From: helbig@asclothestro.multivax.de   
      
   In article , Luigi Fortunati   
    writes:   
      
   > 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.   
      
   > 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?   
      
   This is, of course, the "twin paradox".  As you hint, special relativity   
   is sufficient to explain it.  (One might think that GR has something to   
   do with it, since acceleration is involved, but that is not the case.   
   Note that the time dilation depends on the length of the journey but not   
   on the amount of acceleration.)  There is a huge literature on this   
   topic, and it has certainly been answered better than is possible in a   
   newsgroup post.   
      
   The basic explanation is that the twin who accelerates moves through   
   space while the other doesn't.  While the acceleration is not the CAUSE   
   of the difference, it does allow one to say which twin really moves and   
   which doesn't.  In other words, the motion is NOT relative in this case.   
   (Why there is such a thing as absolute acceleration is, as far as I   
   know, an unsolved problem.  Some believe that Mach's Principle explains   
   it, i.e. acceleration is relative to the bulk of the matter in the   
   universe.  That implies that if there were no matter in the universe   
   other than the two twins, then the resolution of the "paradox" might be   
   different.)   
      
   [[Mod. note -- As the author notes, there is a huge literature on this   
   problem/paradox.  For anyone wanting to explore, Wikipedia provides a   
   nice introduction & overview:   
     https://en.wikipedia.org/wiki/Twin_paradox   
   -- jt]]   
      
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