The phenomenon of inertia is modeled by a new process, which semi-discretely changes the metrical scale of spacetime. This process explains inertia as a curved spacetime phenomenon, since all accelerating trajectories take place on geodesics of general relativity. By this approach inertial reference frames are Minkowskian manifolds imbedded in a five-dimensional space. It would imply a revised interpretation of special relativity, which preserves a universal temporal reference. If acceleration curves spacetime, it would also imply the existence of an energy-momentum tensor with net energy, which under certain conditions could become negative.