becomes in the new theory under the simplifying assumptions we have chosen.
The stress-energy-tensor, which excites the field, degenerates, as a result of our quite special assumptions, into the density of mass:
In the differential equations (2) the second term on the left-hand side is the product of two magnitudes, which, according to the above arguments, are to be regarded as infinitely small quantities of the first order. Thus the second term, being of the second order of small quantities, may be dismissed. The first term, on the other hand, if we omit the terms differentiated with respect to time, as above (i.e. if we regard the gravitational field as "stationary"), reduces to:
The differential equation for
thus degenerates into Poisson's equation:
Thus, to a first approximation (i.e. if one regards the velocity of light as infinitely great, and this is a characteristic feature of the classical theory, as was explained in detail in [§ 3(b)]: if certain simple assumptions are made about the behaviour of the