The Casimir frequency spectrum: Can it be observed?

The frequency spectrum of the Casimir force between real materials is studied with a view to assess possible ways of by which its dramatic oscillatory behaviour might be observed. Two simple attempts are made in which a model material is perturbed so as to become semitransparent in a band of frequencies. It is found that the real frequency formalism of the Casimir Lifshitz force is extremely volatile upon manipulations of optical properties and produces nonsensical results when extreme care is not taken to ensure the physicality of all perturbations, whereas the imaginary frequency formalism is highly robust and behaves well even under unphysical manipulations. The indication is thus that the general physical requirements of response functions preclude the observation of the frequency spectrum.