10. The Length of a Second — The Best-Known Number We Have

A unit taken off the sky

For most of history the second was a subdivision of the day, and the day is set by the rotation of the Earth. That rotation is not steady: tides drag on it, the atmosphere and oceans shift mass around, and earthquakes redistribute it slightly. By the mid-twentieth century clocks had become better than the planet they were calibrated against, making the astronomical definition the limiting error. In 1967 the 13th General Conference on Weights and Measures cut the connection and defined the second as exactly 9,192,631,770 periods of radiation from a specific transition in caesium-133. The number is not round because it was chosen to make the new second match the old one, so that nothing in daily life had to change. Every atom of caesium-133 is identical to every other, which makes the standard reproducible in any laboratory rather than held in a vault.

Ten thousand times better than the definition

Caesium clocks were superseded in the laboratory by optical clocks, which count a far faster oscillation and so slice time much more finely. A strontium optical lattice clock reported in 2024 reached a systematic uncertainty of about 8 × 10⁻¹⁹, and entanglement-enhanced work reported in 2025 pushed fractional precision to around 1.1 × 10⁻¹⁸. At that level the clock resolves general relativity as an everyday effect: raise it by a centimetre and it runs measurably faster, because time passes differently at different heights in a gravitational field. This turns clocks into instruments for geodesy, letting height differences be measured by timing rather than by surveying. It also means the second is now known far more precisely than the definition specifying it, the same situation the metre was in before 1983. Metrologists are working towards redefining the second on an optical transition, though the timetable is a matter for the international committees rather than settled fact.

The quantity everything else leans on

Time is the best-measured quantity in existence by several orders of magnitude, and that is why the other units keep being rebuilt on top of it. The metre is defined through the second and a fixed speed of light; the kilogram, since 2019, through the second and a fixed Planck constant. A measurement system reorganises itself around whatever it can count most reliably, and for the last sixty years that has been the beat of an atom.

Example: A workshop lays out every tool against its straightest edge, and the straightest edge we have is a caesium atom.

Verdict. The second has been defined since 1967 as 9,192,631,770 caesium-133 periods, and laboratory clocks now surpass that definition by orders of magnitude, making time the anchor on which the other units are built.

Overturned if: optical clocks compared across laboratories disagree beyond their stated uncertainties, meaning the claimed precision is not yet real.

Sources. 2024 · "A clock with 8×10⁻¹⁹ systematic uncertainty" — arXiv:2403.10664 · 2025 · Optica, "Unprecedented optical clock network lays groundwork for redefining the second" — Optica