Caesium is a soft, silvery-gold metal that becomes liquid when stored in a warm room. It is mostly found in mineral deposits near a small lake in the

The mind-bending new science of measuring time

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2024-11-24 13:00:25

Caesium is a soft, silvery-gold metal that becomes liquid when stored in a warm room. It is mostly found in mineral deposits near a small lake in the wilds of central Canada. Its main commercial use is as an ingredient in drilling fluids for petroleum exploration. But thanks to quirks of chemistry and history, caesium is also the metronome of the world, the ultimate source of all modern time.

For millennia, celestial phenomena were our timekeepers and calendars, the best clockwork we had. Prehistoric tombs and monuments around the world are perfectly aligned with the sunrise on the solstice. We knew time passed because we saw things change. The Sun rose, seasons turned.

As late as the middle of the 20th century, our time remained tied to the Sun. A second was officially defined as a fraction of the solar year. But in 1967, deep in the atomic age, the 13th General Conference on Weights and Measures in Paris ruled that the second would now be defined according to vibrations of the caesium atom. Ever since, timekeeping has become the domain of physicists, extracted in sunless laboratories with precision optics, synthesised by computers and distributed by satellites.

Caesium atoms, when excited by just the right frequency, resonate, like a wine glass shattered by an opera singer. By measuring this frequency, we measure time. Atoms make for handy clockwork. They don’t have mechanical parts, and they don’t wear out. They are attractively standard. While sunlight and pendulums vary, every caesium atom is identical to any other. And they tick very fast.