Since the exact identity of inertial and gravitational mass is the cornerstone of General Relativity, and since there are generalization of Einsteins theory, which predict a slight violation of this equivalence, it is of utmost importance to test the equivalence principle.

Already Galilei and Newton demonstrated on experiments with pendula, that the equivalence hold with one part in a million. Up to now the equivalence could be tested with an accuracy of one in 10¹³.

The planned experiment STEP (Satellite Test of Equivalence Principle) aims to increase this up to one part in 10¹⁸.