Apeil 24, 1914J 



SCIENCE 



597 



by the principle of relativity, that an essential 

 error was introduced when the formulse of 

 dynamics and those of electromagnetism were 

 assumed to lead to the same conceptions of 

 space and time. According to the new me- 

 chanics, our ideas of time and space, obtained 

 from mechanical notions, are only approxi- 

 mately true, while those derived from the laws 

 of electromagnetism are correct. The result 

 is many physicists of the new school are now 

 seeking for an interpretation of inertia from 

 the laws of electromagnetism rather than to 

 continue to explain the laws of electromagnet- 

 ism by mechanics. These laws of electro- 

 magnetism have the advantage of great sim- 

 plicity of form which may qualify them to 

 serve as the fundamental principle of all 

 physical laws. 



The germ of these new views of electro- 

 magnetism is to be found in the work of Fara- 

 day and Maxwell. Their true experimental 

 foundation is Rowland's experiment, per- 

 formed in 1878, when he found that an electric 

 charge, if it be carried through space with a 

 high velocity, acts like an electric current in 

 that it creates about itself a magnetic field. 

 Three years later, J. J. Thomson showed that, 

 if an electrified body moves through space, 

 it not only creates a magnetic field in the sur- 

 rounding space, but also that this magnetic 

 field is a form of mechanical energy; from the 

 law of the conservation of energy, this energy 

 must be acquired at the expense of an equal 

 amount of energy localized in the free space 

 about the body. Now it can be shown rigor- 

 ously that this " magnetic " energy has all 

 the characteristics of a kinetic energy, since 

 it is proportional to the square of a velocity 

 and contains a factor which corresponds with 

 the mass as given in the third of our former 

 definitions. This supplementary inertia of 

 electromagnetic origin results solely from the 

 fact that the body is electrified, and it is an 

 addition to the stationary or initial mass 

 which was denoted by m^. 



The result is the same if we consider the 

 second definition of mass, as a capacity for 

 impulse or momentum. So soon as a body is 

 electrified and moves, it develops a supple- 

 mentary capacity for momentum which agrees 



with the supplementary capacity for energy 

 which has just been described. Poincare, in 

 order to preserve the fundamental law of the 

 conservation of momentum, has shown that it 

 is necessary to localize a quantity of momen- 

 tum in space just as we were forced to localize 

 a quantity of energy in space. Thus the 

 fundamental consequence of rational mechan- 

 ics, requiring the three definitions of mass to 

 agree, is not satisfied for all cases of motion. 



These ideas have affected our notions of 

 space. Maxwell deduced as a consequence of 

 theory that rapid and periodic variations in 

 the electrical charge of a body should be prop- 

 agated through space with the velocity of light. 

 The existence of these electromagnetic radia- 

 tions was verified experimentally by Hertz, and 

 in the hands of his successors this new form of 

 radiation has attained great importance under 

 the name of wireless telegraphy. A theoretical 

 consequence of this radiation is the now gen- 

 erally accepted belief that light is merely a 

 type of electromagnetic radiation of exces- 

 sively rapid vibration. To transfer the phe- 

 nomena of light from a mechanical to an 

 electromagnetic manifestation of energy was 

 to shake profoundly the belief in the funda- 

 mental and universal nature of mechanical 

 energy. 



Another very important principle of ra- 

 tional mechanics was embodied in Newton's 

 third law of motion, that to every action there 

 is an equal and oppositely directed reaction. 

 But we have not been able to make the forces 

 created by an electrified body in motion con- 

 form to this law. This is especially evident 

 when radiation also occurs. Let us suppose 

 that an incandescent body is giving off light 

 (i. e., electromagnetic radiation) uniformly 

 in all directions. By reason of symmetry this 

 radiation exerts no resultant force on the 

 source. But if the perturbation, in the form 

 of a wave, encounters an obstacle at a distance, 

 then we know, both from theory and from 

 experiment, that the obstacle will be subjected 

 to a force due to its absorbing a part of the 

 radiant energy. This pressure of light pushes 

 the obstacle in the direction of the propagation 

 of the light, and the action thus exerted on the 



