view is really not sound, because the universal time we are endeavouring to define is essentially quantitative in character. The best way 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,[1] 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 τ 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 A's measurements is . 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 can be regulated in a consistent manner, provided light always takes the same time to travel from an observer to an observer .
Let us suppose that a large number of observers find that their observations of one another's experiences give a consistent universal time as far as they are concerned; they can then regard themselves as being at constant distances from one another, the distance between two observers , being defined as , where is the time light takes to travel from to , 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 for the purpose of studying the properties of time by A. Einstein, Ann. der Physik, vol. 17 (1905).