4 Bateman, The Physical Aspect of Time. 



view is really not sound, because the universal time we 

 are endeavouring^ to define is essentially quantitative in 

 character. The best waj' of establishing the existence of 

 a consistent method of comparison is to give an example 

 of one, and so we shall consider Galileo's method of light 

 signals,* which was used in a first but unsuccessful 

 attempt to measure the velocity of light. 



The way in which this method is applied is as follows. 

 An observer situated at a point A observes at time t an 

 event which has taken place at another point B. If r is 

 the time which light takes to travel from B to A, the 

 universal time to be associated with the event at B 

 according to ^'s measurements is / — r. As soon as A 

 has observed the event he makes a signal, and it is clear 

 that by a series of signals the two measures of an interval 

 of time may be compared. 



By means of this rule the clocks belonging to a 

 number of observers A ^, A„,... can be regulated in a 

 consistent manner, provided light always takes the same 

 time to travel from an observer A^ to an observer A^. 



Let us suppose that a large number of observers A,. 

 find that their observations of one another's experiences 

 give a consistent universal time as far as they are con- 

 cerned ; they can then regard themselves as being at 

 constant distances from one another, the distance between 

 two observers A,., A, being defined as Ct,., where r„ is 

 the time light takes to travel from A^ to A,, and C is a 

 constant called the velocity of light. 



These observers may then form a standard system 

 for the measurement of time and distances at other points 

 of space. The measurements of four standard observers 

 should suffice to determine the position and time of any 



* This method is used lor tlie purpose of studying the properties of time 

 by A. Einstein, Ami. der Physik, vol. 17 (1905). 



