 
Summary: physics/99120103Dec1999
ELECTRODYNAMIC FORCES IN ELASTIC MATTER
S. ANTOCI AND L. MIHICH
Abstract. A macroscopic theory for the dynamics of elastic, isotropic
matter in presence of electromagnetic fields is proposed here. We avail
of Gordon's general relativistic derivation of Abraham's electromagnetic
energy tensor as starting point. The necessary description of the elas
tic and of the inertial behaviour of matter is provided through a four
dimensional generalisation of Hooke's law, made possible by the intro
duction of a fourdimensional "displacement" vector. As intimated by
Nordstršom, the physical origin of electrostriction and of magnetostric
tion is attributed to the change in the constitutive equation of elec
tromagnetism caused by the deformation of matter. The part of the
electromagnetic Lagrangian that depends on that deformation is given
explicitly for the case of an isotropic medium and the resulting expres
sion of the electrostrictive force is derived, thus showing how more real
istic equations of motion for matter subjected to electromagnetic fields
can be constructed.
1. Introduction
According to a widespread belief cultivated by presentday physicists, gen
