Conclusions of the Theory of Relativity. £45 



problems of radiation require a modification of the law of 

 inertia if they are to be explained mechanically, so also the 

 static balance of mechanics will have to be abandoned. We 

 find that electromagnetic radiation, when it is absorbed or 

 reflected by matter, exerts a mechanical pressure. This 

 pressure becomes apparent at an interval of time after the 

 emission of energy from the radiating body. There is either 

 no mechanical reaction on the emitting body, or, if there is, 

 no evidence exists that it is a simultaneous one. When light 

 is reflected from a body, is the velocity of light altered, 

 contrary to hypothesis, in accordance with the mechanical 

 law of equivalence of momenta, or must the laws of impact 

 be modified ? The proposed theory of relativity must 

 therefore modify the third law of mot'on as well as the 

 other two. 



If one accepts the opinion of Einstein that classical me- 

 chanics is inadequate because it will rot account for all 

 phenomena, there is probably no doubt of its failure, but it 

 can be added that no logical system can ever be devised 

 which will accomplish that impossible task. Does Einstein 

 suppose that the new mechanics, or rather mechanico-electro- 

 dynamics, is of that all-embracing type ? Apparently he 

 does, if the meaning of the following quotations be clear 

 (p. 15) : "As long as one was convinced that all natural 

 phenomena were capable of representation with the help of 

 classical mechanics, there was no need to doubt the validity 

 of this principle of relativity [the Newtonian]. But in view 

 of the more recent development of electrodynamics and 

 optics, it became more and more evident that classical 

 mechanics affords an insufficient foundation for the physical 

 description of all natural phenomena." Then he adds 

 (p. 16): "The principle of relativity must therefore apply 

 with great accuracy in the domain of mechanics. But that a 

 principle of such broad generality should hold with such 

 exactness in one domain of phenomena, and yet should be 

 invalid for another, is a priori not very probable/' Just the 

 contrary is true, as we always pass from one domain of 

 phenomena to another by means of a ratio or physical 

 coefficient whose meaning is unknown and whose mea- 

 suivment is expressed in units of the first domain, as when 

 we pass from mechanics to electricity a quantity o( elec- 

 tricity is expressed in mechanical units of mass, length, 

 and time, and the meaning of the dielectric constant is 

 unknown. 



If classical mechanics affords us a tool by which we can 

 account for phenomena involving the positions and motions 



