From: richalivingston@gmail.com   
      
   [[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.   
       -During the return trip, the traveling twin is encountering all   
        the light that has been leaving earth during the outbound trip.   
        As a result the traveling twin sees earth spinning rapidly through   
        the telescope, again due to the Doppler effect.   
       -When the traveling twin returns to earth, many years have passed,   
        just as SR calculates.   
   -For the traveling twin, what SR calculates for "now":   
        -During the outbound trip, the calculated "now" on earth advances   
         slowly due to time dilation of the Lorentz transform.  This slow   
         down in apparent time is not as great as what they see in the   
         telescope, however.   
        -During turn-around, the "now" calculated for earth advances   
         almost instantaneously many years.  This is not seen in the   
         telescope, however.  We do not see "now" at a distant object,   
         we only see that object on our past light cone.   
        -During the return trip, time on earth is calculated to advance   
         slowly per the Lorentz time dilation.  The telescope, however   
         shows earth time advancing rapidly.   
   -For the people on earth:   
        -During the outbound trip, the traveling twin does not appear   
         to age at all when viewed through the telescope.  The calculated   
         "now" however has the twin aging slower than "normal" but faster   
         than shown through the telescope.   
        -During turnaround, the traveling twin does not change at all.   
        -During the return trip, the traveling twin appears to age   
         rapidly due to the Doppler effect on the light from the twin.   
         This aging rate is faster than calculated for the "now" time   
         of the twin.   
        -When the traveling twin arrives back on earth, they have aged   
         as SR predicts and is younger than the earth twin.   
      
   Rich L.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)