From: fortunati.luigi@gmail.com   
      
   Il giorno luned=C3=AC 24 luglio 2023 alle 09:06:24 UTC+2 Stefan Ram ha scri=   
   tto:   
   > >What I want to point out is that in (1) there is the=20   
   > >*dilation* of terrestrial time (which runs slower in the=20   
   > >spaceship frame) and in (3) there is the *contraction* of=20   
   > >terrestrial time (which runs faster in the spaceship frame).   
   > This is true from the point of view of the space traveler if=20   
   > he considers himself to be at rest. (But which traveler would=20   
   > seriously consider himself to be at rest?)=20   
      
   I have an answer and it seems extraordinarily reasonable to me.   
      
   The traveling twin can (rightly) be considered at rest if his   
   spaceship travels in space with the engine off and cannot be   
   considered at rest if the engines are running.   
      
   If this is the case (and it seems to me that this is the case),   
   during the outward and return journeys the traveling twin is at   
   rest and special relativity is usable.   
      
   Instead, during the reversal, the engine is running, the motion is   
   not inertial, the traveling twin is not at rest and special relativity   
   cannot be used because it does not apply in accelerated references.   
      
   So, if special relativity isn't usable, what theory can we use to   
   evaluate the behavior of Earth's time in the spaceship frame during   
   U-turn?   
      
   > However, if we don't look at this from a special coordinate system...   
      
   This goes beyond my question which concerns exclusively the traveler   
   twin's reference system.   
      
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