SPACE, TIME 309 



ings without reference to their varying spatio-temporal 

 appearances." * Hence space- time is an absolute four- 

 dimensional scheme. This is simply to say that the location 

 of an event requires four independent values, of which three 

 are spatial and one is temporal. 2 



Homogeneity. Whether space-time is considered to be 

 homogeneous or non-homogeneous depends upon whether 

 it is examined in its abstract form, without regard to systems 

 of reference, or in its more concrete form, relative to systems 

 of reference. As has already been seen, the character of 

 space and time is determined by the systems of reference; 

 space may be shortened or lengthened and time may be 

 slowed down or speeded up by properly chosen systems of 

 reference. In this sense space and time may be said to be 

 non-homogeneous, for the "here" and the "now' of the 

 systems of reference determine the character of the space 

 and time which is measured. But, again, if one recognizes 

 that there is a constancy in the way in which space and time 

 "appear" from given systems of reference he is obliged to 

 conclude that the irregularities disappear if by space and 

 time are meant not their "appearances" but the law accord- 

 ing to which their "appearances'' change for varying sys- 

 tems of reference. If space and time are defined not in terms 

 of given manifestations but in terms of the potentiality of 

 all possible manifestations, the hummocks, condensations, 

 and other heterogeneities disappear. Abstract space-time 

 exhibits a permanent structure of which all deformations 

 are due to selected perspectives. Space-time is therefore 

 homogeneous. 



Isotropy. The dimensional isotropy of space-time is not 

 perfect. So far as the spatial dimensions are concerned com- 

 plete isotropy exists, i.e., there is no distinction between the 

 up-down, the near-far, and the right-left dimensions, but 



1 d'Abro, op. cit., pp. 197-198. 



2 It is not to be denied, of course, that the pre-relativity system of Euclidean 

 space and Newtonian time was also a four-dimensional system. Graphs of space 

 against time were used long before Einstein. But the structure of the relativity 

 system is different from that of the pre-relativity scheme. Roughly, the former 

 involves a greater "interfusion" of space and time than the latter. 



