A pseudo-photon in non-trivial background fields

We show that in the presence of external fields for which either B˙ext≠0 or ∇×Eext≠0 it is not possible to derive the classical Maxwell equations from an action with only one gauge field. We suggest that one possible solution is to consider a second physical pseudo-vector gauge field C. The action for this theory is originally motivated by the inclusion of magnetic monopoles. These particles play no role in this work and our argument is only based in, that the violation of the Bianchi identities, cannot be accounted at the action level with only the standard gauge field. We give a particular example for a periodic rotating external magnetic field. Our construction holds that at classical level both the vector and pseudo-vector gauge fields A and C are regular. We compare pseudo-photon with paraphoton (graviphoton) theories concluding that, besides the mechanisms of gauge symmetry breaking already studied, the Bianchi identities violation is a crucial difference between both theories. We also show that, due to Dirac's quantization condition, at quantum field theory level the effects due to pseudo-photons and photons can be distinguished by the respective contributions to the magnetic moment of fermions and vacuum polarization. These effects may be relevant in astrophysical environments, namely close and inside neutron stars and magnetars.