436 TEE POPULAR SCIENCE MONTHLY 



points A, B, C (Fig. 2), are moving to the right with a uniform un- 

 known velocity through still air, and if a sound wave were sent out from 

 A, it would be exceedingly simple to determine the velocity of the point 

 A by a comparison of the time necessary for sound to travel from A to B 

 and from A to C. But now if the same three points move through sta- 

 tionary ether, and if the wave emanating from A is a light wave, there 

 is absolutely no way in which an observer connected with these three 

 points can determine whether he is moving or not. Thus we are, in con- 

 sequence of the Michelson and Morley experiment, driven to the first 

 fundamental postulate of relativity : The uniform velocity of a body can 

 not be determined by experiments made by observers on the body. 



Consider now one of the fundamental concepts of mechanics, time. 

 Physicists have not attempted to define it, admitting the impossibility 

 of a definition, but still insisting that this impossibility was not owing 

 to our lack of knowledge, but was due to the fact that there are no 

 simpler concepts in terms of which time can be defined. As Newton says : 



Absolute and real time flows on equably, having no relation in itself or in its 

 nature to any external object. 



Let us examine this statement, which embodies fairly our notion of 

 time, in the light of the first fundamental principle of relativity just 

 laid down. Suppose A and B (Fig. 3) are two observers, some distance 



apart, and they wish to set their 



H 



given m- 



A Ij clocks together. At a 



fig. 3. stant agreed upon before hand, A 



sends out a signal, by wireless if 

 you wish, and B sets his clock at this instant. But obviously the signal 

 has taken some time to pass from A to B, so 5's clock is slow. But this 

 seems easy to correct ; B sends a signal and A receives, and they take the 

 mean of the correction. But says the first principle of relativity, both A 

 and B are moving through the ether with a velocity which neither knows, 

 and which neither can know, and therefore the time taken for the signal 

 to pass from A to B is not the same as that taken to pass from B to A. 

 Therefore the clocks are not together, and never can be, and when A'a 

 clock indicates half-past two, B's does not indicate this instant, and 

 worse yet, there is absolutely no way of determining what time it does 

 indicate. Time then is purely a local affair. The well-known phrase, 

 " at the same instant " has no meaning for A and B, unless a definition 

 be laid down giving it a meaning. The " now " of A may be the " past " 

 or " future " of B. To state the case in still other words, two events can 

 no more happen simultaneously at two different places, than can two 

 bodies occupy the same position. 



But doubtless the reader is anxious to say, this matter of adjusting 

 the clocks together can still be settled. Let there be two clocks having 

 the same rate at a point A, and let them be set together. Then let one 



