Experimental evidence for electron spin

Up to now, we have considered quantum particles to have three degrees of freedom, $x$, $y$, and $z$. This, then, leads to three quantum numbers that characterize the states. For example, in a central potential, the three quantum numbers are $n$, $l$ and $m$, the radial, total angular momentum, and $z$-component of angular momentum numbers, respectively.

However, there is substantial experimental evidence to suggest that there are additional degrees of freedom still missing from this simple picture that need to be accounted for. Some of this evidence will be reviewed below.