Page:The Meaning of Relativity - Albert Einstein (1922).djvu/98

86 is so chosen that $$g$$ = constant. From $$R_{\mu\nu}$$ we can form the scalar,

Straightest (Geodetic) Lines. A line may be constructed in such a way that its successive elements arise from each other by parallel displacements. This is the natural generalization of the straight line of the Euclidean geometry. For such a line, we have

The left-hand side is to be replaced by $$\frac{d^2x_\mu}{ds^2}$$, so that we have

We get the same line if we find the line which gives a stationary value to the integral

between two points (geodetic line).