Researched
Atomic Clock
Essen and Parry's caesium clock (1955) counts the vibrations of atoms and is far more stable than any pendulum or quartz clock.
Open in the interactive tree →Since 1967 the SI second has been defined by caesium-133 radiation. Atomic clocks make GPS and network timing possible and test relativity, and the best optical clocks would lose only about a second in 15 billion years.
Prerequisites
- Sundial & Water Clock~1500 BC
- Standard Time & Time Zones1883-1884Atomic clocks now keep UTC, the successor of Greenwich-based world time (1884)
- Quantum Mechanics1925
Unlocks
- Satellite navigation (GPS)1995GPS satellites carry atomic clocks
- SI Redefined by Constants2019