﻿.91 Dr. A. D. Fokker : A Summary oj 



Relativity of Inertia. 



15. When we take into account terms with -, then 



c 



equations of motion may be derived for a moving particle 

 which furnish us with some remarkable conclusions as to 

 the relativity of inertia. 



It was pointed out long ago by Mach* that we cannot 

 speak of mass in an absolute sense. Just as we can only 

 speak of the motions, velocities, and accelerations of a body 

 relative to other existing bodies, so we only come to consider 

 the inertia? of different bodies when we study their mutual 

 action. Mach concluded that we are not justified in thinking 

 of the mass as of something absolute belonging to a particle, 

 but that it may be due to some inducing influence of bodies 

 one upon the other. 



If this is true we should expect first, that the inertia of a 

 particle is increased by heaping up other masses in its 

 neighbourhood, and secondly, that an accelerated mass 

 induces an acceleration of the same direction in other masses. 

 For if two masses are accelerated together (amidst other 

 bodies) their mutual acceleration is zero, and the resistance 

 against the mutual acceleration, which is the inertia that 

 they mutually induce one in the other, ought not to come 

 into play. So that when A and B are accelerated together the 

 force required to give A this acceleration is less than it was 

 when B remained at rest. This is the same as saying that 

 the acceleration of B actually gives rise to an accelerating 

 force in A. 



It is remarkable that these conclusions follow as con- 

 sequences from Einstein's theory. Indeed, by bringing the 

 other masses nearer, the inertia of a particle is expected to 

 increase, and a sudden acceleration of neighbouring masses 

 would cause an acceleration of the particle. 



Unhappily, the amount of the expected effects is so small 

 that there is scarcely any hope of discovering them. 



Concluding Remarks. 



16. In the absence of experimental evidence, certain points 

 may be laid stress upon which distinguish Einstein's theory 

 from other theories, and give it high intrinsic merits. 



It is an advantage that the theory regards the velocitv of 

 light no more as an absolute constant. There was somethmo- 

 unsatisfactory in the unexplained existence of a certain 

 constant critical velocity. 



* 'Prinzipien der Mechanik in ihrer historischen Entwicklung 

 dargestellt.' 



