atomsekunder

Atomsekunder, or atomic second, is the SI unit of time defined by the properties of the cesium-133 atom. Specifically, one atomsekund is the duration of 9,192,631,770 periods of the radiation associated with the transition between the two hyperfine levels of the atom’s ground state. This definition provides a highly stable and reproducible standard for measuring time.

The atomic second was adopted in 1967 by the General Conference on Weights and Measures, replacing the previous definition based on Earth's rotation. It underpins the realization of international time scales, including International Atomic Time (TAI) and Coordinated Universal Time (UTC). Atomic clocks that realize this second use devices such as cesium fountain clocks and hydrogen masers, delivering extremely precise timekeeping.

UTC is the world's standard for civil time and keeps near-synchrony with UT1, a solar-time scale, by adding leap seconds as needed. The ongoing pursuit of greater accuracy has led to the development of optical clocks, which use higher-frequency transitions in atoms like strontium or ytterbium. These optical clocks promise even more precise realizations of the second, with the goal of redefining the second in the future.

Atoms and timekeeping play a crucial role in navigation (for example, GPS), telecommunications, astronomy, and fundamental science. The term atomsekunder is used in some languages to denote this atomic definition of the second.