Just In Time

How do the professional timekeepers of the world determine, to the precise nanosecond, when a new year begins? They simply consult an atomic clock. And last week, just in time to ring in the new, the Hewlett-Packard Co., of Palo Alto, Calif., unveiled the latest in these meticulous timepieces. Twice as accurate as earlier models, the $54,000 device -- the size of a desktop computer -- will remain reliable to the second for the next 1.6 million years, a period far longer than modern humans have existed.

Who could possibly need such precision? Practically everyone, indirectly at least. Telephone and computer networks rely on atomic clocks to synchronize the flow of trillions of bits of information, thus avoiding mammoth electronic logjams. TV and radio stations use the clocks to time their broadcasts. The armed forces employ them in satellite-based navigation systems and smart- missile guidance. 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.

The principle behind all this precision comes from quantum physics. When an atom is bombarded with electromagnetic radiation -- in this case microwaves -- it shifts into a new energy state. Each type of atom responds most readily to a particular frequency. For the cesium-133 atoms in most atomic clocks, the frequency is 9,192,631,770 vibrations per second. When a microwave beam inside the clock is set to that frequency, the maximum number of atoms will undergo the energy switch, signaling the clock's internal computer that the device is correctly tuned. The vibrating microwaves keep time; the atoms just keep them on track.

Theoretically, an atomic clock could keep perfect time; the actual performance, though, depends on the electronics and such engineering details as how the microwaves hit the cesium atoms. Hewlett-Packard will doubtless come up with other refinements, but for now losing a second every 1 1/2 million years will have to do.