I've heard it proposed (by peers in the physics halls) that inertia is the result of the force from all other masses in the universe on the test mass. Is this a valid theory? If it is, I would extrapolate with some predicted consequences. addendum: ok, I have found the name of it: Mach's Principle seems comparable. If this is true, wouldn't it imply that every point in the universe is the center of universe, or else the mass will have a different value depending on where it is in the universe? Also, what would it imply about the strength of the fundamental forces? Does it mean we can break inertia down into a summation of forces? Would it mean that the coupling constants of the gravitational and electromagnetic forces aren't a fair comparison? I have difficult understanding what m is after this, since F = ma, but m is the result of forces: F = ma... it seems circular. I'm having a conceptual brain fart here.