@johncarlosbaez Someone should add a plaque which expresses Maxwell's equations as a Yang-Mills theory (after all it was the motivation for Yang-Mills)[dA=0,~\ast d\ast A = J]In this case, if we want to recover Maxwell's equations, then we should take (A) to be a connection on a line bundle over (\mathbb{R}^4) and equip (\mathbb{R}^4) with the Lorentz metric?