where q μ μ represents the photon momentum (equal in magnitude to E / c, where E is the energy of the photon). The three functions: α α , β β , κ κ are associated to the electric and magnetic form factors for this interaction, and are routinely measured experimentally; these three effective vertices can then be used to check, or perform calculations that would otherwise be too difficult to perform from first principles. This matrix element then serves to determine the transition amplitude involved in the scattering interaction or the respective particle decay—cf. Fermi's golden rule.