From: tjroberts137@sbcglobal.net   
      
   On 5/21/20 2:36 AM, Luigi Fortunati wrote:   
   > [...]   
   > 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?   
      
   The difference between the twins comes from the simplicity that applies   
   only to inertial frames.   
      
   If twin A remains at rest in an inertial frame, it is easy to calculate   
   the age (elapsed proper time) of each twin: simply integrate   
      
   	T = \integral sqrt(1-v^2/c^2) dt   
      
   where T is the elapsed proper time, the integral is taken over the path   
   of the twin relative to A's rest frame, v is the speed of the twin   
   relative to that frame (as a function of t), and t is the time   
   coordinate of the frame. Note that neither position nor acceleration   
   appear in this equation; all that matters is the speed of the twin   
   relative to the inertial frame being used to calculate.   
      
   For twin A, v = 0, giving an age that is simply the frame's total   
   coordinate time of the scenario.   
      
   For twin B, 0