8 DOWNFALL OF CLASSICAL PHYSICS 



Thus we have two legs to stand on. Some will prefer to 

 trust the results because they seem to be well established 

 by experiment; others will be more easily persuaded by 

 the knowledge that the FitzGerald contraction is a 

 necessary consequence of the scheme of electromag- 

 netic laws universally accepted since the time of Max- 

 well. Both experiments and theories sometimes go 

 wrong; so it is just as well to have both alternatives. 



Consequences of the Contraction. This result alone, 

 although it may not quite lead you to the theory of rela- 

 tivity, ought to make you uneasy about classical physics. 

 The physicist when he wishes to measure a length — 

 and he cannot get far in any experiment without meas- 

 uring a length — takes a scale and turns it in the direc- 

 tion needed. It never occurred to him that in spite 

 of all precautions the scale would change length when 

 he did this; but unless the earth happens to be at rest 

 a change must occur. The constancy of a measur- 

 ing scale is the rock on which the whole structure of 

 physics has been reared; and that rock has crumbled 

 away. You may think that this assumption cannot have 

 betrayed the physicist very badly; the changes of length 

 cannot be serious or they would have been noticed. 

 Wait and see. 



Let us look at some of the consequences of the Fitz- 

 Gerald contraction. First take what may seem to be a 

 rather fantastic case. Imagine you are on a planet mov- 

 ing very fast indeed, say 161,000 miles a second. For 

 this speed the contraction is one-half. Any solid con- 

 tracts to half its original length when turned from across 

 to along the line of motion. A railway journey between 

 two towns which was 100 miles at noon is shortened to 

 50 miles at 6 p.m. when the planet has turned through 



