How Does an Atomic Clock Keep Time?

In a caesium atomic clock, caesium-133 atoms are exposed to microwave radiation. When the radiation frequency matches exactly 9,192,631,770 Hz — the natural resonance of the caesium atom's hyperfine transition — the atoms absorb maximum energy. A feedback loop tunes the microwave source to this peak, and a counter tallies the oscillations: every 9,192,631,770 cycles equals exactly one second (by definition since 1967). Modern optical lattice clocks use strontium or ytterbium atoms and laser light at much higher frequencies, achieving accuracy to within one second over the age of the universe — roughly 14 billion years.