, in which
is zero or has some definite non-vanishing value according to whether we are considering a region outside matter or in its interior. Now, this potential distribution in Newton’s law refers to the distribution through space of one same potential, an invariant in space, known as the Newtonian potential. But we know that if Einstein’s ideas are correct, if gravitation is to be attributed to space-time curvature, Newton’s single potential
(an invariant) will have to be replaced at every point by the ten separate
’s of space-time. These, as we know, are no longer invariants, they are second-order tensors;[93] and Newton’s invariant potential
turns out to be none other than one of the