Caesium-133 clock transition
The caesium-133 clock transition is the specific change in energy state of a caesium-133 atom that defines the duration of one second in the International System of Units.
Also known as: cesium-133 clock transition, caesium clock frequency
How big is it?
108.8 ps · one cycle
One cycle · derived
A little perspective
Caesium-133 clock transition vs Muon lifetime: duration comparison
The lifetime of a muon is about 20,200 times one cycle of the caesium-133 clock transition.
ELAPSED TIME
The caesium-133 clock transition is the specific change in energy state of a caesium-133 atom that defines the duration of one second in the International System of Units.
Too small to see at this scale; the bar has not been widened.
Muon lifetime is the average time a free muon exists before decaying into other particles, such as an electron and neutrinos.
Measurements, assumptions & sources +
Caesium-133 clock transition
One cycle: 108.8 ps; derived.
The SI second is defined from exactly 9,192,631,770 periods of the unperturbed caesium-133 ground-state hyperfine transition. The cycle duration and photon energy here are derived from that exact frequency.
BIPM — The International System of Units
The marked one cycle is calibrated. Unmeasured illustration details are representative.
Muon lifetime
Mean lifetime: 2.197 µs; measured; range 2.197 µs–2.197 µs.
A free positive muon has a mean lifetime of about 2.197 microseconds before decay. This is a statistical mean, not an identical lifetime for every muon.
Particle Data Group — Muon properties
The marked mean lifetime is calibrated. Unmeasured illustration details are representative.
Use the visual comparison to explore its scale, then check the measurements and sources.
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Why this measurement?
Why we use 108.8 ps
We compare one cycle of the caesium-133 clock transition. The measurement is labelled derived. Check the source notes for its basis.
The SI second is defined from exactly 9,192,631,770 periods of the unperturbed caesium-133 ground-state hyperfine transition. The cycle duration and photon energy here are derived from that exact frequency.
The picture identifies the subject; the diagram compares the recorded values. How calibrated comparisons work →
Recorded properties
Dimensions and measurements
| Property | Selected value | Recorded range | Basis |
|---|---|---|---|
| 3.802 × 10⁻⁵ eV | Not recorded | derived | |
| 108.8 ps | Not recorded | derived |
Around this scale
Explore nearby values.
Compare examples with lower, similar and higher values. Each card names the measurement being compared.
Nearest lower valueTime and durationAtomic-scale transition1 fsTimeAn atomic-scale transition is a rapid change in the energy state of an atom, often occurring in extremely short intervals.Nearest higher valueTime and durationMuon lifetime2.197 µsMean lifetimeMuon lifetime is the average time a free muon exists before decaying into other particles, such as an electron and neutrinos.Sources and method
Want to check the number?
The source records and measurement method remain available for inspection.
- BIPM — The International System of UnitsThe second is defined by exactly 9,192,631,770 periods of the caesium-133 hyperfine transition.
There’s always another scale
Follow your curiosity.
TimeAtomic-scale transition1 fsAn atomic-scale transition is a rapid change in the energy state of an atom, often occurring in extremely short intervals.Fundamental physicsHydrogen 21 cm hyperfine transition5.873 × 10⁻⁶ eVThis is a change in the energy state of neutral hydrogen atoms that produces or absorbs radio waves, widely used in radio astronomy.TimeMuon lifetime2.197 µsMuon lifetime is the average time a free muon exists before decaying into other particles, such as an electron and neutrinos.TimeCamera flash1 msA camera flash is a brief, intense burst of light used to illuminate a scene for photography.
TimeCinema frame at 24 frames per second41.67 msA cinema frame at 24 frames per second is the time interval between consecutive images in standard film projection.TimeBlink of an eye150 msA blink of an eye is the quick closing and opening of the eyelid, usually happening involuntarily.TimeHeartbeat1 sA heartbeat is the regular contraction of the heart that pumps blood through the body.
GravityParabolic-flight microgravity arc0.02 NA parabolic-flight microgravity arc is a flight manoeuvre that creates short periods of near-weightlessness for people and experiments inside an aircraft.