I've often thought that one way to restate the equivalence principle is that the force of gravity is a manifestation of using a frame that is not in free-fall. In other words, it's an inertial force akin to the Coriolis and centrifugal forces in a rotating frame of reference. However, I just read the following quote from http://plato.stanford.edu/entries/newton-principia/#NewLawMot For some reason, I've never seen the third law being invoked to distinguish real and "apparent" forces. Now, the deflection of objects in a gravitational field is due to the curvature of spacetime, so I should think of it as inertial. But spacetime wouldn't be curved except a second object were doing the curving. It truly is an interaction between the two via the intermediary of the gravitational field. Hence, it is not really inertial in the same sense as the Coriolis force. So I guess gravity stands alone: it's the only inertial force that is also "real". Does this make any sense?