Originally created 02/21/99

Got the time? Atomic timepiece sets standard

BOULDER, Colo. -- It looks more like a torpedo than a Timex.

NIST-7, the world's most accurate clock, lies in repose inside a climate-controlled federal lab at the foot of the Rocky Mountains. It is encased in a gleaming, 10-foot aluminum straitjacket. It doesn't tick.

The $3 million atomic timepiece establishes frequency for the National Institute of Standards and Technology, as well as the Paris-based International Bureau of Weights and Measures, which reports the standard Universal Coordinated Time.

Atomic clocks keep time by precisely counting the vibrations of atoms. The first version was invented in 1949 with the help of physicists who also worked on the Manhattan Project.

The current atomic clock, No. 7, is accurate to within one second every six million years, its operators say.

But why?

For millennia, time has been governed by the rhythms of nature -- a single spin of the planet on its axis equals a day. Trouble is, Earth wobbles.

Such tiny irregularities might not make a difference when you pick up the dry cleaning. But infinitesimal time errors, multiplied millions of times in the highly automated modern world, can add up. Critical operations ranging from routing electricity on the power grid to synchronizing traffic lights call for a degree of punctuality unimagined a century ago.

In an Internet debate in January about humankind's most profound achievement in the past 2,000 years, W. Daniel Hillis, the father of parallel computing, nominated timekeeping.

"The clock," Hillis said in the electronic salon Edge, "embodies objectivity. It gave us a framework in which the laws of nature could be ... quantified."

And it turns out that the atom, nature's fundamental unit, provides the best standard for measuring time.

Inside the atomic clock, a sample of a metallic element, cesium-133, is heated. Individual atoms boil off the sample and are accelerated through microwave fields. The outermost electron of a hot cesium atom oscillates at an extremely regular rate, and detectors count the vibrations.

An atomic second is equal to 9,192,631,770 oscillations.

The system is so precise that every six months, researchers calculate the Earth's wobble, as well as the friction generated by winds and tides and perturbations to the Earth's core.

If the combined forces have changed the planet's rotation, the researchers decide whether the world needs to add or subtract a second. And the atomic clock is reset accordingly.

(EDITOR'S NOTE -- You can synchronize the clock in your personal computer to atomic time through the Internet at www.boulder.NIST.gov/timefreq/.)