LIGHT AND THE QUANTUM 131 



should we make a different assumption regard- 

 ing light? The answer is suggested by the new 

 geometry, which I recall to your mind through 

 Figure 23. We remember that there is a sharp 

 distinction between all the space-like lines, OX 

 and OX', and the time-like lines such as OT and 

 OT'. Between the two classes are the singular 

 lines OL and OL', which belong to neither of 

 these two classes and bear no more resemblance 

 to one class than to the other. The path of a 

 material particle in space-time is always one of 

 the time-like lines such as OT', and the slope of 

 this line with respect to a chosen time line OT 

 represents the velocity of the particle. On the 

 other hand, the slope of a space-like line, such 

 as OX', in no way suggests a velocity of matter 

 or energy. Now in spite of the symmetry de- 

 manded by our geometry, we affiliate the line 

 OL with the time-like lines and say that it 

 represents the velocity of light. This concession 

 to traditional thought certainly does violence to 

 the geometry, and if we continue to make such 

 concessions it must be with the full realization 

 of the unique character of the process of radia- 

 tion. 



Let me recall another remarkable feature of 

 the relativity geometry. Such intercepts of sin- 



