  The total momentum of a thermodynamically closed system is unique, as is the
total energy. Nevertheless, there is continuing confusion concerning the
correct form of the momentum and the energy-momentum tensor for an
electromagnetic field interacting with a linear dielectric medium. Here we
investigate the energy and momentum in a closed system composed of a
propagating electromagnetic field and a negligibly reflecting dielectric. The
Gordon momentum is easily identified as the total momentum by the fact that it
is, by virtue of being invariant in time, conserved. We construct continuity
equations for the energy and the Gordon momentum and use the continuity
equations to construct an array that has the properties of a traceless,
diagonally symmetric energy-momentum tensor. Then the century-old
Abraham-Minkowski momentum controversy can be viewed as a consequence of
attempting to construct an energy-momentum tensor from continuity equations
that contain densities that correspond to nonconserved quantities.
