LIGHT AND THE QUANTUM 127 



and now he expects his arrival to produce an 

 unparalleled sensation. But he finds everything 

 changed; the people hardly understand him; 

 and only after many bewildered questions does 

 he realize that he is a second Rip van Winkle, 

 and that the day of his return is not a decade 

 but a century later than the day of his depar- 

 ture. 



You will say that this is pure romancing, and 

 surely there are grave doubts as to the feasibil- 

 ity of such a machine as I suggest; but, grant- 

 ing such a machine, no one who is acquainted 

 with the elementary theory of relativity will 

 deny that during the traveler's ten years' jour- 

 ney a century will have elapsed on the earth. 

 I am simply putting into a more startling form 

 the conclusion reached in the third chapter that 

 there is no such thing as an absolute measure of 

 the flow of time. In doing so it is with the 

 thought that those who have made this great 

 stride away from the traditional notion of time 

 may not be unwilling to go a little further in 

 the direction that I am about to propose. 



We have seen how kinematics has been com- 

 pletely correlated with a pure geometry in 

 which there is no intrinsic difference between up 

 and do\\Ti, or past and future ; and we have also 



