Common Sense of Relativity. 513 



the real rate change ?" We immediately inquire what the 

 " real rate "■ means. He is at first inclined to assert that it 

 is the rate observed by an observer travelling with the clock, 

 but when we inquire relative to what clock that observer is 

 to measure the rate he becomes uneasy. He cannot compare 

 another clock travelling with him, for if the "real rate'"' of 

 one clock has changed, so has the " real rate " of the other ; 

 and he cannot use a clock which is not travelling with him, 

 because he admits that he does not see such a clock " as it 

 really is." 



Pressing our inquiries, I think w T e shall get an answer of 

 this nature. " If I take a pendulum clock to some place 

 where gravity is different, the rate of the clock will change. 

 It is a change of this nature which I call a change in the 

 ' real rate,' and I want to know whether there is any change 

 of that kind, on the theory of Relativity, when the clock is 

 set in motion." Now why does our objector call a change of 

 the first kind a change in the " real rate " ? The reply is to 

 be found in the history of the word " real." The word is 

 intimately associated with the philosophic doctrine of realism, 

 which holds that the most important thing that we can know 

 about any body is not what we observe about it, but its " real 

 nature," which is something that is independent of observa- 

 tion. Now, of course, a quantity which is wholly independent 

 of observation cannot play any part in an experimental 

 science, but there are quantities which are independent of 

 observation in the more limited sense that they are observed 

 to be the same by whatever observer the observation is made. 

 The term "real" has come to be transferred from the 

 philosophical conception to such quantities. The " real rate " 

 of the clock is said to change when it is transferred to a place 

 where gravitation is different, because all observers agree 

 that the rate of the clock which has been moved has under- 

 gone an alteration relatively to that which has not been moved. 



Now in the conditions which we are considering the 

 observers do not agree. If A and B, each carrying a clock 

 with him, are moving relatively to each other, they will not 

 agree as to the rate of either of their clocks relative to 

 A's standard or to B's standard or to any other standard. 

 The conditions which, in the case of the alteration of 

 gravitation, gave rise to the conception of a " real rate " are 

 not present : in this case there is no " real rate/' and it is as 

 absurd to ask whether it has changed as it would be to ask a 

 question about the properties of round square. However, 

 some people, who in their eagerness to escape the reproach 

 of being metaphysicians have adopted without inquiry the 



