See Inside A Matter of Time

Ultimate Clocks [Preview]

Atomic clocks are shrinking to microchip size, heading for space--and approaching the limits of useful precision

In Brief


  • A renaissance under way in atomic clock building is expected to improve the precision of timekeeping by 1,000-fold. Scientists have invented atomic clocks that are even more precise than cesium clocks--so accurate, in fact, that they can no longer measure time in seconds, which are defined by the properties of cesium.
  • In theory, one can measure time with infinite accuracy. But gravity and motion distort time, imposing a practical limit to clocks' precision.
  • Atomic clocks are short-lived. Engineers are also designing a mechanical clock that could operate through the year 12,000.


DOZENS OF THE TOP CLOCKMAKERS IN THE WORLD CONVENED IN NEW ORLEANS ONE muggy week in May 2002 to present their latest inventions. There was not a mechanic among them; these were scientists, and their conversations buzzed with talk of spectrums and quantum levels, not gears and escapements. Today those who would build a more accurate clock must advance into the frontiers of physics and engineering in several directions at once. They are cobbling lasers that spit out pulses a quadrillionth of a second long together with chambers that chill atoms to a few millionths of a degree above absolute zero. They are snaring individual ions in tar pits of light and magnetism and manipulating the spin of electrons in their orbits.

Thanks to major technical advances, the art of ultraprecise timekeeping is progressing with a speed not seen for 30 years or more. These days a good cesium beam clock, of the kind Symmetricom sells for about $50,000, will tick off seconds true to about a microsecond a month, its frequency accurate to five parts in 1013. The primary time standard for the U.S., a cesium fountain clock installed in 1999 by the National Institute of Standards and Technology (NIST) at its Boulder, Colo., laboratory, is good to five parts in 1016 (usually written simply as 10−16). That is 1,000 times the accuracy of NIST's best clock in 1975. Successful prototypes of new clock designs—devices that extract time from aluminum or mercury ions instead of cesium—have recently attained accuracy in the 10−18 range, a 100-fold improvement in a decade.

Accuracy may not be quite the right word. The second was defined in 1967 by international fiat to be “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.” Leave aside for the moment what that means: the point is that to measure a second, you have to look at cesium. The best clocks now don't—so, strictly speaking, they don't measure seconds. That is one predicament the clockmakers face.

Further down the road lies a more fundamental limitation: as Albert Einstein theorized and experiment has confirmed, time is not absolute. The rate of any clock slows down when gravity gets stronger or when the clock moves quickly relative to its observer—even a single photon emitted as an electron reorients its magnetic poles or jumps from one orbit to another. By putting ultraprecise clocks on the space station, scientists hope to put relativity theory through its toughest tests yet. But now that clocks have achieved a precision of 10−18—proportions that correspond to a deviation of less than half a second over the age of the universe—the effects of relativity are starting to test the scientists. No technology exists that can synchronize clocks around the world with such exactness.

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