Proton-Neutron Mass Difference in Configuration Space

We argue that the canonical structure of light-cone singularities should break down at distances of the order of ${10}^{-16\mathrm{}}$ cm. We propose that this be the separation from the light cone of the boundary of a region where configuration-space structure functions effectively lose their $x^2$ dependence. A new formula for the proton-neutron mass difference is derived as a limit of configuration-space invariant amplitudes. The above conjecture is then used for a numerical estimate which happens to be in excellent agreement with the experimental value. Finally we exhibit bilocal operators in the quark-gluon model responsible for divergences in $\Delta I=1\mathrm{}$ electromagnetic mass shifts.