CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



ON AN ELECTROMAGNETIC THEORY OF GRAVITATION. 



By D. L. Webster. 



Presented by B. O. Peirce, November 8, 1911. Received November 8, 1911. 



The desirability of an electromagnetic theory of gravitation has been 

 pointed out many times, by Maxwell,^ Lorentz,^ and others, who 

 would assume that gravitation is due to a slight excess in the attrac- 

 tion of an electric charge for one of the opposite sign over its repulsion 

 for an equal one of the same sign; and also by Einstein and his fol- 

 lowers, who would have changes of gravitational attraction propagated 

 with the velocity of light. But Maxwell and Lorentz had no proof of 

 the truth of their theory, or reason for belief in it, other than that it 

 might connect gravitation and electricity; and even Einstein had no 

 proof, but only the fact that if the velocity were different from that of 

 light, we should have a means of detecting absolute motion. The ob- 

 ject of this paper is to give some reasons for belief in the electromag- 

 netic theory, and to point out some of its most surprising results. 



Objections have been raised against such a theory, principally on the 

 ground that it would involve gravitational aberration like the aberra- 

 tion of light, but I hope to show that the electromagnetic theory 

 involves no such consequences. Wiechert^ and others have also 

 objected on the ground that gravitation is essentially different from 

 electrostatic force, especially in its ability to penetrate even the densest 

 matter without appreciable change. But it may be proved, with no 

 other assumption than that a dielectric affects electrostatic force only 

 through the displacement of negatively charged particles within it, that 

 the uniform gravitational permeability of all dielectrics is no argu- 

 ment against the electromagnetic theory. And it has been proved by 

 Gans * that the equality of the gravitational permeabilities of vacuum 

 and conductors is also consistent with such a theory. 



* Maxwell, Electricity and Magnetism, Vol. 1, p. 40. 



2 Versl. K. Ac. van Wet., 8, 603 (1900); 8, 616 (1900). 

 ' Wiechert, IJber die Relativitatsprincip und Ather, Phys. Zeitsch., 12, 

 18-19. 



* Phys. Zeitsch., 6, 803. 

 VOL. XLVII. — 36 



