From: fortunati.luigi@gmail.com   
      
   Il giorno luned=C3=AC 24 luglio 2023 alle 18:52:50 UTC+2 Richard Livingston ha   
   scritto:   
   > [[Mod. note -- Please limit your text to fit within 80 columns,   
   > preferably around 70, so that readers don't have to scroll horizontally   
   > to read each line. I have manually reformatted this article. -- jt]]   
   >   
   > On Sunday, July 23, 2023 at 4:59:57=E2=80=AFAM UTC-5, Luigi Fortunati wrote:   
   > > Il giorno mercoled=C3=AC 19 luglio 2023 alle 09:07:26 UTC+2   
   > > Stefan Ram ha scritto:   
   > ...   
   > > What I want to point out is that in (1) there is the *dilation* of   
   > > terrestrial time   
   > >(which runs slower in the spaceship frame) and in (3) there is the   
   > > *contraction   
   > >* of terrestrial time (which runs faster in the spaceship frame).   
   > I think one issue that causes confusion is the difference between   
   > what each observer SEES (with their eyes via light) and what SR says   
   > is happening "now" for each observer.   
   > -For the traveling twin, what he SEES:   
   > -During outbound trip using a telescope to observe the earth,   
   > time on earth appears to go very slow, even slower than SR says.   
   > This is because in addition to the time dilation of SR there is   
   > also the Doppler shift due to the twin receding rapidly at nearly   
   > the speed of light.   
   > -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 *big* change visible on earth, because the light rays from   
   the Earth that previously had a hard time reaching the telescope (and   
   lagged behind), now come all at it at once.   
      
   Luigi.   
      
   [[Mod. note -- By definition, the travelling twin looking through their   
   telescope at Earth sees the event where Earth's worldline intersects the   
   travelling twin's backwards light cone.  The travelling twin's backwards   
   light cone depends only on the event at its apex, NOT on the travelling   
   twin's velocity.  Therefore, the intersection of that light cone with   
   Earth's worldline (i.e., the event that the travelling twin sees when   
   looking at Earth through their telescope) depends on the travelling   
   twin's position but NOT on the travelling twin's velocity.  Therefore,   
   the travelling twin sees little visible change when looking at Earth   
   during their (the travelling twin's) turnaround.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)