"The SI second is defined by a fixed caesium-133 transition frequency"
The SI second is defined by fixing the numerical value of the caesium-133 hyperfine transition frequency to 9,192,631,770 when expressed in hertz (Hz).
This is a human-established fact, not a universal axiom. It is true within INTERNATIONAL as enacted by BIPM.
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Answer
sec:answer-f428301c
The SI second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium-133 hyperfine transition frequency, ??Cs, to be 9,192,631,770 when expressed in hertz (Hz), where Hz = s^-1.
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