1918] on Gravitation and the Principle of Relativity 219 



dignity as to join this conspiracy ? There is certainly not enough 

 evidence for a jury to convict ; but yet I think we shall have to 

 intern it on suspicion. Recently Sir Oliver Lodge, believing that 

 gravitation was innocent of the conspiracy, showed that a very 

 famous astronomical discordance in the motion of Mercury miirht be 

 an effect due to the sun's motion through the ether, and might afford 

 a means of estimating its speed. It is difficult in a brief reference 

 to deal quite fairly with an intricate question, but it seems now tliat 

 we should rather lay stress, not on this single discordance, which can 

 perhaps be otherwise explained, but on the exact agreement of Yenus 

 and the Earth with theory ; for they also should show evidence of 

 the sun's motion through the ether if gravitation had not joined in 

 the conspiracy to conceal all such effects. It may be that the effects 

 on Venus and the Earth are not found because the sun's motion 

 through the ether happens to be very small ; but on the whole it 

 appears more likely that the effect of the motion is null, just as 

 in the Michelson-Morley experiment, because there is a complete 

 compensation in the law of gravitation itself. 



The great advantage of Minkowski's point of view is that it gets 

 rid of all idea of a conspiracy. You cannot have a conspiracy of con- 

 cealment when there is nothing to conceal. We cut Minkowski's 

 space-time world in a certain direction, so as to give us separately 

 space and time as they appear to us. AVe have been imagining that 

 there exists some direction which would separate it into a real and 

 absolute space and time. But why should there be ? Why should 

 one direction in this space-time world be more fundamental than any 

 other ? We do not attempt to cut the space-world in a particular 

 direction so as to give us the reaJ horizontal and vertical. The ^-ords 

 horizontal and vertical have no meaning except in reference to a 

 particular spot on the earth. So for a particular observer th3 space- 

 time world falls apart into its four components, up-and-down, right- 

 and-left-, backwards-and-f or wards, sooner-and -later ; but no observer 

 can say that this division is the one and only real one. 



Our idea of a real space more fundamental than our own was, h )W- 

 ever, not entirely metaphysical ; we had materialized it by filHng it 

 with an ether supposed to be at rest in it. We now deny the existence 

 of any unique framework of that kind. We have failed to obtain 

 experimental knowledge of such a framework since we cannot detect 

 our motion relative to it. Whatever may be the nature of the ether, 

 it is. devoid of those material properties which could constitute it a 

 framework of reference in space. We can perhaps best picture the 

 ether as a four-dimensional fluid filling uniformly Minkowski's space- 

 time continuum, not as a material three-dimensional fluid occupying 

 space and time independently. 



The position we have now reached is known as the Principle of 

 Relativity. In so far as it is a physical theory, it seems to be amply 

 confirmed by numerous experiments (except in regard to gravitation). 



