228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



movement with reference to a supposed stationary ether. This novel 

 hypothesis was announced in its most general form for the first time 

 by Einstein/ who named it the principle of relativity. Starting with 

 this simple principle, Einstein modified slightly the transformation 

 of Lorentz, giving to it a physical basis of very great generality and 

 gathering all the conclusions resulting from it into a group of per- 

 fectly consistent formulae. 



We will not state here all the physical and philosophical conse- 

 quences of this theory of relativity. We will note only the absolute 

 character assumed by the two fundamental postulates of this theory : 

 First, the ether is immovable and penetrates matter perfectly; second, 

 the velocity of light is an absolute invariant and represents a su- 

 perior limit which no other velocity can exceed (whether for matter 

 in motion or the propagation of waves). The theory has been fur- 

 ther developed (principally by Germans) by Einstein (1905-1912), 

 Minkowski (1905-1908), Planck (1907-8), Born (1909), Som- 

 merfeld (1910), Laue (1911), etc. The various points of view 

 which these physicists have adopted are too numerous to be given 

 here in detail ; some have tried to put the transformations of Lorentz 

 into more geometrical and comprehensive form (Minkowski) ; others 

 have deduced the kinetic consequences of the principle, either for a 

 moving point (composition of velocities according to Sommerfeld) 

 or for a solid body in rotation (Born, Laue, etc.). Difficulties and 

 complications quickly arise as soon as the motion of uniform trans- 

 lation originally supposed is departed from and these difficulties have 

 not yet been overcome. The total absence of any experimental basis 

 or confirmation of these later developments deters us from further 

 discussion. We will stop a moment only on one of the most para- 

 doxical consequences of the principle of relativity which will bring 

 out the difficulties which the theory encounters and rebut the abso- 

 lutism of the principles which it uses as bases of the phj^sical 

 sciences. At the start Einstein ^ showed that if the energy of a sys- 

 tem increases by the amount E^ the principle of relativity requires 

 that its TThOss increases at the same time hy E/V^. Only on this con- 

 dition can the principle of the conservation of the movement of the 

 center of gravity as well as the new system of mechanics be main- 

 tained. Accordingly, mass and energy are not really distinct; the 

 principle of the conservation of mass is inseparable from the prin- 

 ciple of conservation of energy. This result, however strange, is 

 nevertheless consistent in itself. 



Einstein himself, basing his deductions on this consequence, tried 

 to bring back to the principle of relativity the absolute value which 



1 Einstein, Annalen der Physik, vol. 17, p. 902, 1906. 



2 Einstein, 1. c. and Annalen der Physik, vol. 20, p. G27, 1906 ; vol. 23, p. 078, 

 1907, etc. 



