EINSTEIN'S PRINCIPLE 21 



I am prepared to admit that frames of space in spite 

 of their present resemblance may in the future turn out 

 to be not entirely indistinguishable. (I deem it unlikely, 

 but I do not exclude it.) The future physicist might 

 find that the frame belonging to Arcturus, say, is unique 

 as regards some property not yet known to science. Then 

 no doubt our friend with the label will hasten to affix 

 it. "I told you so. I knew I meant something 

 when I talked about a right frame." But it does not 

 seem a profitable procedure to make odd noises on 

 the off-chance that posterity will find a significance to 

 attribute to them. To those who now harp on a right 

 frame of space we may reply in the words of Bottom the 

 weaver — 



"Who would set his wit to so foolish a bird? Who 

 would give a bird the lie, though he cry 'cuckoo' never 

 so?" 



And so the position of Einstein's theory is that the 

 question of a unique right frame of space does not arise. 

 There is a frame of space relative to a terrestrial ob- 

 server, another frame relative to the nebular observers, 

 others relative to other stars. Frames of space are rela- 

 tive. Distances, lengths, volumes — all quantities of 

 space-reckoning which belong to the frames — are likewise 

 relative. A distance as reckoned by an observer on one 

 star is as good as the distance reckoned by an observer 

 on another star. We must not expect them to agree; 

 the one is a distance relative, to one frame, the other is 

 a distance relative to another frame. Absolute distance, 

 not relative to some special frame, is meaningless. 



The next point to notice is that the other quantities 

 of physics go along with the frame of space, so that they 

 also are relative. You may have seen one of those tables 

 of "dimensions" of physical quantities showing how 



