PHYSICAL IMPLICATIONS OF THE USE OF PRIMITIVE AND TRACELESS ELECTRIC QUADRUPOLE-MOMENTS

Theories of certain electromagnetic effects, such as chiral phenomena in fluids and crystals and gyrotropic birefringence in antiferromagnet ic crystals, require the inclusion of electric quadrupole contribution s for a full description of the effect. In a number of these theories the electric quadrupole moment is defined to be traceless, as indeed i s the general practice for such moments in nuclear physics. It is show n that, when the traceless quadrupole moment is used in the derivation of the wave equation that describes light propagation through an opti cally active uniaxial medium, this equation and properties derived fro m it, in particular refractive index, depend in general on the arbitra ry origin used to specify the quadrupole moment. This is physically un acceptable for an observable property of a substance. By contrast, thi s defect does not occur if the definition of the primitive quadrupole moment is adopted, namely Sigma qr(alpha)r(beta).