58 TIME 



against that topsy-turvydom of past and future, of which 

 Einstein's theory is sometimes wrongfully accused. 



Expressed in the conventional way this limitation of 

 the speed of signalling to 299,796 kilometres a 

 second seems a rather arbitrary decree of Nature. We 

 almost feel it as a challenge to find something that goes 

 faster. But if we state it in the absolute form that 

 signalling is only possible along a track of temporal 

 relation and not along a track of spatial relation the 

 restriction seems rational. To violate it we have not 

 merely to find something which goes just 1 kilometre 

 per second better, but something which overleaps that 

 distinction of time and space — which, we are all con- 

 vinced, ought to be maintained in any sensible theory. 



Practical Applications. In these lectures I am concerned 

 more with the ideas of the new theories than with their 

 practical importance for the advancement of science. 

 But the drawback of dwelling solely on the underlying 

 conceptions is that it is likely to give the impression that 

 the new physics is very much u up in the air". That is 

 by no means true, and the relativity theory is used in 

 a businesslike way in the practical problems to which 

 it applies. I can only consider here quite elementary 

 problems which scarcely do justice to the power of the 

 new theory in advanced scientific research. Two 

 examples must suffice. 



1. It has often been suggested that the stars will be 

 retarded by the back-pressure of their own radiation. 

 The idea is that since the star is moving forward the 

 emitted radiation is rather heaped up in front of it and 

 thinned out behind. Since radiation exerts pressure the 

 pressure will be stronger on the front surface than on 

 the rear, Therefore there is a force retarding the star 



