26 RELATIVITY 



the relative distances to build on. The quest of the 

 absolute leads into the four-dimensional world. 



A more familiar example of a relative quantity is 

 "direction" of an object. There is a direction of Cam- 

 bridge relative to Edinburgh and another direction rela- 

 tive to London, and so on. It never occurs to us to 

 think of this as a discrepancy, or to suppose that there 

 must be some direction of Cambridge (at present undis- 

 coverable) which is absolute. The idea that there ought 

 to be an absolute distance between two points contains 

 the same kind of fallacy. There is, of course, a differ- 

 ence of detail; the relative direction above mentioned is 

 relative to a particular position of the observer, whereas 

 the relative distance is relative to a particular velocity 

 of the observer. We can change position freely and 

 so introduce large changes of relative direction; but 

 we cannot change velocity appreciably — the 300 miles 

 an hour attainable by our fastest devices being too 

 insignificant to count. Consequently the relativity of 

 distance is not a matter of common experience as the 

 relativity of direction is. That is why we have unfor- 

 tunately a rooted impression in our minds that distance 

 ought to be absolute. 



A very homely illustration of a relative quantity is 

 afforded by the pound sterling. Whatever may have 

 been the correct theoretical view, the man in the street 

 until very recently regarded a pound as an absolute 

 amount of wealth. But dire experience has now con- 

 vinced us all of its relativity. At first we used to cling 

 to the idea that there ought to be an absolute pound 

 and struggle to express the situation in paradoxical state- 

 ments — the pound had really become seven-and-six- 

 pence. But we have grown accustomed to the situation 

 and continue to reckon wealth in pounds as before, 



