From: julio@diegidio.name   
      
   On Tuesday, 25 July 2023 at 09:38:07 UTC+2, Luigi Fortunati wrote:   
   > Il giorno luned=C3=AC 24 luglio 2023 alle 18:52:50 UTC+2 Richard Livingston   
   ha scritto:   
      
   > > -During the turn-around, which we typically assume is almost   
   > > instantaneous, there is little change visible on earth.   
   >   
   > What are you saying? During the turn-around the spacecraft has to brake   
   > hard and then accelerate just as hard backwards and you say that in the   
   > images of his telescope, at this stage, there is little change visible on   
   > earth?   
      
   No, there is in fact no need for any of that: for the simplest form of twins   
   experiment just take SR in flat spacetime and compute the  proper length   
   of the two path segments, and there already you have it: one of the two   
   paths is *shorter* (in terms of proper time/distance) than the other, and   
   that's it.  And that experiment can even be really simulated, with *two*   
   rockets appropriately coordinated, one for each leg of the journey.  So,   
   overall, it is true that acceleration concerns and even more so gravitational   
   effects are just *not germane to the twin paradoxes*.   
      
   > No! There is *big* change visible on earth,   
      
   No, there is no change at all on Earth expect for effects occurring   
   after any light goes from the travelling twin back to the one on Earth.   
   OTOH, the change of direction for the travelling twin does effect an   
   instantaneous change (for him), which is rather related to *rotation*   
   re the (his own) inertial path.  There is a nice clip, by Brian Greene   
   IIRC, "the slices of bread", with the traveller going in a direction and   
   seeing a planet with its advanced technology, then the traveller inverts   
   direction and now sees the same planet in the middle ages: Greene   
   shows in pictures how the whole "jump in time" thing happens while   
   rotating (in space time), not earlier, not later...   
      
   You are totally grasping at straws, and necessarily so since you   
   have systematically disregarded all advice and corrections, all the   
   more so the methodological ones...   
      
   Julio   
      
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