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TIME IS AN ANCIENT and contrary mystery. Augustine of Hippo, writing his Confessions in a North African monastery, asked, "Who can even in thought comprehend it, so as to utter a word about it? But what in discourse do we mention more familiarly and knowingly, than time?"
More than 16 centuries later, many scholars share the feeling, if not the prospect of sainthood. "We don't even know what time is. But we can measure it really, really well," says Chris Oates, a physicist at the U.S. National Institute of Standards and Technology's campus in Boulder, Colorado. Oates' team operates an ytterbium optical lattice clock, one of the latest types of souped-up atomic timepieces. To track the passing seconds, such clocks rely on the fixed frequencies of photons absorbed and emitted by atoms' electrons as they change energy levels.
Recently, scientists have found ways to make these quantum counters even better, by switching from a reliance on microwave frequencies to the faster-paced optical regime and introducing a system of checks that relies on multiple atoms in levitated grids. In a remarkable recent development, the central atomic metronomes are protected from distortion by a method so powerful that physicists formally call it magic.
Oates is a member of a worldwide cadre working with such devices at the frontier of clockmaking. His team's clock loses about one second every few hundred million years.
Such accuracy is why time is not just one dimension among several but a foundation for defining other fundamental units. The metre's definition has been defined with increasing accuracy by such things as one 10-millionth the distance on a circular arc from the equator to the North Pole, and by a precision-made "prototype metre" bar of metal alloy kept in Paris. In 1983 the metre officially became the distance light will travel in a vacuum in 1/299,792,458 of a second. The better the stopwatch, the better such definitions can be applied.
The metrology of time is not holding still. In the April-June 2011 issue of Reviews of Modern Physics, experimental physicist Hidetoshi Katori of the University of Tokyo and theorist Andrei Derevianko of the University of Nevada declared dramatic ambitions for a record-breaking atomic clock
based on emissions from mercury atoms.
"If someone built such a clock at the Big Bang and if such a timepiece survived the 14 billion years, then the clock wouldbe off by no more than a mere second," they note in the paper - and this is actually conservative. In scientific shorthand, the proposed mercury clock would reach a fractional uncertainty of at most one part in 1018 - it would run for 1,000,000,000,000,000,000 seconds before being one second awry. That is one second in about 32 billion years, and is 10 to 100 times better than any existing clocks.
Already, atomic clocks have come a long way. In Australia, the bunch of clocks that keep time in an office in Lindfield, Sydney are accurate to a few parts in 1013, while the official U.S. timekeeper, a device called the NIST-F1, is accurate to a few parts in 1016. It occupies a large first-floor room in Building One at NIST's Boulder campus, where its dominant feature is a shiny steel vacuum chamber 2.43 m high. Inside is a laser-controlled fountain of cesium atoms chilled to near absolute zero.
The atoms rise in clumps about as large as a man's thumb and, responding to gravity, fall back through a cavity in a tuneable microwave generator.
Oscillations within the cesium atoms are akin to the to-and-fro of the balance wheel in an old wristwatch, but it is the microwave generator that communicates with the outside world. Just as the ticking of a watch arises from the escapement mechanism connected to the gears and hands, oscillations within the cavity are recorded electronically. By itself, the microwave generator would drift off time. So with each passage of the atoms, the generator checks to be sure its pulsations exactly match the signal from a chosen energy transition in the atoms' electron clouds - an electromagnetic wave that beats 9,192,631,770 times a second.
