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TIME, IT HAS been said, is nature's way of keeping everything from happening at once. That may sound like a frivolous definition, but it's about the best we can do. Time, like space, is a fundamental part of our world. Ask a physicist or a philosopher to define it more precisely and, if they don't immediately change the subject, they'll probably tell you what you already know: time, they will say, is what clocks measure.
Our species may not be ready to show off its deep understanding of the nature of time at the next intergalactic, interdisciplinary symposium, but we could probably give a decent lecture on how to measure it. From the first stick-in-the- ground sundials to the current crop of atomic clocks, the story of time- keeping parallels the story of civilisation itself. Some historians will even argue that our civilisation advanced because we learned how to measure time. Either way, clocks have become ubiquitous. If we don't know the time, we feel a bit lost.
For proof that we take time-keeping seriously, consider the "leap second". At first, it sounds silly to reset a clock by such a trivial amount. At the end of this year, however, the keepers of the world's atomic clocks will do just that. The reason is fairly simple: the Earth is slowing down. If we kept our clocks on solar time - as we did until about 30 years ago - this wouldn't be an issue. But it's now the atom that marks the flow of time. And left unchecked, atomic time would eventually get out of step with our spinning planet. "Atomic clocks are actually considerably more accurate than the Earth itself," explains Jonathan Betts, curator of horology at the Old Royal Observatory in Greenwich. The leap second, he says, "asks the atomic clocks to hold their breath for one second, so that the Earth can catch up". Indeed, without such corrections, the sun would be overhead at midnight rather than noon in about 3,000 years.
Today's atomic clocks measure time by counting oscillations of the caesium atom, which vibrates 9,192,631,770 times per second. That allows the world's best clocks to keep time to about one billionth of a second (one nanosecond) per day -...