This is the sequel to an earlier paper on physical effects associated with the QED theta angle. Unfortunately, field configurations which would yield theta-dependent effects are not realizable in the lab, based on general considerations discussed in the new paper. There could, however, be small non-perturbative effects sensitive to theta despite the lack of topology in QED (maps from S^3 to U(1) are all topologically trivial).
Theta terms and asymptotic behavior of gauge potentials in (3+1) dimensions
We describe paths in the configuration space of (3+1) dimensional QED whose relative quantum phase (or relative phase in the functional integral) depends on the value of the theta angle. The final configurations on the two paths are related by a gauge transformation but differ in magnetic helicity or Chern-Simons number. Such configurations must exhibit gauge potentials that fall off no faster than 1/r in some region of finite solid angle, although they need not have net magnetic charge (i.e., are not magnetic monopoles). The relative phase is proportional to theta times the difference in Chern-Simons number. We briefly discuss some possible implications for QCD and the strong CP problem.