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Quantum Wonders card QTM-41, Atomic Clocks, with a glowing paper-cut illustration. It reads: One second is defined as 9,192,631,770 vibrations of the microwave that matches a quantum jump in a caesium atom.

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Atomic Clocks

What’s going on

Every caesium-133 atom has two energy levels separated by exactly the same tiny step. Microwaves tuned to that step make the atoms flip, and the clock locks its ticking to that frequency. Since 1967 this has defined the second for the whole world. Your phone’s map works because GPS satellites carry atomic clocks: an error of one millionth of a second would put you about 300 metres off.

Myth

Atomic clocks are radioactive.

Fact

They use no decay, just a steady jump between two energy levels.

Learn more

BIPM, The International System of Units (SI Brochure, 9th ed., 2019); NIST Time and Frequency Division.

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QTM-41 from Quantum Wonders, free quantum physics cards for ages 10 to adult

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