From quantum electrodynamics it can be found that photons cannot couple directly to each other and a fermionic field according to the Landau-Yang theorem since they carry no charge and no 2 fermion + 2 boson vertex exists due to requirements of renormalizability, but they can interact through higher-order processes or couple directly to each other in a vertex with an additional two W bosons: a photon can, within the bounds of the uncertainty principle, fluctuate into a virtual charged fermion –antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon structure, or, somewhat metaphorically, what is "inside" the photon.