What defines how long one second lasts?
Chemistry Inside the Atom
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One second is defined as 9,192,631,770 times the period of the electromagnetic radiation (see the “Physical and Theoretical Chemistry” chapter for more on electromagnetic radiation) corresponding to the difference in hyperfine energy levels in the ground state of a Cesium–133 atom.
What does this mean? To begin, the difference in two closely spaced energy levels of a Cesium–133 atom defines a specific gap in energy. Using the relationship between the energy and frequency of a photon of light, this energy gap can be converted to a frequency of light. Recall that light is electromagnetic radiation. Also recall that the reciprocal of the frequency of light tells us the period of the oscillation of the electromagnetic fields that make up the light. The period tells us how long it takes the electric and magnetic fields to oscillate a single time, and one second is defined as 9,192,631,770 times this (extremely brief) time interval. As those 9,192,631,770 oscillations of the electric field of the light take place, the second hand of each clock on Earth moves 1/60th of a rotation forward.