168 THE LIMITATIONS OF SCIENCE 



illustrate further our confusion as regards the measure- 

 ment of time, suppose two persons wish to record two 

 events, which we shall grant to be simultaneous, 

 but which occur at different places, A and B. An 

 observer at A records the event, when it occurs at A, 

 as being at T A time by his clock, and instantly signals 

 the fact to an observer at B. Whatever the means of 

 signaling, a certain time will elapse before the ob- 

 server at B is cognizant of it. For the reasons given, 

 we shall adopt light signals as the best method. B 

 observes the signal as T B time by a clock placed at 

 B and immediately reflects it back to A, who receives 

 it at T'A time by his clock. Although we have granted 

 that both events were simultaneous, it is evident they 

 will not be so recorded by the two observers. B will 

 record the event at A as being later than the one at B. 

 But if the velocity of light be absolutely independent 

 of all conditions, then we should find that the differ- 

 ences of time going and returning are the same, or 

 T B T A = T' A T B . This relation is satisfied by any 

 two events occurring at a distance from each other, 

 is Professor Einstein's definition of simultaneous 

 events or isochronism. The definition may be put in 

 this form: two events are simultaneous, if the differ- 

 ence of time to flash a signal there and back is equal 

 to twice the distance between the two positions divided 

 by the velocity of light, V. 



Not only does this limitation in our ability to meas- 



