second (SI unit) — LLMpedia

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second (SI unit) The second is the SI base unit of time used internationally in International System of Units conventions, scientific publications, chronometry instruments and standards organizations. It underpins measurements in astronomy observatories, metrology laboratories, satellite systems and navigation agencies, enabling interoperability among institutions such as National Institute of Standards and Technology, International Bureau of Weights and Measures, European Space Agency, NASA and Japan Aerospace Exploration Agency. Its definition, dissemination and improvement involve collaborations between entities like International Telecommunication Union, Comité International des Poids et Mesures, International Astronomical Union, and national metrology institutes such as Physikalisch-Technische Bundesanstalt and National Physical Laboratory (United Kingdom).

Definition and Symbol

The symbol for the unit is s, adopted by the International System of Units and used in publications from bodies such as International Organization for Standardization, Institute of Electrical and Electronics Engineers, American National Standards Institute, IEEE Standards Association and European Committee for Standardization. The formal definition is given by frequency-based realizations tied to transitions of the element used by primary standards at laboratories like National Research Council (Canada), CSIRO, China National Institute of Metrology and Korea Research Institute of Standards and Science. Standards documents from organizations including International Electrotechnical Commission and protocols from Internet Engineering Task Force reference the symbol and practices for expressing the unit in scientific and technical contexts.

Historically, subdivisions of the day were used in civilizations such as Ancient Egypt, Babylon, Ancient Greece and by astronomers associated with institutions like Royal Observatory, Greenwich and figures such as Galileo Galilei and Isaac Newton. The mechanical second emerged with developments at workshops of inventors like Christiaan Huygens and instrument makers allied to patrons including Royal Society. In the 19th and 20th centuries, observatories such as United States Naval Observatory, Paris Observatory and scholars like Friedrich Bessel and Simon Newcomb influenced timekeeping leading to atomic definitions adopted after experiments by scientists at National Physical Laboratory (United Kingdom), Laboratoire national de métrologie et d'essais and institutions where researchers like Isidor Rabi and teams at National Bureau of Standards performed microwave spectroscopy. The shift from astronomical to atomic time involved organizations such as International Astronomical Union and treaties negotiated through representatives of states including United Kingdom, United States, France and Soviet Union.

Modern realizations rely on atomic transitions, particularly the hyperfine transition of caesium-133 used in caesium fountain clocks built and tested at National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, Time and Frequency Division (NIST), SYRTE and laboratories like BIPM collaborating with teams at JET PROPULSION LABORATORY. Optical lattice clocks based on elements such as strontium, ytterbium and ions like aluminium ion are developed by groups at National Institute of Standards and Technology, University of Tokyo, Paris Observatory and University of Colorado Boulder to achieve higher stability. Time transfer techniques include two-way satellite time and frequency transfer used by Global Positioning System, Galileo (satellite navigation), GLONASS and BeiDou, and fiber-optic links realized in networks connecting institutions like CERN, CNRS and Max Planck Society labs. Measurement protocols reference metrology committees including Consultative Committee for Time and Frequency and implementations use equipment from manufacturers like Rohde & Schwarz and Keysight Technologies.

As a base unit of the International System of Units, the second provides the temporal foundation for derived units such as the hertz, metre (via speed of light), newton (through dynamics), watt and electrical units standardized by International Electrotechnical Commission and international treaties administered by United Nations agencies. The unit’s stability affects legal metrology overseen by institutions like Organisation internationale de métrologie légale and underpins global infrastructures operated by entities such as International Maritime Organization, International Civil Aviation Organization and World Meteorological Organization. Revisions to its definition engage conferences of parties represented at meetings of Comité International des Poids et Mesures and technical committees convened by Bureau International des Poids et Mesures.

Precise seconds are essential in satellite navigation systems like Global Positioning System, Galileo (satellite navigation), GLONASS and BeiDou, in particle physics experiments at CERN and observatories such as Arecibo Observatory and European Southern Observatory. Telecommunications networks managed by companies like AT&T, Deutsche Telekom, NTT and standards bodies including 3GPP and International Telecommunication Union require synchronization to the second for protocols and billing. Financial markets in centers such as New York Stock Exchange, London Stock Exchange, Tokyo Stock Exchange and Hong Kong Stock Exchange depend on timestamping aligned with national time scales produced by agencies like NIST, NPL and ANSTO. Scientific research in fields represented by Max Planck Society, Lawrence Berkeley National Laboratory, Fermilab and universities such as Harvard University and University of Cambridge uses the unit for experiments, simulations and publications.

Maintaining the second’s accuracy is the remit of metrology institutes including BIPM, NIST, PTB, NPL and VNIIMS which coordinate via comparisons and publications such as Circular T and reports to the Comité International des Poids et Mesures. Uncertainty budgets for primary standards are assessed using procedures from organizations like International Organization for Standardization and evaluated in intercomparisons with facilities at SYRTE, Physikalisch-Technische Bundesanstalt and National Research Council (Canada). Improvements in optical clock research at institutions including University of Colorado Boulder, National Institute of Standards and Technology and RIKEN have reduced systematic uncertainties and prompted international discussions about future redefinitions coordinated by committees such as the Consultative Committee for Time and Frequency and overseen by Comité International des Poids et Mesures.

Category:Units of time