How do scientists measure how long a second is using vibrating atoms?

The old way, and the way most of us still use to measure a second, is derived from the time it takes for a complete day to pass.

For centuries we have split a day into twenty-four hours, an hour into sixty minutes and a minute into sixty seconds. But though the Earth’s movements from day to day are constant enough to schedule business lunches by, they do vary minutely.

Military defense systems, communications satellites and space travel require literally split-second timing. Without it, for example, a few instants of the Academy Awards would be lost in space instead of beaming from Hollywood to the network satellite, to the network affiliate, and to you. The ramifications in defense and space travel are significantly more sobering.

In the late 1940s, Isidor Rabi, a physicist at Columbia University, found that certain atoms vibrate consistently at exactly the same rates. He suggested that atoms could make good clocks.

Electronically counting the tiny vibrations of millions of atoms in fourteen-foot vacuum tubes, U.S. and British scientists found that the atom the isotope of cesium with the atomic weight 133 was one of the most consistent and easy to measure. After hundreds of tests carried out over several years, they determined that it vibrates exactly 9,192,631,770 times in what they considered an average second.

The General Conference of Weights and Measures, an international treaty organization, met in Paris in 1967 and officially changed its definition of a second. No longer measured by the Earth’s movements, the second became the exact length of time occupied by 9,192,631,770 vibrations of a cesium atom.

The beam of vibrating cesium atoms was essentially the pendulum of an incredibly accurate clock.

How accurate? The best atomic clocks in the world will be off by one second after 300,000 years of steady running, compared with a quartz wristwatch, which will (theoretically, of course) miss the mark by at least a million seconds, or 278 hours, over the same time period.