  The Einstein equations, apart from being the classical field equations of
General Relativity, are also the classical field equations of two other
theories of gravity. As the experimental tests of General Relativity are done
using the Einstein equations, we do not really know, if gravity is the
curvature of a torsionless spacetime, or torsion of a curvatureless spacetime,
or if it occurs due to the non-metricity of a curvatureless and torsionless
spacetime. However, as the classical actions of all these theories differ from
each other by boundary terms, and the Casimir effect is a boundary effect, we
propose that a novel gravitational Casimir effect between superconductors can
be used to test which of these theories actually describe gravity.
