﻿78 Dr. A. 1). Fokker : .1 Summary oj 



The largest effect occurs in the case of the bending of light- 

 rays in a strong gravitational field. The strongest field 

 available is the sun's. A light-ray passing near the sun's 

 surface should suffer a bending through an angle of 0*83 

 seconds of arc, so that the position of a star when the sun is 

 nearly touching it should be shifted away from the sun's 

 centre. Perhaps the next eclipse may reveal such an effect*. 

 Another consequence of the theory is the influence of the 

 gravitation potential on the rate of action of physical pro- 

 cesses. For example, the vibrations in the atoms should be 

 slower at the sun's surface than on the earth. As a matter of 

 fact, there has been observed a general shift of solar spectral 

 lines to the red side of the spectrum as compared with lines 

 from terrestrial sources t, but the solar conditions are so 

 complicated that no definite conclusion can yet be drawn 

 from this. 



2. Notwithstanding the fact that the theory cannot give 

 much hope for new discoveries in experimental physics, it 

 cannot be said to be chiefly a mathematical speculation. For 

 throughout its development the lines followed are lines of 

 physical thought, and Einstein's intuition has only trusted 

 truly physical principles. The reason why these had to be 

 so few is that a century of experimenting has failed to bring 

 to light an appreciable influence of gravitation on other 

 phenomena. 



Indeed, when the theory of relativity had concluded that 

 gravitation was to be propagated with the speed of light {, 

 it was difficult to look for an extension of Newton's theory, 

 which henceforth had to be considered as a first approxima- 

 tion, without any new experimental indications as to the 

 direction in which this extension was to be sought. There 

 seemed to be too much freedom, the number of possible 

 assumptions seemed not to be restricted enough to point in 

 any definite direction. 



Einstein considers of fundamental importance the fact 

 that all bodies fall with the same acceleration ; combined 

 with the assumption of the identity of gravitating and inertial 

 mass, it led him to work out the consequences of his ' Aequi- 

 valenz-Hypothese/ which will be described further on. 



Next he bases himself on the principle of the conservation 



* War lias made this impossible. 



t Compare E. Freundlich, Pkys. Zeitschr. xv. p. 369 (1914). 



X For the consistency of this propagation with astronomical observa- 

 tions cp. H. A. Lorentz, Proc. R. Ac. Sc. Amsterdam, viii. p. 603 

 (1900) ; H. A. Lorentz, Pkys. Zeitschr. xi. (1910). 



