Null covector
= Null covector
{title2=$g^{\alpha\beta}\ell_\alpha\ell_\beta=0$}
A nonzero <covector> $\ell$ is null when its squared dual <Lorentzian inner product> is zero: $g^{\alpha\beta}\ell_\alpha\ell_\beta=0$. Raising its index gives a <null vector>. Null covectors describe normals to characteristic <light cones> and are the test directions in the <classical null condition for wave equations>.