A machian request for the equivalence principle in extended gravity and nongeodesic motion

Starting from the origin of Einstein’s general relativity (GR), the request of Mach on the theory’s structure has been the core of the foundational debate. That problem is strictly connected with the nature of the mass-energy equivalence. It is well known that this is exactly the key point that Einstein used to realize a metric theory of gravitation having an unequalled beauty and elegance. On the other hand, the current requirements of particle physics and the open questions within extended gravity theories request a better understanding of the Equivalence Principle (EP). TheMOND theory byMilgrom proposes a modification of Newtonian dynamics, and we consider a direct coupling between the Ricci curvature scalar and the matter Lagrangian showing that a nongeodesic ratio mi/mg can be fixed and that Milgrom’s acceleration is retrieved at low energies.