A FORGOTTEN ARGUMENT BY GORDON UNIQUELY SELECTS ABRAHAMS TENSOR AS THE ENERGY-MOMENTUM TENSOR FOR THE ELECTROMAGNETIC-FIELD IN HOMOGENEOUS,ISOTROPIC MATTER

Given the present status of the problem of the electromagnetic energy tensor in matter, there is perhaps use in recalling a forgotten argume nt given in 1923 by W. Gordon. Let us consider a material medium which is homogeneous and isotropic when observed in its rest frame. For suc h a medium, Gordon's argument allows to reduce the above-mentioned pro blem to an analogous one, defined in a general relativistic vacuum (i. e. in the presence of a suitably determined metric gamma(ik)) For the latter problem the form of the Lagrangian is known already, hence the determination of the energy tensor is a straightforward matter. One ju st performs the Hamiltonian derivative of the Lagrangian chosen in thi s way with respect to the true metric g(ik). Abraham's tensor is thus selected as the electromagnetic energy tensor for a medium which is ho mogeneous and isotropic in its rest frame.