Fractional Aharonov-Bohm effect for retarded potentials

It has been suggested that the magnetic Aharonov-Bohm effect can be interpreted equally well as being due to a phase shift associated with an electron in an interferometer enclosing a magnetic flux, or as a phase shift associated with the electrons in the solenoid that generates the field. Here the Aharonov-Bohm effect is derived using second-quantized field theory to describe all the electrons as well as the electromagnetic field in a consistent way. The results are in agreement with the usual expression for the Aharonov-Bohm effect when the retardation of the electromagnetic field is negligible, but they predict the possibility of a fractional phase shift when retardation effects are significant.