represented the density of the matter exciting the field at each point of space. It followed that it was solely the mass of a body which was active in creating a gravitational field. Likewise, it was the mass of a body which was the sole cause of its being attracted in a gravitational field, so that the weight of a body at a given point near the earth was fully determined when we knew the mass of the body and the mass of the earth.
Now, in Einstein’s theory, the gravitational equations become
where
and
refer to the curvatures of space-time, or, if we prefer, to the potential or
