SIseconde

The second, symbol s, is the base unit of time in the International System of Units (SI). It is defined by a fixed number of cycles of radiation: the period corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.

Historically, time standards were tied to celestial and Earth-based processes, but atomic properties provide far greater stability. In 1967, the second was defined as 9,192,631,770 periods of the radiation corresponding to the cesium-133 transition, establishing a precise and reproducible reference for clocks.

In 2019 the SI redefinition fixed the numerical value of the cesium-133 transition frequency, ΔνCs, at 9,192,631,770 hertz. The second is thus defined as the duration of 9,192,631,770 periods of this radiation. This makes the second an exact unit, independent of any physical artifact, and ties time measurement directly to atomic properties.

In practice, the second is realized by atomic clocks, including cesium fountain clocks and more recent optical clocks that achieve even higher stability. The second underpins submultiples such as the millisecond (10^-3 s), microsecond (10^-6 s), and nanosecond (10^-9 s), and it serves as the basis for timekeeping, navigation, telecommunications, and many fields of science and engineering.

See also: time in physics, atomic clock, base units of the SI.