On a Characterization of Geodesic Trajectories and Gravitational Motions
  We shall here discuss a characterization of geodesics trajectories. We shall show that the action of the gravitational field on mass particles can be essentially identified with the force that cannot be absolutely eliminated. This leads to an alternative formulation of equivalence principle. 
L.Fatibene, M.Francaviglia, G.Magnano
gr-qc