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 cut off, the total field has an infinite energy density, with a spectral energy density ( per unit frequency per unit volume ) [ 2h / c 3 ] f 3 where h is Planck's constant.
