246 CONTEMPORARY SCIENCE 



dictions made from Einstein's formulae have been inves- 

 tigated can much be said; and further, it must be seen 

 whether any other lines of argument will lead to the same 

 conclusions. But without waiting for all this there is at 

 least one difficulty which is apparent at this time. We 

 have discussed the laws of nature as independent in their 

 form of reference axes, a concept which appeals strongly 

 to our philosophy ; yet it is not at all clear, at first sight, 

 that we can be justified in our belief. We cannot imagine 

 any way by which we can become conscious of the trans- 

 lation of the earth in space ; but by means of gyroscopes 

 we can learn a great deal about its rotation on its axis. 

 We could locate the positions of its two poles, and by 

 watching a Foucault pendulum or a gyroscope we can ob- 

 tain a number which we interpret as the angular velocity 

 of rotation of axes fixed in the earth ; angular velocity 

 with reference to what? Where is the fundamental set 

 of axes? This is a real difficulty. It can be surmounted 

 in several ways. Einstein himself has outlined a method 

 which in the end amounts to assuming the existence on 

 the confines of space of vast quantities of matter, a propo- 

 sition which is not attractive. deSitter has suggested a 

 peculiar quality of the space to which we refer our space- 

 time coordinates. The consequences of this are most in- 

 teresting, but no decision can as yet be made as to the 

 justification of the hypothesis. In any case we can say 

 that the difficulty raised is not one that destroys the real 

 value of Einstein's work. 



In conclusion I wish to emphasize the fact, which should 

 be obvious, that Einstein has not attempted any explana- 

 tion of gravitation ; he has been occupied with the deduc- 

 tion of its laws. These laws, together with those of 

 electromagnetic phenomena, comprise our store of knowl- 

 edge. There is not the slightest indication of a mecha- 

 nism, meaning by that a picture in terms of our senses. 

 In fact what we have learned has been to realize that our 

 desire to use such mechanisms is futile. 



