Editing Caesium standard (section)

{{short description|Primary frequency standard}}

[[File:NIST-F2 cesium fountain atomic clock.jpg|thumb|A caesium [[atomic fountain]] used as part of an [[atomic clock]]]]

The '''caesium standard''' is a primary [[frequency standard]] in which the [[Absorption (electromagnetic radiation)|photon absorption]] by transitions between the two [[hyperfine level|hyperfine]] [[ground state]]s of [[caesium-133]] [[atom]]s is used to control the output frequency. The first caesium clock was built by [[Louis Essen]] in 1955 at the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] in the UK<ref>{{cite journal |author=L. Essen, J.V.L. Parry |year=1955 |title=An Atomic Standard of Frequency and Time Interval: A Caesium Resonator |journal=[[Nature (journal)|Nature]] |volume=176 |pages=280–282 |doi=10.1038/176280a0 |issue=4476 |bibcode=1955Natur.176..280E |s2cid=4191481 }}</ref> and promoted worldwide by [[Gernot M. R. Winkler]] of the [[United States Naval Observatory]].

Caesium [[atomic clock]]s are one of the most accurate [[time]] and [[frequency]] standards, and serve as the [[primary standard]] for the definition of the [[second]] in the [[International System of Units]] (SI), the modern [[metric system]]. By definition, [[radiation]] produced by the transition between the two hyperfine ground states of caesium-133 (in the absence of external influences such as the [[Earth's magnetic field]]) has a frequency, {{math|Δ''ν''<sub>Cs</sub>}}, of exactly {{val|9192631770|ul=Hz}}. That value was chosen so that the '''caesium second''' equaled, to the limit of measuring ability in 1960 when it was adopted, the existing standard [[ephemeris second]] based on the [[Earth]]'s orbit around the [[Sun]].<ref>{{Cite journal | last1 = Markowitz | first1 = W. | last2 = Hall | first2 = R. | last3 = Essen | first3 = L. | last4 = Parry | first4 = J. | title = Frequency of Cesium in Terms of Ephemeris Time | doi = 10.1103/PhysRevLett.1.105 | journal = Physical Review Letters | volume = 1 | issue = 3 | pages = 105 | year = 1958 |bibcode = 1958PhRvL...1..105M }}</ref> Because no other measurement involving time had been as precise, the effect of the change was less than the experimental uncertainty of all existing measurements.

While the second is the only [[SI base unit|base unit]] to be explicitly defined in terms of the caesium standard, the majority of SI units have definitions that mention either the second, or other units defined using the second. Consequently, every base unit except the [[Mole (unit)|mole]] and every [[SI derived unit|named derived unit]] except the [[coulomb]], [[Gray (unit)|gray]], [[sievert]], [[radian]], and [[steradian]] have values that are implicitly at least partially defined by the properties of the caesium-133 hyperfine transition radiation. And of these, all but the mole, the coulomb, and the [[dimensionless quantity|dimensionless]] radian and steradian are implicitly defined by the general properties of [[electromagnetic radiation]].