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 transition period · derived
A little perspective
Muon lifetime is about 20,200 times larger on this measurement.
Muon lifetime: 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
The SI second is defined from exactly 9,192,631,770 periods of the unperturbed caesium-133 ground-state hyperfine transition. Period and photon energy here are derived from that exact frequency.
BIPM — The International System of Units
The marked one transition period is calibrated. Unmeasured illustration details are representative.
Muon lifetime
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.
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Why this measurement?
Why we use 108.8 ps
For comparison, this page uses one transition period. The record labels the measurement derived; that status is retained wherever the value is compared.
The SI second is defined from exactly 9,192,631,770 periods of the unperturbed caesium-133 ground-state hyperfine transition. Period and photon energy here are derived from that exact frequency.
The identifying illustration and the numerical comparison remain separate. How calibrated comparisons work →
Recorded properties
Caesium-133 clock transition 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
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These links use compatible time records, ordered around the selected value.
Nearest lower value · TimeAtomic-scale transition1 fsAn atomic-scale transition is a rapid change in the energy state of an atom, often occurring in extremely short intervals.Nearest higher value · TimeMuon lifetime2.197 µsMuon lifetime is the average time a free muon exists before decaying into other particles, such as an electron and neutrinos.Sources and method
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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.