g-factor

The g-factor is a plain number that says how strong a particle's magnetic moment is for a given amount of spin. If spin and magnetism were related in the simplest classical way you might expect, the g-factor would be exactly one. It turns out that nature does not follow that naive expectation, and the deviation of g from the simple value is itself deeply informative.

For the electron's spin the g-factor is very close to two, not one. This factor of two was a genuine puzzle that classical thinking could not supply; Dirac's relativistic equation for the electron predicted it cleanly, which was one of that theory's great triumphs. The g-factor is therefore a bridge between the measured magnetism of a particle and the deeper theory that explains where it comes from.

Measured with exquisite care, the electron's g-factor is not exactly two but a little larger, by about one part in a thousand. That small excess comes from the electron constantly interacting with the seething quantum vacuum around it, and quantum electrodynamics predicts it to a dozen decimal places. The agreement between that prediction and experiment is one of the most precise tests in all of science, which is why this humble number is so prized.