RELATIVE AND ABSOLUTE QUANTITIES 23 



divide energy by time to give horse-power. We can set 

 ourselves the mathematical problem of constructing in 

 this way quantities which shall be invariant, that is to 

 say, shall have the same measure whatever frame of 

 space may be used. One or two of these invariants 

 turn out to be quantities already recognised in pre- 

 relativity physics; "action" and "entropy" are the best 

 known. Relativity physics is especially interested in 

 invariants, and it has discovered and named a few more. 

 It is a common mistake to suppose that Einstein's theory 

 of relativity asserts that everything is relative. Actually 

 it says, "There are absolute things in the world but 

 you must look deeply for them. The things that first 

 present themselves to your notice are for the most part 

 relative." 



Relative and Absolute Quantities. I will try to make 

 clear the distinction between absolute and relative quan- 

 tities. Number (of discrete individuals) is absolute. It 

 is the result of counting, and counting is an absolute 

 operation. If two men count the number of people in 

 this room and reach different results, one of them must 

 be wrong. 



The measurement of distance is not an absolute 

 operation. It is possible for two men to measure the 

 same distance and reach different results, and yet neither 

 of them be wrong. 



I mark two dots on the^ blackboard and ask two 

 students to measure very accurately the distance between 

 them. In order that there may be no possible doubt as 

 to what I mean by distance I give them elaborate 

 instructions as to the standard to be used and the pre- 

 cautions necessary to obtain an accurate measurement 

 of distance. They bring me results which differ. I ask 



