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by Ciro Santilli (@cirosantilli, 37)

Caesium standard (1967, 3.26 cm)

 ... List of systems of units International System of Units Dimension of the International System of Units Time Clock Atomic clock
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Tags: Caesium
Uses the frequency of the hyperfine structure of caesium-133 ground state, i.e spin up vs spin down of its valence electron 6s1, to define the second.
International System of Units definition of the second since 1967, because this is what atomic clocks use.
TODO why does this have more energy than the hyperfine split of the hydrogen line given that it is further from the nucleus?
Why caesium hyperfine structure is used:
  • physics.stackexchange.com/questions/191871/why-do-atomic-clocks-only-use-caesium

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  • Hyperfine structure
  • Kibble balance

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