I keep seeing articles and posts that say that atomic clocks are based on the vibrational frequency of atoms. I even see very specific, but still incorrect, claims that the current definition of the second is the time needed for cesium atoms (sometimes even getting the isotope right) to complete 9,192,631,770 vibrations.
This obviously can't be true. Atomic vibrations are temperature dependent, so they can't be the basis of accurate clocks or, even worse, a standard of time. So why does this keep getting repeated? I don't know who first got this wrong but I suspect it doesn't matter. This error is so easy to make that it would occur spontaneously even if all examples were eliminated. The standard for a second, and atomic clocks in general, rely on atoms and are based on a vibrational frequency. But it isn't the atoms that are doing the vibrating.
Atoms are held together by the electromagnetic forces between their components. Different configurations have different energy. In the case of the cesium atomic standard, we consider the lowest possible state of all of the electrons in the cesium atoms. This will result in a single unpaired electron. That's what gives cesium its high chemical reactivity. The nucleus of the particular isotope of cesium (Cs-133) acts like a tiny magnet. The electron does as well and those two magnetic fields can orient, with respect to each other, in only two ways. Normal magnets can be in many orientations to each other but not these. This seemingly absurd statement is one of the fundamental aspects of Quantum Mechanics. If you take a Cs-133 atom in its overall ground state and expose it to just the right frequency of EM radiation it will absorb some of it and flip to the other electron-nucleus state. These two states are very close in energy so they are called a hyperfine state. This EM radiation needs to be, you guessed it, 9,192,631,770 Hz. When the Cs-133 atoms drop into the lower state they emit EM radiation at this frequency.
The Cs-133 atoms in atomic clocks are NOT what is vibrating. It is the electromagnetic field of the radiation involved in their state change.
