L. Page — A Century's Progress in Physics. 343 



The principle of relativity proposed by Einstein was 

 by no means new to students of dynamics. Newton's 

 first two laws of motion express very clearly the fact that 

 in mechanics all motion is relative. Force is propor- 

 tional to acceleration, and the relation between the two 

 is the same whether the motion under consideration is 

 referred to fixed axes or to axes moving with a constant 

 velocity. But in connection with the phenomena of light 

 and electromagnetism the case seemed to be quite differ- 

 ent. There everything was referred to a fixed ether, and 

 even though Lorentz had found a set of transformations 

 which left the electrodymanic equations practically 

 unchanged, he continued to think in terms of an ether. 

 So physicists were not a little startled when Einstein 

 postulated that no experiment, practical or ideal, could 

 ever distinguish between two systems in such a manner 

 as to warrant the assertion that one of them is at rest 

 and the other in motion. All motion is relative, and the 

 laws governing physical, chemical and biological phe- 

 nomena are the same in terms of the units of one system 

 as in terms of those of any other. 



Einstein next considers some very fundamental ques- 

 tions. What do we mean when we say that two events, 

 one at A and the other at a point B far from A, occur at 

 the same time? Obviously the expression has no signi- 

 ficance unless synchronous clocks are stationed at the 

 two points. But how is it to be determined whether or 

 not these two clocks are synchronous ? If instantaneous 

 communication could be established between A and B 

 the matter would be simple enough. Since no infinite 

 velocity of transmission is available, however, let a light 

 wave be sent from A to B and returned to A immediately 

 upon its arrival. If the time indicated by the clock at 

 B when the signal is received is half way between that at 

 which it left A and the time at which it arrives on its 

 return, then the two clocks may be considered syn- 

 chronous. Now if it desired to measure the length of a 

 bar which is moving parallel to the scale with which the 

 measurement is to be made, it is necessary to note the 

 positions of the two ends of the bar at the same instant. 

 So even the measurement of the length of a moving body 

 depends upon the condition of synchronism at different 

 points in space. 



The principle of relativity requires that the velocity 



