Spin component variance
= Spin component variance
{title2=$\operatorname{Var}(S_\alpha)=\langle S_\alpha^2\rangle-\langle S_\alpha\rangle^2$}
The uncertainty of a spin projection is measured by its variance. In a maximally polarized <spin coherent state>, the longitudinal variance is zero and each transverse variance is $S/2$ in units $\hbar=1$. The transverse fluctuations do not imply finite <magnon> occupation in an isotropic ferromagnet's polarized vacuum.