  Off-shell effects in proton electromagnetic vertices can be constrained from
their effects on known processes. In particular, parameters in models for the
off-shell effects can be determined by fitting to the proton electric and
magnetic polarizabilities measured in low-energy Compton scattering. There has
been recent speculation that off-shell effects contribute enough energy to the
muonic hydrogen Lamb shift to explain the discrepancy between muonic and
electronic measurements of the proton radius. We find that the constraints
discussed here make the off-shell effects about two orders of magnitude smaller
than needed for this purpose.
