Scientists had long realized that atoms (and molecules) have resonances; each chemical element and compound absorbs and emits electromagnetic radiation at its own characteristic frequencies. These resonances are inherently stable over time and
space. An atom of hydrogen or cesium here today is exactly like one a million years ago or in another galaxy. Here was a
potential "pendulum" with a reproducible rate that could form the basis for more accurate clocks.

The development of radar and extremely high frequency radio communications in the 1930s and 1940s made possible the
generation of the kind of electromagnetic waves (microwaves) needed to interact with the atoms. Research aimed at developing
an atomic clock focused first on microwave resonances in the ammonia molecule. In 1949 NIST built the first atomic clock,
which was based on ammonia. However, its performance wasn't much better than existing standards, and attention shifted
almost immediately to more-promising, atomic-beam devices based on cesium.

In 1957 NIST completed its first cesium atomic beam device, and soon after a second NIST unit was built for comparison
testing. By 1960 cesium standards had been refined enough to be incorporated into the official timekeeping system of NIST.

In 1967 the cesium atom's natural frequency was formally recognized as the new international unit of time: the second was
defined as exactly 9,192,631,770 oscillations or cycles of the cesium atom's resonant frequency replacing the old second that
was defined in terms of the earth's motions. The second quickly became the physical quantity most accurately measured by
scientists. The best primary cesium standards now keep time to about one-millionth of a second per year.

Much of modern life has come to depend on precise time. The day is long past when we could get by with a timepiece accurate
to the nearest quarter hour. Transportation, communication, manufacturing, electric power and many other technologies have
become dependent on super-accurate clocks. Scientific research and the demands of modern technology continue to drive our
search for ever more accurate clocks. The next generation of cesium time standards is presently under development at NIST's
Boulder laboratory and other laboratories around the world.