  The important point in all this is that the potential field U = −GM/r is only one of very many potential fields compatible with the inverse square law for forces F = GM/r2. Since the forces F are recovered from the potential field U by comparing the values of U at neighboring points in space, we can add a constant amount—K say—to U everywhere and still get the same forces. When we compare the potential field U at neighboring points, the Ks at each point cancel out. 