The total momentum of a thermodynamically closed system is unique, as is thetotal energy. Nevertheless, there is continuing confusion concerning thecorrect form of the momentum and the energy-momentum tensor for anelectromagnetic field interacting with a linear dielectric medium. Here weinvestigate the energy and momentum in a closed system composed of apropagating electromagnetic field and a negligibly reflecting dielectric. TheGordon momentum is easily identified as the total momentum by the fact that itis, by virtue of being invariant in time, conserved. We construct continuityequations for the energy and the Gordon momentum and use the continuityequations to construct an array that has the properties of a traceless,diagonally symmetric energy-momentum tensor. Then the century-oldAbraham-Minkowski momentum controversy can be viewed as a consequence ofattempting to construct an energy-momentum tensor from continuity equationsthat contain densities that correspond to nonconserved quantities.
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