The total field acting on the dipole has two parts, E 0 (t) and E RR (t). E 0 (t) is the free or zero-point field acting on the dipole. It is the homogeneous solution of the Maxwell equation for the field acting on the dipole, i.e., the solution, at the position of the dipole, of the wave equation [ ∇ ∇ 2 − − 1 c 2 ∂ ∂ 2 ∂ ∂ t 2 ] E = 0 satisfied by the field in the (source free) vacuum. For this reason E 0 (t) is often referred to as the "vacuum field", although it is of course a Heisenberg-picture operator acting on whatever state of the field happens to be appropriate at t = 0. E RR (t) is the source field, the field generated by the dipole and acting on the dipole.