The second is defined by caesium atoms ticking 9.19 billion times
Atomic clocks keep time by watching atoms' resonant frequencies. The SI second fixes the caesium-133 ground-state hyperfine transition at exactly 9,192,631,770 hertz. Ensembles of such clocks worldwide maintain International Atomic Time beneath civil UTC.
Every clock counts some regular oscillation: Earth's spin for a sundial, a swinging pendulum, a vibrating quartz crystal. Those all drift with temperature. Atoms are steadier because their energy levels are fixed, and only one precise frequency of radiation will push them from one level to the next. An atomic clock prepares atoms in the lower state, bathes them in microwaves, and tunes the signal until the largest number flip, locking the frequency to nature. Satellite navigation depends on this: a timing error of a single nanosecond means a position off by about 30 centimetres, which is why GPS and Europe's Galileo carry atomic clocks.
James Clerk Maxwell suggested in 1873 that light vibrations would make a more universal time unit than the Earth's rotation, and Lord Kelvin and Isidor Rabi pursued the idea; Rabi built atomic beam resonance equipment in the 1930s. An ammonia-based prototype followed in 1949, the same year Alfred Kastler and Jean Brossel invented optical pumping, and Britain's National Physical Laboratory built the first practical caesium clock. The National Radio Company sold more than 50 Atomichrons in the 1950s, and Hewlett-Packard released its rack-mounted 5060 in 1964.
Before 1968 the second was a fraction of the tropical year 1900; since then it has been defined by caesium, later specified as an atom at rest at absolute zero. International Atomic Time averages clocks worldwide, and UTC adds leap seconds to stay within a second of Earth's rotation. America's primary standard, the NIST-F2 caesium fountain, is uncertain to about one part in 10 to the 16th. In 2004 NIST built a chip-scale clock the size of a rice grain drawing 125 milliwatts, sold commercially from 2011.
Lasers and optical frequency combs in the 1990s opened the far higher frequencies of light. NIST's aluminium-ion quantum logic clock beat caesium in 2010, JILA's strontium clock reached 10 to the minus 18th in 2015, and in July 2025 a new NIST aluminium clock achieved about 19 decimal places of accuracy. After six countries compared optical clocks in June 2025, metrologists expect to redefine the second around 2030 to 2034.
Source: Atomic clock
