How do the professional timekeepers of the world determine, to the precise nanosecond, when a new year begins They simply consult an atomic clock. At the end of last month, just in time to ring in the new year, the Hewlett-Packard company, of Palo Alto, California, unveiled the latest of these meticulous time- pieces. For nearly 30 years, the firm has been supplying military and scientific clients with atomic clocks; the most advanced models neither gain nor lose more than a second every 800,000 years. But the newest version, a $54,000 device the size of desktop computer, is accurate to one second in 1.6 million years — far longer than all of human history to date.
It is natural to wonder who could possibly need such precision. The answer: practically everyone, at least indirectly. Telephone and computer networks rely on atomic clocks to synchronize the flow of trillions of bits of information around the nation and the world, thus avoiding mammoth electronic logjams. Television and radio stations use the clocks to time their broadcasts. Satellite- based navigation systems depend on the devices to measure the arrival time of radio signals to within a tiny fraction of a second, allowing users to gauge their location to within a few feet. The armed forces use atomic clocks to help steer smart missiles and time secret calls to nuclear submarines around the world. And scientists depend on atomic clocks to help track the almost imperceptible motions of continents across the surface of the earth and galaxies and stars across the sky. Even the people who dropped the ball in New York City’s Times Square to signal the start of 1992 relied on a timekeeping source that was pegged ultimately to an atomic clock.
The principle that lies behind all this precision comes out of quantum physics. When an atom is bombarded with electromagnetic radiation — in this case, microwaves — its electrons shift into a new energy state. Each type of atom responds most readily to a particular frequency of radiation. That means that when a microwave beam inside the clock is set exactly to that frequency, the maximum number of atoms will undergo the energy shift. This signals the clock’s internal computer that the device is correctly tuned. And in fact, it is the vibrating microwaves that keep time; the atoms are used just to keep them on track.
Theoretically, an atomic clock could keep perfect time, but the actual performance depends on engineering details — exactly how the microwaves hit the cesium atoms, how sophisticated the electronics are and so on. It was by improving factors like these that Hewlett-Packard boosted its clocks’ performance from incredibly good to even better. The next generation of clocks should do better still, but no one is sure when that generation will come along. For now, a second every million and a half years will have to do.

The newest atomic clock is accurate to

A. a second in 30 years.
B. a second every 800,000 years.
C. a second every million years.
D. a second in 1,6 million years.