> while two light beams do not [interact] Photons do interact! There is no two-photon vertex, that’s true, since there is no fundamental interaction between tw

> while two light beams do not [interact] Photons do interact! There is no two-photon vertex, that’s true, since there is no fundamental interaction between two photons. But put four photon/charged particle vertices together and you’ll get the 4-photon box diagram [1], whose contribution is nonzero. [1]: Since I’m not good at painting diagrams in ASCII on HN, here’s a link to the Wikipedia article with the box rendered nicely: https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics