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



and time according to its motion tlirouo-b the etlier. The space 

 and time of one stai- will not fit the experience of individuals on 

 another star. 



The exact relation between the appropriate space and time of 

 one star and the space and time of another was first Ijrought out 

 clearly by Minkowski ; it is a very remarkable one. We recognize 

 three dimensions of space, which we may take as up-and-down, 

 right-and-left, backwards-and-forwards. If we go over to Ireland 

 we still have the same space, but Ireland's up-and-dow^n no longer 

 corresponds to ours. The directions are inclined ; and what is 

 vertical to them is partly vertical and partly horizontal to us. Now 

 lee us add a fourth dimension, imaginary* time, at right angles to 

 the other three. There is no room for it in the model, but we 

 must do our best to imagine it in four dimensions. In Ireland the 

 three space-dimensions will have rotated, as I have said ; but the 

 time will be just the same. But if we go to Arcturus, or to any 

 body moving with a velocity different from our own, the time- 

 dimension also has rotated. What is time to them is partly time and 

 partly imaginary space to us. It is a change in the space-time world 

 of four dimensions just analogous to the change in the space-world 

 between liere and Ireland. That is Minkowski's great result ; space- 

 time is the same universally, but the orientation — the resolution into 

 space and time separately — depends on the motion of the individual 

 experiencing it, just as the resolution of space into horizontal and 

 vertical depends on his situation. In Minkowski's own famous 

 words—" Henceforth Space and Time in themselves vanish to 

 shadows, and only a kind of union of the two preserves an inde- 

 pendent existence." 



From our original point of view it seems very remarkable that in 

 the Miche'son-Morley experiment the contraction should Lave been 

 of just the right amount to annul the expected effect of our motion 

 through the ether. Many other experiments, which seemed likely 

 to show such an effect, have been tried since then, but in all of them 

 the same kind of compensation takes place. It looks as though all 

 the forces of nature had entered on a conspiracy together with the 

 one design of preventing us from measuring or even detecting our 

 motion through the ether. It is still an open question whether one 

 force, the force of gravitation, has joined the conspiracy. Hitherto 

 gravitation has stood aloof from all the other interrelated phenomena 

 in majestic isolation. We have become almost reconciled to leaving 

 it outside every physical theory. A new model of the atom is put 

 forward which accounts for a whole host of abstruse and recently 

 discovered properties ; but it would l)e considered unfair to suggest 

 that it ought to account for the simple and universal property of 

 gravitation. Dare we think that gravitation has so far forgotten its 



Imaginary in the mathematical sense, i.e. involving V - 1. 



