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Axion electrodynamics is based upon the Lagrangian of the electromagnetic(EM) field plus its interaction with the axions, and is accordingly aphysically open system. It means that the four-divergence of the EMenergy-momentum tensor is different from zero, implying in turn that the totalEM energy and momentum (when integrated over all space) do not constitute afour-vector. The EM force is in principle accessible to experimental detection,just analogous to what is the case in ordinary electrodynamics. In the firsthalf of this paper the energy-momentum aspects of axion electrodynamics areworked out in general, when the surroundings are allowed to be a medium withconstant permittivity and permeability. In the second half, two examples arediscussed. The first is a static situation, where a block of uniform materialcontaining axions is exposed to external strong electric and magnetic fields.Assuming the axion amplitude $a(x)$ (i.e. its density) to increase linearly inone direction, we calculate the axion-generated forces. As a second example, weconsider axions varying not with position but instead harmonically with time;this is the constellation usually assumed in astrophysics. Assuming a Gaussianprofile for the EM wave emitted from the Earth towards an axion cloud in outerspace, we make a calculation of the 'axion echo', the return signal.

Axion electrodynamics: Energy-momentum tensor, and possibilities for experimental tests