Vacuum torque

A classical and a fully quantum mechanical formalism are introduced for the calculation of the torque experienced by a macro- and a microdipole immersed in an anisotropic homogeneous medium. This is done by obtaining first the frequency shift, and thereby evaluating the change in the energy of the dipoles. It is shown that the shift in energy depends on the orientation of the dipoles and thus introduces a vacuum torque. Although the classical and quantum approaches are different, we attempt to cast the final expressions into a unified form by making the use of a classical Green tensor