Atomic time

(horology) Any time system standardized with reference to an atomic resonance, such as the international standard cesium-133 transition.

Time based on the frequency determined by the energy of quantum transitions. Atomic time is obtained from the continuous operation of atomic clocks since mid-1955. The only relativistic correction to atomic time is for gravitational potential (related to height above sea level). See also Atomic clock.

An atom which drops from an energy level, E₂, to a lower one, E₁, emits radiation of frequency f = (E₁ − E₂)/h, where h is Planck's constant. The second in the International System of Units (SI) was defined in 1967 as the duration of 9,192,631,770 cycles of the radiation from a selected transition of cesium-133. See also Atomic structure and spectra; Quantum mechanics.

International Atomic Time (TAI) is generated by the International Bureau of Weights and Measures (BIPM) in Sèvres, France. It is based on approximately 200 commercial atomic clocks and up to 8 laboratory-built, cesium primary-frequency standards at approximately 44 laboratories around the world. The clock data are collected through use of the Global Positioning System (GPS) with a process called common-view time transfer. The primary physical frequency standards, with accuracy as good as 2.2 parts in 10¹⁵, provide the accuracy and long-term stability of TAI, which reproduces the SI second to better than 1 part in 10¹⁴. The highest-accuracy standards now use laser-cooled atoms. The function of the commercial clocks is to provide redundancy and short-term stability. See also Atomic clock; Satellite navigation systems.

TAI is a highly precise atomic time used, for example, in determining variations in the Earth's speed of rotation, computing orbits, and tracking celestial objects, including spacecraft. TAI provides not only the SI unit of time but also the unit of length, the meter, now defined as the distance that light travels in 1/299,792,458 of a second in vacuum. See also Physical measurement.