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Yet even the most accurate clock in the world can't answer the question of what all these atoms are actually measuring. What is time anyway? According to Isaac Newton, both space and time were fixed attributes of the universe, a God-given stage on which events unfolded. But Albert Einstein torpedoed that idea with his theories of special and general relativity: the only thing that's fixed in the cosmos, he showed, is the speed of light.
Everything else--including space and time, which he melded into a single entity called space-time--is relative, as malleable as rubber. The Big Bang theory further established that space-time came into existence at a definite point in the past. Talking about what happened "before" the Big Bang is meaningless--as absurd as talking about what lies north of the North Pole.
This elasticizing of space-time means, for example, that observers might disagree over which of two events happened first--and both could be right. Even more bizarrely, physicists including Stephen Hawking have seriously discussed the possibility that relativity might make it feasible (though not with any technology we know of today) to send objects backward in time.
Does that mean you could go back, kill your own grandfather and keep yourself from being born--a seeming absurdity? Maybe not, say some physicists. In one interpretation of quantum physics, the world splits at each moment into an infinite number of universes that proceed in parallel; if you killed your grandfather, it might be in an alternate universe, so it would have no effect on the universe you inhabit.
Quantum physics demolishes the conventional concept of time in its own peculiar ways. Measured at short enough durations, space-time loses its apparently smooth, continuous structure, devolving into what Princeton physicist John Wheeler calls "quantum foam." The orderly flow of events may really be as much an illusion as the flickering frames of a movie. And according to independent physicist Barbour's new book, even the apparent sequence of the flickers is illusory.
It gets even worse. In some versions of M theory--the latest rage in physics, which attempts to meld relativity and quantum theory--there may be more than three dimensions of space and more than one dimension of time. What does that mean? Even the experts have no clue. "We're trying to understand it," says Harvard theorist Cumrun Vafa. "It's quite mysterious."
For physicists, then, time is an exceedingly complex and slippery concept. No wonder St. Augustine couldn't explain it. But when the month, the year, the century and the millennium end next week, it's a fair bet that theoretical physicists, like the rest of us, will be partying to welcome in the year 2000--whether it really exists or not.
--Reported by Mairi Brahim/London and Dick Thompson/Washington