Science: Clock for the Space Age

For centuries, measuring time was simple. A day was one full rotation of the earth, and a second, the smallest unit needed, was 1/86,400th of a day. But as scientists’ standards became more exacting, they were bothered by the fact that the earth’s rotation fluctuates slightly from day to day and slows one second a century as tides and other influences slow the earth’s spin. Such tiny variations become important in space-age problems. Example: plotting the position of a long-range missile, where widely separated stations have to coordinate their observations within millionths of a second.

Looking around for a better measuring stick, scientists found that quartz can be made to vibrate electrically at very constant frequency. A quartz disk will keep time for short periods with the accuracy of one part in 10 billion (the equivalent of a one-second error in 300 years). But after a week or so, quartz changes its frequency in an unpredictable way.

Last week the National Bureau of Standards laboratories at Boulder, Colo, announced that it has settled on a system of time measurement that presumably will not change at all in thousands or millions of years. It is based on the convenient fact that atoms of cesium vibrate at an absolutely constant rate: about 9,200 million times per second. A cesium “clock” has neither a face nor hands. Instead, cesium atoms are shot down a vacuum tube, and radio waves are directed across the cesium beam. When the radio waves are at precisely the frequency at which cesium vibrates, they are absorbed. The operator of the cesium clock need only tune his waves until he gets absorption. Then he will know accurately the frequency (i.e., vibrations per second) of his waves, which can be displayed on an oscilloscope and used as reference scale. In effect, the cesium clock permits scientists to chop time into exact fractions of microseconds.

Time measured by the rotation of the earth will continue to be used, in spite of its inconstancy, for catching trains or getting to the church on time. But the cesium clock will be the arbiter for super accuracy. It will have no cumulative drift and can be read with an error of less than three parts in 100 billion (one second in a thousand years).