MATTER IN MOTION 



95 



This law of Galileo may be readily pictured 

 in our space-time map. Let A (Figure 17) be a 

 spot on the surface of the earth, and B and C 

 two bodies immediately above it. (Since we have 

 allowed extension 



m 



B'C 



Figure 17 



time to usurp the ver- 

 tical direction, we shall 

 have to twist our heads 

 a bit in order to visual- 

 ize B and C directly 

 over A.) The spot A 

 traces the time-space 

 path AA', and the two 

 bodies the paths BB' 

 and CC. The law of Galileo now states that no 

 matter what bodies are represented by B and C, 

 their space-time paths, if they start together 

 will remain together throughout. The curvature 

 of these paths represents the acceleration of 

 falling bodies. 



Galileo did not associate this phenomenon of 

 falling bodies with the motion of the planets, 

 nor did Kepler, whose stupendous researches led 

 to as complete a knowledge of orbital motion 

 as anyone possessed before the present century 

 began, and who attributed planetary motion as 

 well as terrestrial tides to an attracting force. 



