Poynting's theorem and luminal total energy transport in passive dielectric media - art. no. 046610

Without approximation the energy density in Poynting's theorem for the gene rally dispersive and passive dielectric medium is demonstrated to be a syst em total dynamical energy density. Thus the density in Poynting's theorem i s a conserved form that by virtue of its positive definiteness prescribes i mportant qualitative and quantitative features of the medium-field dynamics by rendering the system dynamically closed. This fully three-dimensional r esult, applicable to anisotropic and inhomogeneous media, is model independ ent, relying solely on the complex-analytic consequences of causality and p assivity. As direct applications of this result, we show (1) that a causal medium responds to a virtual, "instantaneous" field spectrum, (2) that a ca usal, passive medium supports only a luminal front velocity, (3) that the s patial "center-of-mass" motion of the total dynamical energy is also always luminal and (4) that contrary to (3) the spatial center-of-mass speed of s ubsets of the total dynamical energy can be arbitrarily large. Thus we show that in passive media superluminal estimations of energy transport velocit y for spatially extended pulses is inextricably associated with incomplete energy accounting.