6 DOWNFALL OF CLASSICAL PHYSICS 



rod can be altered merely by pointing it different ways. 

 You expect them to remain unchanged. But which rod 

 are you thinking of? (You remember my two tables.) 

 If you are thinking of continuous substance, extending in 

 space because it is the nature of substance to occupy 

 space, then there seems to be no valid cause for a change 

 of dimensions. But the scientific rod is a swarm of 

 electrical particles rushing about and widely separated 

 from one another. The marvel is that such a swarm 

 should tend to preserve any definite extension. The 

 particles, however, keep a certain average spacing so 

 that the whole volume remains practically steady; they 

 exert electrical forces on one another, and the volume 

 which they fill corresponds to a balance between the 

 forces drawing them together and the diverse motions 

 tending to spread them apart. When the rod is set in 

 motion these electrical forces change. Electricity in 

 motion constitutes an electric current. But electric 

 currents give rise to forces of a different type from those 

 due to electricity at rest, viz. magnetic forces. More- 

 over these forces arising from the motion of electric 

 charges will naturally be of different intensity in the 

 directions along and across the line of motion. 



By setting in motion the rod with all the little electric 

 charges contained in it we introduce new magnetic forces 

 between the particles. Clearly the original balance is 

 upset, and the average spacing between the particles 

 must alter until a new balance is found. And so the 

 extension of the swarm of particles — the length of the 

 rod — alters. 



There is really nothing mysterious about the Fitz- 

 Gerald contraction. It would be an unnatural property 

 of a rod pictured in the old way as continuous substance 

 occupying space in virtue of its substantiality; but it is 



