DEVELOPMENTS IN ELECTROMAGNETISM— BLOCH. 229 



had been attributed to it since 1905.^ He has tried to include in 

 the electromagnetic synthesis of the universe the phenomenon of 

 gravity, hitherto so rebellious against all our efforts at explanation. 

 He noted that a uniform gravitational field of constant accelera- 

 tion, f, is equivalent to a medium free from gravitation in which the 

 reference axes are supposed acting with a uniform acceleration 

 — y. Next we must generalize the principle of relativity and pass 

 from the case considered until now of a uniform velocity of trans- 

 lation to that of a uniform acceleration. In the earlier case we 

 were led to attribute to energy a mass m=E/V-', now, if we wish 

 to preserve the principle in its entirety we must attribute to the 

 same energy the weight my. As a particular case, radiant energy, 

 light, must have weight ; a beam of light must then be deviated by 

 the masses close to which it may pass. Einstein's calculation showed, 

 for example, that the angular distance between a star and the center 

 of the sun must be decreased by about one second when the star 

 appears close to the sun. The measurement could be attempted at 

 a total eclipse of the sun. 



There is no need of calling attention to the strangeness of these 

 conclusions. The important thing from a philosophical point of 

 view is that we are obliged to give up the absolute invariability 

 of the velocity of light, V, considered at the start as an unassailable 

 axiom. This invariability is only true in a system where the gravi- 

 tational potential S remains constant. For variable potentials the 

 velocity of light must vary according to the formula, V=:Vo 

 (l+^/V^). So it is only in the case of uniform motion of 

 translation that the transformation of Lorentz represents the phe- 

 nomena of a system in movement. In the more general case the 

 group of transformations is more complicated and as yet undeter- 

 mined ; the equations to be substituted for those of the classic elec- 

 tromagnetism are also undetermined. 



This new point of view of Einstein has at least one incontestable 

 utility: It makes us realize that the postulates which were at the 

 basis of the earlier principle of relativity (the invariability of F, 

 etc.) are perhaps only approximate affirmations, susceptible of modi- 

 fication, and not first truths. It has led us from metaphysics to 

 physics. And since the discussion became opened anew concerning 

 the foundations proposed by Einstein we will not be surprised to 

 find that Max Abraham, adopting this new conception of mass and 

 weight, has developed a new theory of gravitation, different in many 

 respects from that of Einstein. Abraham ^ renounces the generaliza- 



1 Einstein, Jahrbuch der Rad. and Electronik, vol. 4, p. 4 ; Annalen der Physik, vol. 35, 

 p. 898, 1911, etc. 



-Max Abraham, Phys. -Zeitschr., vol. 13, No. 1, 1912; Annalen dpr Physik, vol. 38, p. 

 10.56. vol. 39, p. 444, 1912; Nuovo Oimento, January, 1913. 



