16 president's address — SECTION A. 



The Maxwell equations fit readily into the relativity scheme, and it 

 is found that there exists an Electro-magnetic Energy Tensor which 

 must be added to the Material Energy Tensor when electric systems 

 are present. The final statement of the Conservation Law then shows 

 that the total momentum and total energy remain constant when 

 reckoned for the complete system consisting of Material System, Electro- 

 dynamic System, and fields. . 



Electro-magnetic Energy, like Material Energy, exerts and is subject 

 to gravitational effects. 



Up to this point the Principle of Eelativity fulfils all the requirements 

 of a general scientific principle. It is founded on reasonable assump- 

 tions, its predictions are confirmed by experiment, and it co-ordinates 

 under one general scheme a large number of facts and hypotheses. 

 Furthermore, it satisfies what Freundlich calls the two fundamental 

 postulates of mathematical physics, namely : — 



(1) The statement of physical laws must exclude action at a dis- 



tance, and must tlierefore be made by means of differential 

 relationships. 



(2) Causal relationship can be assumed only between entities 



capable of being perceived. 



There is one difficulty, however, which exists in the Principle of 

 Relativity, as it existed under Newton's ideas of an absolute space and 

 time. We can form no conception of the linear v-locity of a body, 

 apart from observations of the bodies determining the frame of reference, 

 but the phenomena associated with rotation do provide us with a 

 measure of angular velocity independently of any outside frame. 

 In Eddington's words, "It is clear that the equivalence of axes in 

 relative rotation is in some way less complete than the equivalence 

 of axes having different translations." From the analytical standpoint 

 it is possible to classify the various systems of axes, distinguishing 

 those for which complete relativity holds from those for which it fails. 

 If rectangular co-ordinates are used to specify the Minkowski continuum, 

 three of the axes being ordinary space axes of the observer, the 

 nterval is given by 



where the g ^^, satisfy the Einstein Gravitation Equations with the 

 conditions, at an infinite distance from the gravitating matter, 

 gii = (J22 = .933 = - 1 ; 5'44 = 1 ; g^n' = ^^^n /^ and V are unequal. 



If the axes are transformed to any other rectangular set the differential 

 quations are co-variant, but the conditions at infinity may be changed. 



