The transitions of which element are used to define the international standard for the length of a single second?
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Since 1967, the official definition of a second is based on the vibrations of the caesium-133 atom. Specifically, it is the time it takes for a caesium atom to transition between two energy states exactly 9,192,631,770 times. Photo: National Institute of Standards and Technology - Physics Laboratory: Time and Fr, Wikimedia Commons, Public domain.
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Source: Wikipedia (CC BY-SA)
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