LIGHT AND THE QUANTUM 133 



represents the event of the emission of a light 

 particle by the one atom, and if L represents the 

 event of the receipt of the particle by the other 

 atom, these two events may, in our ordinary 

 way of speaking, be thousands of years apart, 

 but any such statement depends upon an arbi- 

 trary choice of an axis of time. If in our figure 

 we take, not OT, but other time axes lying 

 nearer and nearer to OL, not only the time 

 elapsing between O and L, but also their spatial 

 distance, approaches zero, according to Ein- 

 stein's principles. 



I have spoken of virtual contact as distin- 

 guished from ordinary contact, but I do not 

 wish to imply that it is any less a physical con- 

 tact in the one case than in the other. I claim 

 that my eye touches a star as truly as my finger 

 touches this table. When two atoms are in ordi- 

 nary contact we do not inquire how one ascer- 

 tains that the other is in a position to receive 

 energy, nor need we so inquire in the case of 

 virtual contact. It will suffice to discover and to 

 describe the conditions of relative orientation 

 between the atoms, and the like, that permit the 

 exchange of energy. These, however, are techni- 

 cal questions, and I cannot discuss here the 

 methods by which the new idea may be shown 



