  Electromagnetic waves propagation on either rotating or anisotropic spacetime
backgrounds (such as Kerr and G\"odel metrics, or Bianchi-I metric) produce a
reduction of the magnitude of Casimir forces between plates. These curved
spacetimes behave as chiral or birefringent materials producing dispersion of
electromagnetic waves, in such a way that right- and left-circularly polarized
light waves propagate with different phase velocities. Results are explicitly
calculated for discussed cases. The difference on the wavevectors of the two
polarized electromagnetic waves produces an abatement of a Casimir force which
depends on the interaction between the polarization of electromagnetic waves
and the properties of the spacetime.
