electron spin discovery

By 1925 atomic spectra showed fine details, such as single spectral lines splitting into closely spaced pairs, that the known quantum numbers could not explain. Two young Dutch physicists, Samuel Goudsmit and George Uhlenbeck, made a bold proposal: the electron carries a fixed intrinsic angular momentum of its own, as if it were spinning, plus a matching magnetic moment. This new property neatly accounted for the puzzling extra splittings.

Their idea was almost retracted before it was published. A simple estimate suggested that a literal spinning ball of an electron would have to whirl faster than light at its surface to give the required angular momentum, which was clearly impossible. The senior physicist Paul Ehrenfest sent their paper off anyway, reasoning that they were young enough to survive a mistake, and the idea proved right even though the spinning-ball picture was wrong.

Spin turned out to be a genuinely new, non-classical property with no need for any little ball to rotate. Within a few years Wolfgang Pauli gave it a proper quantum description and Paul Dirac showed it emerged naturally when quantum mechanics was made consistent with special relativity. The discovery is a fine example of an inspired guess that captured a real truth while its motivating image had to be discarded.