Exposing ‘‘hidden momentum’’

It is shown that ‘‘hidden momentum’’ is a real physical entity found in a continuous matter under pressure when observed from an improper inertial frame. Precisely the same results are obtained from an analysis of the mechanical motion of microscopic particles that create pressure. Analogous properties hold in a pressure created by standing electromagnetic waves enclosed between two parallel mirrors. The significance of the energy-momentum tensor and its divergence is pointed out. It is also shown that in certain cases there exists an ensemble of particles where the sum of the energy–momentum 4-vectors of its constituents does not transform as a 4-vector. Using these results, it is shown that a stationary classical system made of a magnet and electric charges has null linear momentum.