SPACE, TIME 30' 



regard to its elements, i.e., space-time is capable of infinite 

 subdivision. It is unbroken, i.e., there are no holes. Con- 

 cerning the question of infinity, nothing in the operational 

 derivation, so far as the special theory of relativity is in- 

 volved, would lead one to call it into question. From the 

 point of view of the general theory, however, certain con- 

 siderations suggest that space-time should be considered as 

 finite. The topic is too extensive to be discussed here. 1 



But reference should be made in this connection to the 

 fact that the three-dimensional space and one-dimensional 

 time of the classical physics have now become merged into 

 a four-dimensional whole. The reason for this intimate 

 union of what seem to be independent entities lies in the 

 difficulties which arise in the attempt to correlate the 

 various private space systems and private time systems. 

 This is done, as was seen, through the medium of common 

 spatial events and coexistent temporal events. Consider 

 for illustration the process of determining the coexistence 

 of two events which are located at points A and B, widely 

 separated in space. They may be said to happen at the same 

 time if they do so according to the readings of the respective 

 clocks located at the events. But how can one know that 

 the clocks have been properly set with reference to one an- 

 other? One might carry the clock located at point A to 

 point B and compare it with the clock at B. But the move- 

 ment would almost certainly change its reading. Instead, 

 one might send a light signal from A to B, and adjust the 

 clocks accordingly. But this supposes that one knows what 

 the velocity of light is. This difficulty might be avoided by a 

 somewhat more complicated operation of sending a signal 

 from A to B where it is immediately reflected back to A. 

 Then if one records the time of leaving A (ti) and the time 

 of returning to A (t 2 ), the fraction (t 2 — ti)/2 would repre- 

 sent the time of the light's journey from A to B. If one 

 started the clock at B when the signal reached this point, 

 and then set it back by an amount (t 2 — U)/2 the clocks 



1 See d'Abro, op. cit., Chap. XXXIV. 



