, together with the negative energy of the system, so that (49) expresses the conservation principles in their integral form. That this gives the right conception of energy and the conservation principles will be seen from the following considerations.
PHENOMENOLOGICAL REPRESENTATION OF THE ENERGY TENSOR OF MATTER.
Hydrodynamical Equations. We know that matter is built up of electrically charged particles, but we do not know the laws which govern the constitution of these particles. In treating mechanical problems, we are therefore obliged to make use of an inexact description of matter, which corresponds to that of classical mechanics. The density
, of a material substance and the hydrodynamical pressures are the fundamental concepts upon which such a description is based.
FIG. 3.
Let
be the density of matter at a place, estimated with reference to a system of co-ordinates moving with the matter. Then