NON-EMPTY SPACE 155 



The conception of frames of space and time, and of the 

 non-emptiness of the world described as energy, momen- 

 tum, etc., is bound up with the survey by gross ap- 

 pliances. When they can no longer be supported by 

 such a survey, the conceptions melt away into meaning- 

 lessness. In particular the interior of the atom could 

 not conceivably be explored by a gross survey. We 

 cannot put a clock or a scale into the interior of an atom. 

 It cannot be too strongly insisted that the terms dis- 

 tance, period of time, mass, energy, momentum, etc., 

 cannot be used in a description of an atom with the 

 same meanings that they have in our gross experience. 

 The atomic physicist who uses these terms must find 

 his own meanings for them — must state the appliances 

 which he requisitions when he imagines them to be 

 measured. It is sometimes supposed that (in addition 

 to electrical forces) there is a minute gravitational 

 attraction between an atomic nucleus and the satellite 

 electrons, obeying the same law as the gravitation 

 between the sun and its planets. The supposition seems 

 to me fantastic; but it is impossible to discuss it without 

 any indication as to how the region within the atom is 

 supposed to have been measured up. Apart from such 

 measuring up the electron goes as it pleases "like the 

 blessed gods". 



We have reached a point of great scientific and philo- 

 sophic interest. The ten principal coefficients of cur- 

 vature of the world are not strangers to us; they are 

 already familiar in scientific discussion under other 

 names (energy, momentum, stress). This is comparable 

 with a famous turning-point in the development of elec- 

 tromagnetic theory. The progress of the subject led to 

 the consideration of waves of electric and magnetic force 

 travelling through the aether; then it flashed upon 



