CLASSICAL AND NEW MECHANICS 175 



mean a mere change of position, one body is emitting 

 energy in the form of heat, light, or electricity, and the 

 other receiving it. True relativity requires a strictly 

 symmetrical arrangement Now the mechanical mo- 

 tions of two bodies with respect to one another are 

 symmetrical. If a body has a velocity, v lf to the right 

 and another a velocity, v 2 , to the left, they approach 

 each other with a velocity, Vi '+' v 2 . The same result is 

 obtained if the velocities of the bodies are interchanged. 

 This symmetry is not true if the first body is emitting 

 energy and the second receiving it. The behavior of 

 each is then conditioned by the behavior of the other 

 and in an asymmetrical manner. As an illustration: 

 let a body by a periodic disturbance of its parts send 

 out a wave of heat; this proceeds, according to theory, 

 through space in all directions and on encountering a 

 second body produces a periodic disturbance in it. The 

 effect of this energy is twofold; the motions of the 

 parts of the two bodies are symmetrical but the wave 

 itself does not proceed from the second body in all 

 directions but only in the one already impressed upon 

 it by the first body. To make this clearer, we may start 

 a wave along a stretch of string by plucking aside one 

 part of it; the wave will travel from the initial point 

 along the string in two directions, causing successive 

 parts to vibrate symmetrically, but these parts will 

 send the wave in one direction only. So we may say 

 the velocities of two bodies are relative because the 



