Conclusions of tlie Theory of Relativity. 843 



been developed more slowly and more carefully than have 

 those of: any other branch of physics and, until recently, we 

 would have agreed with Rankine that they are the nearest 

 approach to an exact science that human reason has been able 

 to devise. 



But, with the advance in the sciences of optics and elec- 

 tricity a different plan has been adopted, the phenomena of 

 electricity and radiation are now considered as fundamental ; 

 matter and energy become secondary attributes of an ?ether 

 or of a substance, electricity; but, oddly enough, the ex- 

 planation of this ?etherial or electrical substance is still 

 mechanical. Let me quote Einstein (p. 52): "Classical 

 mechanics required to be modified, before it could come into 

 line with the demands of the special theory of relativity. 

 For the main part, however, this modification affects only 

 the laws for rapid motions, in which the velocities of matter v 

 are not very small as compared with the velocity of light. 

 We have experience of such rapid motions only in the case 

 of electrons and ions : for other motions the variations from 

 the laws of classical mechanics are too small to make them- 

 selves evident in practice." 



It is quite evident that, in Einstein's opinion, the classical 

 mechanics based on the Galileo-Newton coordinate system 

 is adequate only for static problems. When motion is 

 involved, the dimensions of length, mass, and time must 

 be determined by applying the Lorentz-FitzGerald trans- 

 formation. This modification is theoretically necessary for 

 all velocities, but it becomes practically important only when 

 matter is electrically charged and moving with a velocity 

 comparable to light. This condition is reached only when 

 matter is reduced in size to a sub-atomic dimension, which 

 is itself admittedly below all our powers of sense perception, 

 and when matter is radiating non-mechanical energy. In 

 other words, the classical mechanics of tangible bodies must 

 be theoretically discarded, not because its equations will not 

 adequately define the positions, velocities, and energies of 

 bodies of a perceptible size, but because we wish to explain 

 the motion and energy of sub- atomic bodies and to discuss 

 the energy of radiation in vacuo during the interval of time 

 when it is emitted and afterwards absorbed by matter. In 

 all conscience, the obvious thing to do would be to hold fast 

 to classical mechanics as a satisfactory groundwork for our 

 conception of the objective world and to make the subject 

 of radiant energy a separate and distinct branch of science. 

 Certainly, there is a fundamental difference between the 

 reality of a bodv of tangible and perceptible proportions and 



3 K '2 



