Tidal effects in space experiments to test the equivalence principle: implications on the experiment design

Experiments to test the equivalence principle (EP) in low Earth orbit require to detect the effects of an extremely small non-classical differential acceleration between test masses of different composition. In all proposed experiments the test masses are concentric coaxial cylinders, so as to reduce classical tidal effects which are differential too. Perfect centring being impossible, tidal effects need to be carefully investigated as they impose severe constraints on the basic features of the experiment design. The present analysis shows that with free flying (uncoupled) test masses an EP violation signal could be detected if the initial conditions of the masses were finely adjusted for them to remain at a fixed distance relative to each other while orbiting around the Earth. However, such an experiment is severely limited by non-gravitational effects. If the test cylinders are weakly coupled in 2D in the plane perpendicular to their symmetry axis (close to the orbit plane), while rapidly spinning around it, a position of relative equilibrium is provided by physical laws which makes tidal effects widely separated from the signal. Weak coupling in ID along the symmetry axis (to lie and slowly rotate in the orbit plane) is viable but less advantageous. (C) 2003 Published by Elsevier B.V.