From: tjroberts137@sbcglobal.net   
      
   On 8/6/17 8/6/17   5:13 AM, LuigiFortunati wrote:   
   > The "surprising causal effect" (real not apparent) [...]   
      
   This is NOT "real" in the sense that it is part of a valid model   
   of the world.  Causality is not involved, as at the instant the   
   traveling twin measures (or computes) the home twin's age they are   
   separated by a spacelike interval.   
      
   Consider how the traveling twin would measure the home twin's age:   
   there would have to be an assistant at rest in the traveling twin's   
   inertial frame pre- positioned to be co-located with the home twin   
   at the instant of the measurement, who observes the home twin's   
   clock; this assistant would then send the measurement data to the   
   traveling twin. Do this immediately before and immediately after   
   the traveling twin turns around, and the assistants are located and   
   moving very differently -- it's no surprise that their measurements   
   are very different, as the assistants' clocks are synchronized very   
   differently with the traveling twin's clock. It is, of course, that   
   change in clock synchronization that is the underlying effect.   
      
   The home twin, of course, is completely unaffected and notices   
   nothing (except the traveling twin's assistants, who pass by in   
   opposite directions many years apart).   
      
   The whole notion of "what does the traveling twin measure" is highly   
   artificial, requiring outrageously impractical assistants; Fontenot's   
   "CADO frame" is merely a way to compute the highly artificial value   
   that such assistants would measure.  As "all physics is local"   
   [Einstein and others], this is not part of any valid model of the   
   world; it is a measurement over a spacelike interval. The aspect   
   of this that proves its artificiality is the fact that it is   
   coordinate dependent -- coordinates are arbitrary human constructs   
   which Nature does not use, and no natural phenomenon can possibly   
   depend on such an arbitrary human choice....   
      
   What is real (i.e. unambiguous) is the comparison of the twins'   
   ages when they reunite. That is both independent of coordinates and   
   very non-local.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)