The equivalence principle (EP) lies at the foundation of Einstein’s general theory of relativity;
testing this fundamental assumption with the highest possible sensitivity is clearly important,
particularly since it may be expected that the EP will not hold in quantum theories of
gravity [25]. In general relativity, test objects in free-fall follow geodesics of space-time,
and what we perceive as the force of gravity is instead a result of our being unable to follow
those geodesics of space-time, because the mechanical resistance of matter prevents us from
doing so. One can test both the validity of the EP and of the field equations that determine
the geometric structure created by a mass distribution. There are two different ‘flavors’ of