PRESIDENT S ADDRESS — SECTION A. 5 



Under the Minkowski scheme the familiar three-dimensional dyna- 

 mical picture of the physical universe is replaced by a statical four- 

 dimensional representation. Ideas of propagation are dispensed with 

 and the universal constant c becomes a mere dimensional multiplier 

 to bring the time co-ordinate into agreement with the spacial co- 

 ordinates. The restricted Principle of Relativity can be summed up 

 in the statement that the behaviour of a physical system is independent 

 of its orientation in the Minkowski world. 



In a communication to the Royal Society of London, Larmor main- 

 tains that the Relativity scheme, complete as it is as a description 

 of electrodynamic relationships, breaks down when applied to the 

 underlying entities, electrons, between which these relationships exist. 

 He contends that inasmuch as a permanent entity exists in space and 

 independently of time the essential distinction between space and time 

 must be maintained. To what extent can this contention be upheld ? 

 In the sense that the properties of the electron are independent of the 

 position in space and the instant of time at which they are considered 

 it can be said that the electron exists independently of both space 

 and time and the distinction between the two breaks down. There is, 

 however, a distinction in this sense : at any instant of time there is a 

 point in space at which a permanent electron can be found, but, given 

 a definite point in space, there is not necessarily an instant of time at 

 which the electron will be found at that point. It seems, however, 

 that this distinction does not constitute an essential difference between 

 space and time, but is introduced from our older ideas with the notion 

 of permanence. What we call a pefmanent entity in our ordinary 

 statements is an entity of which the world line in the space-time four- 

 fold goes off to infinity in both directions along the time axis. It rnay 

 be urged that the fact that such entities exist while we have no know- 

 ledge of world lines going to infinity along the space axes constitutes 

 a distinction between space and time. To this contention I would 

 reply that the observer's world line is approximately a straight line 

 in the fourfold, and he instinctively takes the direction of his own world 

 line as the direction of the time axis. It is difficult to see how he could 

 have any knowledge of entities whose world lines did not lie close to 

 his own. Hence the absence of knowledge of entities, except those with 

 world lines along the time axis leads to no essential discrimination between 

 time and space. It merely emphasizes the fact, realized long before 

 the days of relativity, that an observer chooses his co-ordinates so that 

 one is differentiated from the other three. The conception ot the 

 electron as a point singularity in a three-dimensional continuum seems 

 to be no more fundamental than its conception as a line singularity 

 in the Minkowski fourfold, and there appears to be no valid reason for 

 excluding the electron from the Relativity scheme. 



The passage from the Restricted Theory of Relativity to the General 

 Theory depends on the fact that all exact physical measurements are 

 made by the observations of coincidences. Thus all our physical 



