191S] on Gravitation and the Principle of Relativity 228 



more "real " ? In effect tliev are so much alike iliat even in scien- 

 tific work we speak of tlieni in one hreatli. What is called the value 

 of gravity in London, 1)81 -17 cm. /sec-, is really made up partly of 

 the trae attraction of the earth and partly of the centrifugal force. 

 It is not considered worth while to make any distinction. Surely, 

 then, it is not a great stretch of the imagination to regard gravitation 

 as of the same nature as centrifugal force, being merely our percep- 

 tion of the crookedness of the scatfoMing that we have chosen. 



If gravity and centrifugal force are manifestations of the same 

 underlying condition it must l)e possible to reduce them to the same 

 laws ; but we must express the laws in a manner which will render 

 them comparable. There is a convenient form of Newton's law, 

 which was given by Laplace and is \\ell known to mathematicians, 

 which describes how the intensity at any point is related to the 

 intensity at surrounding points — or, according to our interpretation, 

 how the distortion of space at any point fits on to the distortion at 

 surrounding points. It is evidently an attempt to express the general 

 laws of the strains in space and time which occur in nature. If we 

 are correct in our assumption that gravitation involves nothinfj more 

 than strain of space-time," so that its law expresses merely the rela- 

 tion between adjacent strains which holds by some natural necessity, 

 clearly the strains which give the centrifugal force must obey the 

 same general law\ Here a very interesting point arises. We cannot 

 reconcile the Newtonian law of gravitation with this condition. 

 Newton's law and the law of centrifugal force are contradictory. 



To put the matter another way, if we determine the strains by 

 Newton's law, we get results closely agreeing with observation^ pro- 

 vided Minkowski's space-time is used ; but if we avail ourselves of 

 our right to use a transformed space -time, the results no longer 

 agree vrith observation. That means that Newton's law involves 

 sometliing which is not fully represented by strains, and so does not 

 agree ^7ithour assumption. We must either abandon our assumption, 

 or abandon the famous law which has been accepted for over 200 

 years, and find a new law of gravitation which will fall in with our 

 requirements. 



This amended law has been found by Einstein, It appears to be 

 the only possible law that meets our requirements, and in the 

 limited applications which come under practical observation is suffi- 

 ciently close to the old law that has served so well. In practical 

 applications the two laws are indistingtiishable, except for one or two 

 crucial phenomena to which reference will be made later. Bat in 

 gravitational fields far stronger than any of v;hich we have experi- 



* The idea is that matter represents a seam or nucleus of strain, and the 

 strains at other points link themselves on according to laws inherent in the 

 continuum Q.ndi quite independent of the matter. The matter starts the strain, 

 but does not control it as it goes outwards. 



