Page:The Meaning of Relativity - Albert Einstein (1922).djvu/17

for every orientation of the interval, then the space of reference is called Euclidean, and the co-ordinates Cartesian.^[1] It is sufficient, indeed, to make this assumption in the limit for an infinitely small interval. Involved in this assumption there are some which are rather less special, to which we must call attention on account of their fundamental significance. In the first place, it is assumed that one can move an ideal rigid body in an arbitrary manner. In the second place, it is assumed that the behaviour of ideal rigid bodies towards orientation is independent of the material of the bodies and their changes of position, in the sense that if two intervals can once be brought into coincidence, they can always and everywhere be brought into coincidence. Both of these assumptions, which are of fundamental importance for geometry and especially for physical measurements, naturally arise from experience; in the theory of general relativity their validity needs to be assumed only for bodies and spaces of reference which are infinitely small compared to astronomical dimensions.

The quantity $s$ we call the length of the interval. In order that this may be uniquely determined it is necessary to fix arbitrarily the length of a definite interval; for example, we can put it equal to I (unit of length). Then the lengths of all other intervals may be determined. If we make the $x_{\nu }$ linearly dependent upon a parameter $\lambda$,

$x_{\nu }=a_{\nu }+\lambda b_{\nu },$

1. This relation must hold for an arbitrary choice of the origin and of the direction (ratios $\Delta x_{1}:\Delta x_{2}:\Delta x_{3}$) of the interval.