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| SI second | |
|---|---|
| Name | SI second |
| Quantity | time |
| Units1 | SI base unit |
| Units2 | derived SI units |
SI second The SI second is the base unit of time in the International System of Units. It is defined by a specific physical process and underpins precision timing in science, technology, navigation, astronomy, and telecommunications. The second connects national metrology institutes, international organizations, space agencies, and standards bodies to maintain global temporal coherence.
The second (symbol: s) is defined by a transition in the ground-state hyperfine structure of the caesium-133 atom as established by the International Committee for Weights and Measures, International Bureau of Weights and Measures, Comité International des Poids et Mesures, and adopted through resolutions of the General Conference on Weights and Measures. The symbol s is used in publications by the International Electrotechnical Commission, Institute of Electrical and Electronics Engineers, International Organization for Standardization, and scientific journals such as Nature (journal), Science (journal), and Physical Review Letters. National bodies like the National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, National Physical Laboratory (United Kingdom), Bureau International des Poids et Mesures coordinate to ensure the symbol and definition are uniformly applied in metrology, engineering, and industry standards such as those from International Telecommunication Union and European Telecommunications Standards Institute.
The modern definition evolved from astronomical references such as the mean solar day and the Greenwich Meridian, through the use of the ephemeris second adopted by organizations including the International Astronomical Union and the International Geophysical Year collaborations. Historical proposals involved figures and institutions like Isaac Newton, John Flamsteed, George Airy, and observatories such as the Royal Observatory, Greenwich and the Paris Observatory. Advances in atomic physics by scientists at National Physical Laboratory (United Kingdom), National Institute of Standards and Technology, Laboratoire national de métrologie et d'essais, and laboratories led by researchers influenced by work at Harvard University, Massachusetts Institute of Technology, and Université Paris-Sud culminated in the 1967 redefinition endorsed by the General Conference on Weights and Measures. The term “second” traces to sexagesimal subdivisions from Babylon and further usage in texts by Claudius Ptolemy and medieval scholars tied to institutions like University of Bologna and University of Paris.
Primary realizations of the unit rely on caesium fountain atomic clocks developed at laboratories including National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, National Physical Laboratory (United Kingdom), and research centres tied to École Normale Supérieure and Caltech. Alternative frequency standards under development employ optical lattice clocks using atoms and ions studied at University of Colorado Boulder, National Institutes of Natural Sciences (Japan), Max Planck Institute for Quantum Optics, University of Oxford, and Universität Wien using elements such as strontium, ytterbium, and aluminium. Time dissemination uses systems like Global Positioning System, Galileo (satellite navigation), GLONASS, and BeiDou, coordinated by agencies such as European Space Agency, National Aeronautics and Space Administration, and the Russian Space Agency. Measurement techniques involve microwave spectroscopy, Ramsey interrogation, frequency combs originating from work at Nobel Prize in Physics–associated groups, and comparison campaigns among institutes using fibre links and satellite two-way time transfer implemented by collaborations including European Space Agency projects and the International Telecommunication Union.
International adoption of the definition and maintenance of Coordinated Universal Time is organized by the International Bureau of Weights and Measures, the International Telecommunication Union, and the International Astronomical Union. National metrology institutes such as National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, National Research Council (Canada), Centro Nacional de Metrología (Mexico), and National Metrology Institute of Japan implement standards and contribute to the compilation of International Atomic Time. Legal and regulatory frameworks referencing the unit appear in legislation and standards from the European Union, national statutes in countries like the United States, United Kingdom, and Japan, and technical regulations from the International Electrotechnical Commission and the International Organization for Standardization.
Precise realization of the unit enables technologies and sciences including satellite navigation (used by Airbus, Lockheed Martin, Boeing), high-frequency trading overseen by regulatory bodies such as the Securities and Exchange Commission, telecommunications infrastructure standardized by 3GPP and ITU-R, fundamental physics experiments at institutions like CERN, Fermilab, and KEK, radio astronomy conducted with arrays such as the Atacama Large Millimeter Array and the Very Large Array, and geodesy networks involving the European Space Agency and NOAA. Precise timing supports tests of general relativity by teams associated with LIGO Scientific Collaboration, timing of pulsars by researchers at Jodrell Bank Observatory, and synchronization for large scientific collaborations including the Event Horizon Telescope.
Uncertainty budgets for primary clocks are evaluated in intercomparisons organized by the International Bureau of Weights and Measures and by inter-laboratory comparisons involving National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, National Research Council (Canada), and other national institutes. Calibration techniques include frequency link evaluations via optical fibre networks, satellite time transfer systems coordinated through the International Telecommunication Union, and absolute frequency measurements traceable to standards maintained at the International Bureau of Weights and Measures. Metrological traceability and uncertainty statements adhere to guidelines from the Joint Committee for Guides in Metrology and are published in reports by national institutions and international organizations such as the International Committee for Weights and Measures and International Organization for Standardization.
Category:Units of time