Some Further Remarks on the Geodesic Hypothesis in the Theory of Gravitation.

in which the WA are the components of the unit time-like velocity vector and the quantities hAB are the components of the metric tensor of the space. The scalar p in these equations is the material density and is assumed to satisfy a conservation law; finally, the quantity p is a scalar which is commonly called the pressure although it is not to be confused with a hydrodynamical pressure. A stress-energy tensor of the type (1) was originally used by Einstein2 in his cosmological theory and would appear to be the simplest such tensor which will effectively represent the energetic relationships inside a material body in the absence of electromagnetic forces. A. H. Taub3 investigated the possibility of applying the procedure, used in the above note, to the case of a relativistic fluid having a stress-energy tensor with components