Null electromagnetic field
= Null electromagnetic field
{wiki}
A null field has both electromagnetic invariants zero:
$$
\mathbf E\cdot\mathbf B=0,
\qquad
E^2=c^2B^2.
$$
For a nonzero constant null field, the mixed tensor is nilpotent of index three,
$$
F^\mu{}_\rho F^\rho{}_\sigma F^\sigma{}_\nu=0.
$$