Just to report back on my last post… Delbruck scattering is actually scattering of light by the Coulomb fields of nuclei. Scattering of light by light is predicted but according to my copy of Sakurai (Advanced QM, not Modern) was too small to detect as of the mid 60s, when the book was written. I have no idea if resolution has advanced enough so that this could be detected; he quotes a cross section of 4 x 10[sup]-31[/sup] cm[sup]2[/sup] at [symbol]w[/symbol] = m, where m is (I believe) the mass of the virtual particle that carries the interaction.