The quantities
and
are expressions which are related in a simple manner to the components of the stress-energy-tensor (which plays the part of the quantity exciting the field in the new theory in place of the density of mass).
is essentially equal to the gravitational constant of Newton's theory.
The differential equations (1) and (2) are the fundamental equations of the new theory. The derivation of these equations is carried out in detail in the tract by A. Einstein, "The Foundations of the General Principle of Relativity," J. A. Barth, Leipzig, 1916.
2. In order to obtain a connection between these equations and Newton's theory, we must make several simplifying assumptions. We shall first assume that the