Speed limit: in Einstein's universe, time and distance may stretch like rubber, but the speed of light remains immutable.(UNIVERSE)(Albert Einstein)

Publication: Natural History

Publication Date: 01-FEB-05

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COPYRIGHT 2005 Natural History Magazine, Inc.

Other than the space shuttle and Superman, not much else in life travels faster than a speeding bullet. But nothing moves faster than the speed of light in a vacuum. Nothing.

But as fast as light moves, its speed is decidedly not infinite. Because light has a speed, astrophysicists know that looking out in space is the same as looking back in time. And with a good estimate for the speed of light, we can come close to a reasonable estimate for the age of the universe.

These concepts are not exclusively cosmic. True, when you flick on a wall switch, you don't have to wait around for the light to reach the floor. Some morning while you're eating breakfast and you need something new to think about, though, you might want to ponder the fact that you see your kids across the table not as they are but as they once were, about three nanoseconds ago. Doesn't sound like much, but stick the kids in the nearby Andromeda Galaxy, and by the time you see them spoon their Cheerios they will have aged more than 2 million years.

Minus its decimal places, the speed of light through the vacuum of space, in Americanized units, is 186,282 miles per second--a quantity that took centuries of hard work to measure with such high precision. Long before the methods and tools of science reached maturity, however, deep thinkers had thought about the nature of light: Is light a property of the perceiving eye or an emanation from an object? Is it a bundle of particles or a wave? Does it travel or simply appear? If it travels, how fast and how far?

In the mid-fifth century B.C. a for. ward-thinking Greek philosopher, poet, and scientist named Empedocles of Acragas wondered if light might travel at a measurable speed. But the world had to wait for Galileo, a champion of the empirical approach to the acquisition of knowledge, to illuminate the question through experiment. He describes the steps in his book Dialogue Concerning Two New Sciences, published in 1638: In the dark of night, two people, each holding a lantern whose light can be rapidly covered and uncovered, stand far apart from each...

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