Stochastic electrodynamics (SED) is a variant of classical electrodynamics (CED) of theoretical physics. SED consists of a set of controversial theories that posit the existence of a classical Lorentz invariant radiation field having statistical properties similar to that of the electromagnetic zero-point field (ZPF) of quantum electrodynamics (QED). Classical background field The background field is introduced as a Lorentz force in the (classical) Abraham–Lorentz–Dirac equation (see: Abraham–Lorentz–Dirac force), where the classical statistics of the electric and magnetic fields and quadratic combinations thereof are chosen to match the vacuum expectation values of the equivalent operators in QED. The field is generally represented as a discrete sum of Fourier components each with amplitude and phase that are independent classical random variables, distributed so that the statistics of the fields are isotropic and unchanged under boosts. This prescription is such that each Fourier mode at frequency (f) is expected to have an energy of hf/2, equaling that of the ground state of the vacuum modes of QED. Unless cut off, the total field has an infinite energy density, with a spectral energy density (per unit frequency per unit volume) [2h/c3]f3 where h is Planck's constant. Consequently, the background field is a classical version of the electromagnetic ZPF of QED, though in SED literature the field is commonly referred to simply as 'the ZPF' without making that distinction. Any finite cutoff frequency of the field itself would be incompatible with Lorentz invariance. For this reason, some researchers prefer to think of cutoff frequency in terms of the response of particles to the field rather than as a property of the field itself. Brief history Stochastic electrodynamics is a term for a collection of research efforts of many different styles based on the ansatz that there exists a Lorentz invariant random electromagnetic radiation. The basic ideas have been around for a long time; but Marshall (1963) and Brafford seem to have been the originators of the more concentrated efforts starting in the 1960s. Thereafter Timothy Boyer, Luis de la Peña and Ana María Cetto were perhaps the most prolific contributors in the 1970s and beyond. Others have made contributions, alterations and proposals concentrating on the application of SED to problems in QED. A separate thread has been the investigation of an earlier proposal by Walther Nernst attempting to use the SED notion of a classical ZPF to explain inertial mass as due to a vacuum