Weak equivalence principle

Freely falling test particles with identical initial position and velocity follow the same trajectory independently of their material composition. The principle neglects appreciable self-gravity and nongravitational forces. The Einstein equivalence principle extends the local statement to nongravitational experiments; the strong equivalence principle also includes gravitational experiments and self-gravitating bodies.

A satellite experiment testing the universality of free fall. Electrostatic control keeps test masses made from titanium and platinum alloys on matched orbital motion; the required differential forces constrain composition dependence. A reference sensor uses masses of the same composition. The experiment tests the weak equivalence principle, rather than the full strong equivalence principle.^[ref]

A torsion-balance comparison of the gravitational acceleration of bodies of different composition, testing equality of inertial and passive gravitational mass up to a universal proportionality factor. A universal acceleration law independent of test-body mass and composition agrees with this weak equivalence principle test. It does not by itself establish the strong equivalence principle for self-gravitating bodies.