( 2^7 ) 



ill tlie electron i( is zero. The conliimil)- of llie niolioii of llic energy 



reciiiires that in tlie cle(;tron the transport of energy in the dii-eetion 



— A' takes place, and that therefore also an anionnt of nioinentnni 



in that direetion exists. This trans|)(»rl of energy in the elei'tron is 



oeeasioned by the elastic forces. In the ehM'tron e.vists namely a 



tension, and this is always accompanied with a transport of energy 



opposite to the motion of the hody, in the same way as a pressure 



is accompanied with a transport in the direction of the motion. 



So we see that OV and f' mnst be augmented by several amounts 



which are at present unknown. It is therefore impossible to decide 



1 

 whether the equation ///„ := — f^ is satisfied. It is not even certain 



(■" 



that this question has a real meaning. For in mechanics the energy 

 is never perfectly determined, Init contains an arbitrary constant. 

 And though for some kinds of energy no reasonable doubt can exist 

 as to the absolute amount, as for the kinetic, the electric, and the 

 magnetic energy, it is by no means certain that for all kinds of 

 energy we have a sulïicient reason for the determination of the zero 

 of energy. So we must content ourselves with observing that it is 

 certainly also impossible to prove that the equation does not hold good. 



The explanation of the relation between the energy, the mass, and 

 the momentum of a mo\ing body given here diifers from that of 

 Einstein ^), who assumes that the energy of a moving body varies 

 if a system of equal and opposite forces is applied, although they 

 influence neither the velocity noi' the shape of the body and accord- 

 ingly do not change the energy when evaluated from a system of 

 coordinates which shares the motion of the l)ody. According to 

 LoRENTZ ■) these forces brin<>- about also a variation of the momentum. 



It appears however to me that this view cannot be maintained. 

 In the tirst place the existence of a rigid body is assumed, and the 

 existence of such a body would be at variance with the fundamental 

 hypothesis of the theory of relativity ^). Hut the increase of energy 

 and momentum would not be found even if we assumed the existence 

 of such a body. 



For a body cannot be rigid with respect to every system of 

 coordinates. If it has that (piality with i-espect to a coordinate system 

 which shares its motion, it cannot have it with respect to other 

 coordinate systems. A disturbance wiiicli propagates with infinite 

 velocitv when evaluated from a svstem which shares the motion of 



1) A. Einstein, Aun. d. Phys. XXllI, p. 371. 19U7. 



3) H. A. LoRENTz. Veisl. Kon. Ak. Anist. Juiii 1911, p. 'S,. 



a) M. Laue. Phys. Zeilschr. 12, p. 4S, Anno 1911. 



