one of these columns I wrote about the strange new state of matter known as Bose-Einstein Condensates, or BECs. These are super-cold assemblages of atoms, just a hundred billionths of a degree above absolute zero. At this temperature, all the atoms are at their lowest possible energy level and they become indistinguishable, in practice and in principle. There are no separate atoms, just one big one.

I also wrote, "What comes next? In the near-term, meaning the next ten to twenty years, I anticipate nothing better than more experiments, developing BECs of larger and larger sizes and simply studying their properties." Also: "BECs will probably be playing vital technological roles - don't ask me what - by 2050." I will call these statements Mistake Number Two.

Since I wrote those words, Eric Cornell, Wolfgang Ketterle, and Carl Wieman were awarded the 2001 Nobel Prize in Physics for their success in creating the first Bose-Einstein Condensates. The award was based on their work of 1995, although the possible existence of BECs had been predicted by Einstein way back in 1925.

Not surprisingly, since 1995 physicists have rushed to experiment with this intriguing new state of matter. In 1999, one of the things researchers did was shoot paired laser beams into BECs. A "probe beam" and a "coupling beam" work together on the BEC, making the electrons within it unable to absorb light of a very specific wavelength. This makes the BEC completely transparent to light of that wavelength, and this in turn slows such light when it enters the BEC. The mechanism for this has nothing to do with the slowing of light caused by a higher refractive index. Moreover, the slowing is not the mere factor of 2.5 that I was describing earlier. The slowing is huge, from 300,000 kilometers a second to 17 meters a second. That is a factor of close to twenty million. You could drive your car past a pulse of light traveling at 17 meters a second, and not be pulled over on any freeway - it is less than 40 miles an hour.

New results keep coming in, faster than ever. Last year, researchers using BECs were able to bring light to a complete halt, storing it and then releasing it only when the coupling beam laser was turned back on. Early this year, the same trick was performed using not a BEC, but a crystal of the element yttrium. The experiment still calls for low temperatures, but a few degrees above absolute zero, rather than a hundred billionths of a degree.

When a pulse of light is slowed or stopped in this way, something else happens to it. It shrinks in size. For example, a flash of light that lasts for only a thousandth of a second will be two hundred miles long. When it has been slowed and is traveling inside a BEC at 17 meters a second, the same pulse is less than an inch long. This reduction in size does not distort the pulse, which when it emerges from the BEC has exactly the same properties as when it went in. This is important, because light, like radio signals, can be used to transmit digital data. Slowed light, traveling within a BEC, could be a new and efficient means of compressing information by means of light pulses. Also, because the light when released will be completely unchanged, information stored in this way is of great interest to another new and exotic field of science, quantum computing.

All this work is in its early stages. However, it has already gone far enough that I am no longer willing to state with total confidence that Bob Shaw's idea of slow glass is impossible. Also, my other assertion, that practical use of BECs is as much as half a century away, appears hopelessly pessimistic. Five years looks more like it - and at the current rate of progress, even that may be on the long side.

All this makes me read over what I have just written, and wonder how many statements, written here with confidence, will prove to be wrong. I have my favorite candidate, not because I think it is wrong but because I wish it were: "Nothing in the universe can travel faster than light."

Feel free to pick your own favorite. Your chances of being right are as good as mine.

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