From: fortunati.luigi@gmail.com   
      
   Twins A and B are stationary at point O of space:   
   (-4)----(-3)----(-2)----(-1)----O----(+1)----(2+)----(+3)----(+ 4)   
      
   If a twin moves away at constant relativistic speed, there is no   
   difference between them, in the sense that twin A can safely say that it   
   is B that is moving away from him, just as twin B can say (just as   
   safely) which is A who is moving away.   
      
   So, in the reference of the twin A, it is B goes, arrives at point +4 (4   
   light-years away) and then returns to O.   
      
   Instead, in the reference of the twin B, it is he who remains in O while   
   the twin A goes, arrives at the point -4 and then returns.   
      
   If the speed corresponds to gamma=2, the travel the other twin (for BOTH   
   twins) lasts 16 years, 8 for the outward journey and 8 for the return.   
      
   Both will be 16 years old!   
      
   Where is the conceptual difference that should make the time of a twin   
   different from that of the other, if we are talking about Special   
   Relativity ONLY?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)