International Atomic Time

International Atomic Time
Name	International Atomic Time
Abbreviation	TAI
Established	1972
Governing body	Bureau International des Poids et Mesures
Basis	Caesium atomic frequency standard
Relation to UTC	TAI = UTC + 37 seconds (as of 2024)

International Atomic Time. It is a high-precision time scale that forms the basis for global timekeeping. Calculated by the Bureau International des Puits et Mesures using data from atomic clocks worldwide, it provides a continuous and stable reference. This time scale is fundamental to the generation of Coordinated Universal Time, the standard used for civil time across the globe.

Definition and basis

The definition relies on the precise frequency of the caesium-133 atom, as established by the International System of Units. This definition states that one second is the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state. The practical realization is achieved through an ensemble of atomic clocks operated by institutions like the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt. These devices, including advanced hydrogen maser and optical lattice clock technologies, measure this invariant quantum phenomenon to generate an extremely stable frequency.

History and development

The quest for a time standard based on atomic transitions began in the mid-20th century, following the invention of the first practical caesium beam atomic clock at the National Physical Laboratory (United Kingdom). By 1967, the 13th General Conference on Weights and Measures officially redefined the second in terms of atomic resonance, replacing the ephemeris second based on Earth's orbit. International Atomic Time was formally instituted in 1972, with its computation starting retrospectively from 1958. Key developments were driven by laboratories such as the United States Naval Observatory and the Paris Observatory, leading to its adoption by the International Astronomical Union.

Calculation and dissemination

The calculation is performed by the Bureau International des Poids et Mesures in Sèvres, France, using a weighted average of data from over 400 atomic clocks in more than 80 national laboratories, including the National Metrology Institute of Japan and the All-Russian Scientific Research Institute for Physical-Engineering and Radiotechnical Metrology. This process, known as time and frequency transfer, often utilizes signals from the Global Positioning System and Two-Way Satellite Time and Frequency Transfer. The resulting scale is published monthly in the BIPM Circular T, providing the definitive reference for participating institutions like the European Space Agency.

Role in Coordinated Universal Time (UTC)

It serves as the continuous heartbeat for Coordinated Universal Time, which is derived directly from its readings. However, UTC is adjusted by the insertion of leap seconds to remain within 0.9 seconds of Universal Time (UT1), which is based on the Earth's rotation. These leap second decisions are made by the International Earth Rotation and Reference Systems Service. Therefore, the relationship is defined as UTC = TAI – (current number of leap seconds), a system ratified by the International Telecommunication Union.

Comparison with other time scales

It differs from astronomical time scales like Universal Time and Terrestrial Time, which are tied to the Earth's variable rotation and relativistic frame, respectively. Unlike Geocentric Coordinate Time, used in the theories of motion for solar system bodies, it is a realized time scale measured on the geoid. Its principal distinction from Global Positioning System time is that the latter does not include leap seconds and was set to match it in 1980, but has since diverged by several seconds due to different steering policies.

Applications and significance

Its extreme stability is critical for the operation of the Global Positioning System, Galileo (satellite navigation), and GLONASS, enabling precise geolocation. It is fundamental to scientific endeavors such as very-long-baseline interferometry conducted by the Event Horizon Telescope and tests of general relativity by missions like the Gravity Probe B. Within the International System of Units, it underpins the definition of other units, including the volt and the metre. Its precision supports technologies from high-frequency financial trading on the New York Stock Exchange to the synchronization of power grids and telecommunications networks operated by companies like AT&T.

Category:Time scales Category:Metrology