Absolute Axes and Tidal Forces

IT has been shown¹ that at any point of space-time, whether empty or occupied by matter, it is possible to define a system of absolute axial directions, independently of the Ricci tensor, and, in general, uniquely. The method makes use of a tensor which has since been proved by Ruse² to be identical with the conformal curvature tensor of Weyl. It may be supposed that this system of axes is of mathematical interest only, but it appears possible to give it a, simple physical interpretation.