  In 1+3 (threading) formulation of general relativity spacetime behaves
analogous to a medium with a specific index of refraction with respect to the
light propagation. Accepting the reality of zero point energy, through the
equivalence principle, we elevate this analogy to the case of virtual photon
propagation in a quantum vacuum in a curved background spacetime. Employing
this new idea one could examine the response of vacuum energy to the presence
of a stationary gravitational field in its different quantum field theoretic
manifestations such as Casimir effect and Lamb shift. The results are given
explicitly for a Casimir apparatus in the weak field limit of a Kerr hole.
