is the density of matter from place to place). But, written in this form, the law is indeterminate; for it yields a number of laws differing from Newton’s law of the inverse square. Mathematicians have proved that in order to obtain Newton’s law, we must supplement our differential equation by specifying that the potential
tends to a constant finite limit at spatial infinity.
Now, in Einstein’s theory, the analogue of Poisson’s law is the law of space-time curvature,
Just as Poisson’s law was the differential law of the Newtonian potential distribution, so now Einstein’s law is the differential law of distribution of the ten potentials, the ten
’s; and
is the energy and momentum of matter at every point. Again, as in the case of Poisson’s equation, we must supplement these differential equations of the gravitational field by specifying the values of the