According to the International System of Units (SI), one second is
the duration of \(9\,192\,631\,770\) periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium 133 atom.
This definition is permissible because of the principle of relativity, according to which all physical laws are the same in every inertial reference frame. In particular, this principle asserts that given the inertial reference frame of any Cesium 133 atom in the universe, the law of how fast the transitions between two hyperfine levels of its ground state occur, is always the same.
Definitions: 1
Proofs: 2
Propositions: 3