Abstract

As emphasized by Milne, an observer ultimately depends on the transmission and reception of light signals for the measurement of natural lengths and periods remote from his world point. The laws of geometry which are obeyed when these lengths and periods are plotted on a space-time depend, inevitably, on assumptions concerning the dependence of light velocity on the spatial and temporal coordinates. A convention regarding light velocity fixes the geometry, and conversely. However, the convention of flat space-time implies nonintegrable “radar distances” unless the concept of coordinate-dependent units of measure is employed. Einstein's space-time has the advantage of admitting a special reference system $\hat R$ with respect to which the aether fluid is at rest and the total gravitational field vanishes. A holonomic transformation from $\hat R$ to another reference systemR belonging to the same space-time introduces a nonpermanent gravitational field and holonomic aether motion. A nonholonomic transformation from $\hat R$ to a reference systemR* which belongs to a different space-time introduces a permanent gravitational field and nonholonomic aether motion. The arbitrariness of geometry is expressed by extending covariance to include the latter transformation. By means of a nonholonomic (or units) transformation it is possible, with the aid of the principle of equivalence, to obtain the Schwarzschild and de Sitter metrics from the Newtonian fields that would arise in a flat space-time description. Some light is thrown on the interpretation of cosmological models