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Caesium Seconds
The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical frequency ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s − 1 NIST F2 and friends Imagine a single caesium atom (or "cesium" atom if you stick to the American English spelling). Imagine this atom alone, at rest, undisturbed, at a temperature of absolute zero. Counterfactually, suppose that the unpaired electron of this atom in its ground state were to be irradiated with the precise frequency of light to bump the electron from its lowest energy state into the next lowest energy state, and then that the resulting radiation released as the electron returned to its lowest energy state is recorded. Call the frequency of the resulting radiation ΔνCs. By fiat, set ΔνCs = 9,192,631,770 Hz. Since the unit Hz expresses cycles per second, this means that, by definition , the duration o...
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