The second is a number of wiggles
Since 1967 the second has been defined as 9,192,631,770 periods of the radiation from a particular transition in caesium-133.
Established2 min read
The Earth’s spin wobbles. A second that is 1/86,400 of a solar day wobbles with it. Metrologists moved the definition to an atomic transition that can be counted anywhere. The integer 9,192,631,770 was chosen so the new second matched the old ephemeris second.
Atomic clocks now disagree at the eighteenth decimal place. Leap seconds were the apology to the wobbly planet. The apology is being retired; the atom will not be. When someone says ‘a second’, they are, whether they know it or not, talking about caesium.
Caveats
- Optical clocks are already more stable than caesium. A redefinition is likely. The principle — time as a count of a transition — will stay.
Sources
- 1Bureau International des Poids et Mesures. The International System of Units (SI). BIPM, 2019.