= Null current of a commuting Majorana spinor
{title2=$K^\mu K_\mu=0$}
In four dimensions a single commuting <Majorana spinor> is specified by a two-component complex spinor $\xi$. Its future <Dirac current> is proportional to $(\xi^\dagger\xi,\xi^\dagger\boldsymbol\sigma\xi)$, and the <Pauli matrix multiplication law> implies $|\xi^\dagger\boldsymbol\sigma\xi|^2=(\xi^\dagger\xi)^2$. Thus it is null. A Dirac spinor or a sum of independent Majorana currents can be timelike. This distinction matters when applying a timelike-current classification to minimal real supersymmetry.
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