second

second
ЮК · Public domain · source
Name	second
Quantity	time
Unit	SI base unit
Defined	"The oscillation of the ground-state hyperfine transition of the caesium-133 atom"
First defined	1967
Derived from	"caesium frequency"

second

The second is the SI base unit of time and a fundamental measure used across physics, astronomy, engineering, navigation, and chronometry. It underpins standards maintained by institutions such as the International Bureau of Weights and Measures, the International Committee for Weights and Measures, and national laboratories including the National Institute of Standards and Technology and the National Physical Laboratory (United Kingdom), enabling synchronization of systems like the Global Positioning System, astronomical observatories such as the European Southern Observatory, and telecommunications networks run by corporations like AT&T and Deutsche Telekom.

Definition and SI Unit

The modern definition of the second is internationally codified: it is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition, adopted by the General Conference on Weights and Measures in 1967, replaced astronomical definitions based on the mean solar day and the tropical year. The symbol for the unit is "s" as established by the International System of Units, and the second functions alongside other SI base units such as the metre, the kilogram, and the ampere in forming derived units like the hertz and the newton.

History and Development

Earlier measures of the second derived from subdivisions of the mean solar day and were influenced by calendrical authorities like the Julian calendar and reformers such as Pope Gregory XIII. Astronomical timekeeping advanced through work at observatories including the Royal Greenwich Observatory and the Paris Observatory, where mean time and ephemeris time were developed to address irregularities in Earth's rotation noted by scientists like Simon Newcomb and institutions like the U.S. Naval Observatory. The shift to atomic time began with experiments by physicists such as Isidor Isaac Rabi and was accelerated by laboratories including the National Bureau of Standards and research groups at Harvard University and the University of Paris (Sorbonne). The establishment of International Atomic Time and the adoption of the caesium standard were formalized through resolutions of the International Astronomical Union and the International Telecommunication Union.

Measurement and Standards

Measurement of the second today relies on atomic clocks: cesium beam standards, hydrogen masers, and optical lattice clocks pioneered by teams at the National Institute of Standards and Technology, the Physikalisch-Technische Bundesanstalt, and the National Research Council (Canada). Timekeeping agencies form the Bureau International des Poids et Mesures-coordinated ensemble that produces Coordinated Universal Time by combining inputs from laboratories like the Laboratoire National de Métrologie et d'Essais and the Chinese Academy of Sciences. Techniques such as frequency combs developed by researchers including John L. Hall and Theodor W. Hänsch enable comparisons between microwave and optical standards, while satellite-based systems like the Global Positioning System and the Galileo (satellite navigation) constellation disseminate time signals. International agreements and protocols by bodies like the International Civil Aviation Organization and the European Telecommunications Standards Institute ensure interoperability of time distribution.

Applications and Usage

Precise realization of the second is essential for technologies and scientific fields: synchronization in telecommunications operated by companies such as Vodafone and Verizon, time stamping in financial markets regulated by authorities like the Financial Stability Board, and navigation using receivers connected to the GLONASS and BeiDou systems. Astronomy and space missions run by agencies such as NASA, the European Space Agency, and Roscosmos depend on accurate seconds for mission timing, orbital dynamics, and pulsar timing arrays studied at facilities like the Arecibo Observatory and the Jodrell Bank Observatory. Metrology applications include calibrating instruments at the International Electrotechnical Commission and supporting experiments in particle physics at laboratories like CERN and Fermilab where time-of-flight measurements and synchrony are critical. Everyday devices—from wristwatches by manufacturers such as Rolex and Seiko to smartphones by Apple Inc. and Samsung Electronics—trace their timekeeping lineage to standards established for the second.

Cultural and Linguistic Aspects

The concept and word for the second have permeated languages and cultures, appearing in idioms and measurements codified by institutions like the Oxford English Dictionary and linguistic studies at universities such as Cambridge University and University of Oxford. Historical timekeeping artifacts in museums such as the Smithsonian Institution and the British Museum reflect evolving social rhythms shaped by industrialization associated with companies like Siemens and transport networks like the London Underground. The second figures in legal and regulatory texts drafted by bodies such as the European Commission and national parliaments, affecting standards for broadcast timing and sports adjudication overseen by organizations including the International Olympic Committee and the Fédération Internationale de Football Association.

Category:Units of time