  The field inside a Casimir cavity has an effective negative mass, which acts
as a buoyancy force in a gravitational field. Can this render the total mass of
the cavity negative, making it "float" in the vacuum ? Recent theoretical
arguments indicate that this is impossible. We provide support to this
conclusion discussing a concrete simple model of cavity, with plane parallel
metallic plates kept in mechanical equilibrium by a spring and placed in a weak
gravitational field. We show that basic facts about the structure of matter
imply that the total mass of the cavity is always positive. This has
implications for the hypothetical relation between vacuum energy and
cosmological constant.
