Mass shell
= Mass shell
{title2=$p^2=\pm m^2$}
The <mass shell> is the locus of <four-momenta> satisfying the relativistic dispersion relation for a fixed <mass> $m$. With mostly-minus <Minkowski metric>, $p^2=m^2$; with mostly-plus <Minkowski metric>, $p^2=-m^2$. The positive-energy sheet represents particle <four-momenta>. For $m=0$ this is the light cone. The condition of being <on shell> refers more generally to satisfying the field equations.