A theory of electromagnetism with uniquely defined potential and covariant conserved spin

A theory of electromagnetism is proposed that is based on the Fermi Lagrangian, which is symmetric under electromagnetic spin rotation. Its features are: - the four-potential is unambiguously determined by the inhomogeneous wave equation and boundary conditions at infinity. - the Lorenz condition and minimal coupling then follow from charge conservation. - electromagnetic spin is conserved and spin operators can be defined without sacrificing covariance. - quantisation is fully covariant, without redefining the metric. - all experimental predictions are the same as in the standard theory. This result proves that electromagnetic gauge invariance is redundant as a fundamental principle of physics.