From: nicolaas.vroom@pandora.be   
      
   On Thursday, 21 May 2020 09:36:24 UTC+2, 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.   
      
   If one twin physical moves away from point O (and returns back to point O)   
   it is only his clock that will show a different clock count compared to   
   a reference clock which stayed at point O.   
   (if that is what actually is measured when he returns back to point O.).   
      
   > So, in the reference of the twin A, it is B goes, arrives at point +4 (4   
   > light-years away) and then return 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.   
      
   Reference frames are not important. What physical happens is important.   
      
   > 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!   
      
   Only in the case that both twins physical move away, it is possible that   
   the clock count of both clocks don't coincide with the reference clock   
   (count) at point O.   
      
   In that case what the clock at O shows is also important.   
      
   > 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?   
      
   In order to understand better, the exact details of the physical experiment   
   are most important to known and to predict the outcome and to compare   
   if the prediction is correct.   
   If the comparison between prediction and actual are wrong the error   
   can be both in the prediction (calculation) or in the experiment.   
      
   Nicolaas Vroom.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)