j.^20] The Theory of Kelativity 29 



he represented mathematically, and how any equation referred to these 

 four axes could be transformed to any other set of axes provided the 

 second set is at rest or moving uniformly with respect to the first. 

 Einstein used many of Minkowski's ideas, and developed what he calls 

 his Special Relativity Theory, which he describes as follows : The 

 Special Relativity theory is the application to any natural process of 

 the following- propositions : 



1. Every law of nature which holds good with respect to a co- 

 ordinate system K must also hold good for any other system K' pro- 

 vided that K and K' are in uniform motion of translation. 



2. The second proposition is that light has a constant velocity in 

 a vacuum, quite independent of the velocity of its source. 



There is much experimental evidence for the truth of this second 

 postulate, and the special theory, resting only on the two postulates 

 just given, was eagerly studied by physicists. 



It was immediately seen that the most important fields of study 

 were those of acceleration and energy. The conception that inert mass 

 is nothing but latent energy was developed, the law of conservation 

 of mass lost its independence and became merged with the law of 

 conservation of energy, and new laws of motion, differing from New- 

 ton's, were worked out for masses moving with great velocity. Many 

 startling and unexpected results were found, and Einstein pushed his 

 investigations vigorously. Please remember that his special theory 

 dealt only with axes in uniform motion, and he next tried to find a 

 more generalized theory dealing with axes in any kind of motion. 

 Again the obstacles in his path seemed to defy his highest skill. But 

 he persisted, for he asked himself, whj^ must the independence of 

 physical laws with regard to a system of co-ordinates be limited to 

 a system of co-ordinates in uniform motion of translation with regard 

 to one another? What has Nature to do with the co-ordinate systems 

 which we propose, or with their motions? "We must, of course, use 

 arbitrarily chosen systems to describe Nature's operations, but they 

 ought not to be limited as to their state of motion. So he worked on 

 until he found the necessary transformation formulae for any hind 

 of motion of the axes, and_this is what he calls his General Theory 

 of Relativity, the single postulate of which may be stated as follows : 

 The laws of nature must remain invariant for all transformations of 

 co-ordinates. But he further says that "a generalized theory of 



