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Atomic clocks
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Transitions of caesium define the second.
Caesium-133 hyperfine transition at 9,192,631,770 Hz defines the second; optical clocks using strontium reach 10⁻¹⁸ accuracy, losing a second in billions of years. They underlie GPS.
Relativistic time dilation measured at 1 cm height.
Interrogate atoms in vacuum.
In more detail
Atomic clocks use the very stable frequency of atomic transitions to keep time, currently based on caesium-133 hyperfine transitions. They are accurate to about one part in 10¹⁶ or better.
Real-world uses
- GPS satellites carry atomic clocks for navigation.
- Telecommunications networks synchronise using atomic time.
- Atomic clocks based on caesium keep time for GPS, telecommunications and the internet.
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Frequently asked questions
How accurate are they?
Modern optical clocks are accurate to parts in 10¹⁸ or better.
Why use atoms?
Atomic transitions are the same everywhere, giving reproducible time.
How is the second defined?
As the duration of 9,192,631,770 periods of the radiation from the hyperfine transition of caesium-133.
Related topics in Atomic & Nuclear Physics
Hydrogen-like ions · Positronium · Muonic atoms · Laser cooling of atoms