On Thursday, May 21, 2020 at 2:36:24 AM UTC-5, Luigi Fortunati wrote:   
   > 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?   
      
   You are ignoring the effects of acceleration when one twin reverses   
   direction.  You can't ignore that, it is the key to understanding the   
   "paradox".   
      
   Rich L.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)