# OK FINE, magnetism is explained away by relativity

Afaik there are configurations of the electric and magnetic field (dependent on the sign of $$E^2 - B^2$$, which cannot be transformed into purely electric configurations by a Lorentz transformation.

George Jones
Staff Emeritus
$E \cdot B$ and $E^2 - B^2$ are both Lorentz-invariants. If $B=0$ and $E \neq 0$ in one frame, then $E \cdot B = 0$ and $E^2 - B^2 > 0$ in that frame, and hence in all frames.
Also, if $E \cdot B \neq 0$ or if $E \cdot B = 0$ and $E^2 - B^2 < 0$ in one frame, then there does not exist a frame in which $B = 0$.