A Theorem Connecting the Energy Momentum Tensor with the Velocity of Propagation of Waves

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Abstract
It is shown that every wave phenomenon which is described by a symmetrical energy momentum tensor of the second order, propagates with the velocity of light, provided that the four dimensional divergence and the diagonal sum of the energy momentum vanish. This theorem is applied to the two different expressions given for the energy momentum tensor of electromagnetic phenomena in material bodies by Minkowski and Abraham.