DEFINITIONS I37 



*A body in motion which is not influenced by any force 

 travels in a straight line and its velocity is invariable. There- 

 fore planets follow a curved trajectory because they are 

 continually deviated from their course by forces which act 

 upon them', Einstein simply answers: 'Planets follow their 

 orbits not because they are deviated from their course, but 

 because such is their natural course.' 



What, then, becomes of the straight-line motion? 'The 

 straight-line motion,' answers Einstein, 'is possible only in a 

 Euclidean space; we have never observed it.' We must then 

 admit that our universe is not Euclidean; it is a curved space- 

 time continuum. 



The important point which must be grasped here is that 

 this reasoning makes sense only if, as I explained above, the 

 intimate welding of time and space is conceived. If all idea 

 of motion and time could be suppressed, Euclidean space 

 could easily be conceived. But there exists an unbridgeable 

 chasm between this pure, abstract, static geometrical concept 

 and our universe which is essentially 'motion'. It is the chasm 

 which separates the pure idea from reality. All the obscurities 

 invoked, the revolt and the repugnance which have been 

 expressed on the subject of Relativity derive, I think, from 

 our old habit of separating time and space. The idea must 

 become well rooted in our minds that this is an unconscious 

 metaphysical reasoning, whereas Relativity brings us down 

 to earth, face to face with the real, tangible universe. It is 

 above all a practical theory. 



This does not mean that the theory itself is true in all its 

 details, nor that it will remain immutable. It never had this 

 pretension. It will evolve, be transformed and adapted to 

 new facts. This is the role of a hypothesis. But it has given 

 us a certain number of new concepts which have made it 

 possible to interpret with greater precision an important 

 number of phenomena. Above all, it has renewed our way of 

 thinking, cleared up many obscurities (has it not suppressed 

 that paradoxical and disturbing entity. Ether?) and steered the 

 science of our epoch in a fruitful direction. 



