RECENT ADVANCES IN SCIENCE 557 



ether or " space " (in the absolute sense of the word). This 

 has long been recognised in the dynamical sphere, and this 

 recognition is implicit in Newtonian dynamics, embracing 

 the three laws of motion and law of gravitation. It was the 

 repeated failure of optical and electrical experiments to detect 

 relative motion to the ether which prompted Einstein, in 

 1905, to depart from the method pursued by earlier physicists, 

 highly perfected as it had been by Larmor and Lorentz, and 

 work out the logical consequences of the following postulate : 



The laws according to which the states of physical systems 

 are changing are the same whether the phenomena are referred 

 to the system of co-ordinates (S) or to any other system of 

 co-ordinates (S') moving uniformly with respect to the former. 



This postulate was in fact a very broad generalisation of the 

 fact that the only motion which gave any positive effects in optical 

 experiments was relative motion of matter to matter, of object 

 observed to observer. Recently Einstein has extended its scope 

 by deleting the word " uniformly " ; but more of that presently. 



The first application which Einstein made of this postulate 

 was to work out the connection between the spatial and tem- 

 poral measurements of the observers in the system of refer- 

 ence S, and those in S' . He assumed that the velocity of a 

 moving source added nothing to the velocity of light, a well- 

 known assumption in earlier work (its opposite being con- 

 sistent only with some form of emission theory of light). As, 

 according to Relativity, motion of observer to or from a source 

 is experimentally indistinguishable from motion of source to 

 or from the observer, it followed that light had the same speed 

 to all observers whatever their motion might be relative to 

 the source. In support of this deduction, we can appeal to a 

 number of experiments made in the nineteenth century, cul- 

 minating in the famous one carried out in 1886 by Michelson 

 and Morley, and repeated with further refinements by Morley 

 and Miller in 1905. Such a view cannot, of course, be recon- 

 ciled with the theory that light consists in the transmission 

 of mechanical or electromagnetic oscillations though a fixed, 

 stagnant ether ; but the belief entertained in some quarters 

 that Relativity is inconsistent with ethereal transmission in 

 some form, appears to have little or no foundation. 



The relations which Einstein worked out are embraced in 

 a set of equations arrived at by Lorentz in the first place (but 

 interpreted by him in quite a different sense). The S-observers 

 measure the time of an event (idealised as instantaneous) to 

 be t units of time after a chosen original instant, and the place 

 of its occurrence (idealised as a point) to have co-ordinates 

 x, y, 2, relative to their frame of axes (relatively fixed to them). 

 The S'-observers record their observations of this event as 



