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laws. In the new development he assumes the equivalence of 

 a homogeneous field of gravitation and a system of reference 

 with uniform acceleration. The former is a region where the 

 observer is at rest, relative to a stellar body whose attraction 

 causes all free bodies to move towards it with an acceleration 

 uniform throughout the region considered. The latter is a 

 system in which the observer is at rest, but freed from the attrac- 

 tion of the star, system and observer moving with a uniformly 

 accelerated velocity relative to the star. Einstein's hypothesis is 

 that these two systems are physically identical, and that it will 

 be impossible to distinguish between them by the formulation of 

 the laws concerning phenomena as observed from each system. 

 This amounts in fact to a denial of the possibility of detecting 

 absolute acceleration, just as the relativity principle denies 

 the possibility of detecting absolute velocity. The equivalence 

 of the two systems is obvious enough, so long as we confine 

 ourselves to Newtonian Dynamics, just as a limited case of 

 relativity was in constant use even from the time of Galileo. 

 Thus, to take the illustration which has been frequently pressed 

 into service to assist the beginner in grasping Einstein's idea, 

 consider an observer situated in a closed lift, observer and lift 

 being free from the attraction of any gravitating matter. If the 

 lift were in uniform motion, there would be no pressure between 

 the lift floor and the observer's feet, in fact by a gentle spring 

 the observer could " float up " to the ceiling of the lift ; bodies 

 placed anywhere in the lift would remain there without support 

 — in short there would be no " up " or " down." If now 

 the motion of the lift was accelerated all the mechanical pheno- 

 mena which we associate with a field of gravitation would 

 supervene. If the acceleration is maintained in a direction 

 which is " upwards " (in the sense from feet to head), the lift 

 floor would exert a pressure on the observer's feet in proportion 

 to the magnitude of the acceleration ; bodies would " fall " 

 {i.e. move towards the lift floor) with an acceleration relative 

 to the lift equal in magnitude and opposite in direction to the 

 acceleration of the lift. Now Einstein's hypothesis amounts to 

 this : suppose the observer can observe phenomena through 

 windows in his lift and make precise measurements, then all 

 events, optical, electro-magnetic, etc., as well as mechanical, 

 will follow the same course for him as they do to the observer 

 stationed on the gravitating star. The first deduction from 



