where E x , E y , E z and B x , B y , B z denote respectively the components of the electric and magnetic fields, as measured by an inertial observer (at rest in our coordinates). As usual in relativistic physics, we will find it convenient to work with geometrised units in which c = 1 . In the " Index gymnastics " formalism of special relativity, the Minkowski metric η η is used to raise and lower indices.