From: tjroberts137@sbcglobal.net   
      
   On 6/23/19 11:37 PM, Sylvia Else wrote:   
   > On 24/06/2019 5:15 am, rockbrentwood@gmail.com wrote:   
   >> Therefore, when it comes to the twin paradox, acceleration is not   
   >> ONLY the one thing that matters here, it is the ONLY thing that   
   >> matters or counts! Everything else is a red herring and is   
   >> therefore irrelevant.   
   >   
   > Yet if we perform an experiment that doesn't accelerate anything, but   
   > instead transfers clock readings between observers travelling in   
   > different directions, we still get a "twin paradox" style result.   
      
   Moreover, in General Relativity one can easily set up a situation in   
   which two twins, each with ZERO proper acceleration, have different   
   elapsed proper times between meetings: put one twin in circular orbit   
   around a mass, put the other in a highly elliptical orbit around the   
   same mass, arrange for their orbital periods to have a ratio that is a   
   rational number, and orient them so the orbits periodically intersect.   
      
   Moreover, those two twins can periodically exchange roles: arrange for   
   them to have two intersections per orbit of the elliptical twin, then   
   between one pair of successive meetings the elliptical twin will age   
   more, and between the other pair the circular twin will age more.   
      
   So I would not say "acceleration is the only thing that matters".   
      
   Indeed, the twin paradox is merely the observation that different paths   
   through spacetime can have different elapsed proper times between   
   meetings. So it is really the paths that matter, accelerated or not.   
      
   Tom Roberts   
      
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