222 Professor A. S. Eddington . [Feb. 1, 



same rule -of shortest distance as measured in the uudistorted state 

 — is to hold ill all cases. This is a mode of reasoning which has often 

 been fruitful in scientific g'eneralizations. A magnetic needle turns 

 towards the end of a bar-magnet ; it also turns towards a spot near 

 the pole of the earth ; hence the suggestion that the earth is a magnet. 

 AVe assume the essential identity of the two modes of deflecting the 

 needle. It is a daring step to apply the analogy, and assume the 

 essential identity of the two \yays of deflecting world-lines ; but at 

 any rate we shall make this assumption and see what comes of it. 



You will see that according to this view^ the earth moves in a 

 curved orbit, not because the sun exerts any direct pull, but because 

 the earth is trying to find the shortest way through a space and time 

 which have been tangled up by an influence radiating from the sun. 

 AVe can continue to describe this indirect influence of the sun on the 

 earth's motion as a " force " ; but, assuming that it makes itself felt 

 as a modification or strain of space and time, we are able to bring 

 the discussion of the laws of this force into line with the discussion 

 of the laws of space and time, i.e. the laws of geometry. Needless to 

 say we could not determine a physical law^ like the law of gravitation 

 by geometrical reasoning without making some assumption. 



I am afraid that to talk of a force as being a distortion of space 

 and time must at first appear to you hopeless jargon. But it must 

 be remembered first that we are not concerned with any metaphysical 

 space and time. We mean by space and time simply a scaffolding 

 that we construct as the result of our measures ; and if anything 

 queer happens to our measuring apparatus, the scaffolding may easily 

 go crooked. Taking our everyday conception of space, we should 

 say that this room is at rest ; we have been told that it is being 

 carried round the earth once a day, but in practical life we never pay 

 any attention to that. The space that we naturally use is thus 

 different from, and it is not difficult to show that it is distorted as 

 compared Ayith, the more fundamental astronomical space in which 

 this room is travelling at a great velocity. So our scaffolding is 

 crooked. But, it may be asked, in Avhat way can this distortion of 

 our space-scaffolding be regarded as a force ? The answ^er is quite 

 simple. We perceive it as a force, and that is the only way in which 

 we do perceive it. We do not perceive that this room is being carried 

 round by the earth's rotation, but we perceive a certain force — the 

 earth's centrifugal force. It is rather difficult to demonstrate this 

 force, because graviation predominates overwhelmingly ; but if gravity 

 were annihilated we should have to be tied down to the floor to pre- 

 vent our flying up to the ceiling, and we should certainly feel our- 

 selves pulled i:)y a very vigorous centrifugal force. That is our only 

 perception of the crookedness of our scaffolding. 



We often call the centrifugal force an " unreal " force, meaning 

 that it arises simply from a transformation of the framework of 

 referf.nce. Can we feel confident that gravitation is in any sense 



