Clock — Atomic

Since 1967, the International System of Units (SI) has defined one second as exactly 9,192,631,770 oscillations of the radiation produced by this transition in a cesium-133 atom. Why It Matters

At the heart of the clock is a physical constant: the "resonant frequency" of an atom. When atoms are exposed to specific frequencies of microwave or light energy, their electrons jump between energy levels. In a standard cesium atomic clock: Atomic Clock

An atomic clock is the world’s most accurate timekeeping device. While a grandfather clock relies on a swinging pendulum and a digital watch relies on a vibrating quartz crystal, an atomic clock uses the ultra-stable vibrations of atoms—usually cesium or strontium—to measure the passage of time. How It Works Since 1967, the International System of Units (SI)

If the microwave frequency is exactly right, the atoms change states. In a standard cesium atomic clock: An atomic

You likely interact with atomic clocks every day without realizing it:

The best modern atomic clocks are so stable that they wouldn't lose or gain a single second in —a span of time longer than the age of the universe.

GPS satellites carry atomic clocks. Because radio signals travel at the speed of light, even a billionth of a second of error could result in your GPS being off by several miles.