  The form factor that yields the light-by-light scattering contribution to the
muon anomalous magnetic moment is computed in lattice QCD+QED and QED. A
non-perturbative treatment of QED is used and is checked against perturbation
theory. The hadronic contribution is calculated for unphysical quark and muon
masses, and only the diagram with a single quark loop is computed.
Statistically significant signals are obtained. Initial results appear
promising, and the prospect for a complete calculation with physical masses and
controlled errors is discussed.
