Frame indifferent formulation of Maxwell’s elastic-fluid model and the rational continuum mechanics of the electromagnetic field

We show that the linearized equations of the incompressible elastic medium admit a ‘Maxwell form’ in which the shear component of the stress vector plays the role of the electric field, and the vorticity plays the role of the magnetic field. Conversely, the set of dynamic Maxwell equations are strict mathematical corollaries from the governing equations of the incompressible elastic medium. This suggests that the nature of ‘electromagnetic field’ may actually be related to an elastic continuous medium. The analogy is complete if the medium is assumed to behave as fluid in shear motions, while it may still behave as elastic solid under compressional motions. Then the governing equations of the elastic fluid are re-derived in the Eulerian frame by replacing the partial time derivatives by the properly invariant (frame indifferent) time rates. The ‘Maxwell from’ of the frame indifferent formulation gives the frame indifferent system that is to replace the Maxwell system. This new system includes terms already present in the classical Maxwell equations, alongside terms that are the progenitors of the Biot–Savart, Oersted–Ampere’s, and Lorentz-force laws. Thus, a frame indifferent (truly covariant) formulation of electromagnetism is achieved from a single postulate: the electromagnetic field is a kind of elastic (partly liquid, partly solid) continuum.