E&M is a Vector Theory

A more detailed study of the E&M field shows that the free fields in the Lagrangian are the four components of the vector potential $\bgroup\color{black}$ A_\mu$\egroup$ . They couple to the current (charge) density vector $\bgroup\color{black}$ j_\mu$\egroup$ . Its a vector theory.

Jim Branson 2012-10-21