Sometimes Simple Things Are Complicated

In quantum physics, some interactions that seem simple, like two photons that interact in a manner that ultimately produces two photons, are actually wickedly complex. This is because you have to consider every conceivable way consistent with the laws of quantum mechanics that you can get from the starting state to the end state.

In this simple scenario, called light by light scattering, that means considering the possibility that the two photons have paths that include all sorts of mesons (two quark particles) that are created and destroyed in intermediate steps in the interactions, either as "real" particles or "virtual" ones (called "off shell").

A recent paper tackling some of the deep complexities of this seemingly simple interaction by analyzing fifteen of the most important intermediate meson paths is here.