130 THE ANATOMY OF SCIENCE 



emitting source and the newly made molecule of 

 chlorophyl? Again, let us suppose this same 

 star to be the source of light in Figure 22. By 

 uncovering both mirrors we prevent a particle 

 of light from reaching the point C. Do we thus 

 prevent its original emission in just this direc- 

 tion? If so it means that we can, in a trivial 

 way, but nevertheless in principle, affect the 

 course of what we call past events. 



Offensive as such an idea is to all our estab- 

 lished notions of causality and temporal se- 

 quence, we must remember that these notions 

 have arisen from the observation of processes 

 which are very different from the elementary 

 reversible processes which we are now consider- 

 ing. Unless the result of some actual fact of 

 experiment or observation can be brought 

 against the new view, we need not be deterred 

 by this conflict with common notions, especially 

 since we shall see that by this means certain 

 inconsistencies between prevailing physical 

 ideas and the geometry which so admirably 

 interprets the kinematics of relativity are re- 

 moved. 



We admit that a radioactive substance emits 

 its particles in all directions without regard to 

 their later absorption by other matter, and why 



