The theory that explains the three major interactions of elementary particle physics—the strong interaction responsible for nuclear forces, the weak interaction responsible for radioactive decay, and the electromagnetic interaction—in terms of a common physical picture. The model for this picture is quantum electrodynamics, the fundamental theory underlying electromagnetism. In that theory, electrons, viewed as structureless elementary constituents of matter, interact with photons, structureless elementary particles of light. The standard model extends quantum electrodynamics to explain all three interactions of subnuclear physics in terms of similar basic constituents.