Null crossed electromagnetic field
= Null crossed electromagnetic field
Perpendicular fields with $\mathbf E\cdot\mathbf B=0$ and $E^2-c^2B^2=0$ form a null electromagnetic field. The constant-field Lorentz-force generator is nilpotent, so its exponential terminates as a polynomial.
Both invariants of the <electromagnetic field tensor> vanish for this field class.