12 DOWNFALL OF CLASSICAL PHYSICS 



could not be nil on both occasions. Thus the contraction 

 was demonstrated and its law of dependence on velocity 

 verified. But the actual amount of contraction on either 

 occasion was unknown, since the earth's true velocity 

 (as distinct from its orbital velocity with respect to the 

 sun) was unknown. It remains unknown because the 

 optical and electrical effects by which we might hope 

 to measure it are always compensated by the contraction. 

 I have said that the constancy of a measuring scale is 

 the rock on which the structure of physics has been 

 reared. The structure has also been supported by sup- 

 plementary props because optical and electrical devices 

 can often be used instead of material scales to ascertain 

 lengths and distances. But we find that all these are 

 united in a conspiracy not to give one another away. 

 The rock has crumbled and simultaneously all the other 

 supports have collapsed. 



Frames of Space. We can now return to the quarrel 

 between the nebular physicists and ourselves. One of us 

 has a large velocity and his scientific measurements are 

 seriously affected by the contraction of his scales. Each 

 has hitherto taken it for granted that it is the other 

 fellow who is making the mistake. We cannot settle 

 the dispute by appeal to experiment because in every 

 experiment the mistake introduces two errors which just 

 compensate one another. 



It is a curious sort of mistake which always carries 

 with it its own compensation. But remember that the 

 compensation only applies to phenomena actually ob- 

 served or capable of observation. The compensation 

 does not apply to the intermediate part of our deduc- 

 tion — that system of inference from observation which 

 forms the classical physical theory of the universe. 



