From: jos.bergervoet@xs4all.nl   
      
   On 7/29/2017 1:10 PM, LuigiFortunati wrote:   
   > The traveling twin starts and goes back to Earth where he finds his   
   > 80-year-old brother aged as he is only 10 years old.   
   >   
   > The wristwatch on the twin traveler's wrist marks (of course) the   
   > 10-year time.   
   >   
   > The earth twin during the years of the trip has been flashing every 10   
   > years (his).   
   >   
   > The last lightning is emitted at the time of the return of the ship,   
   > when the Earth clock marks the 80-year time and the wrist watch on   
   > the twin traveler marks 10 years.   
   >   
   > Calculation (perhaps) impossible is this: at the ejection of the   
   > penultimate lightning (when the Earth clock marked 70 years) what time   
   > was the watch on the wrist of the traveling twin?   
      
   You assume constant speed except for the instantaneous   
   change at turn-around and at departure and arrival, I   
   assume?   
      
   And you want the answer for the simultaneous events   
   in the Earth's reference frame coordinates, I assume?   
   Then it is 8.75 years (if the watch shows, years..)   
      
   The answer is different if you ask for simultaneous   
   events in the traveling twin's reference frame. In   
   that case it is 5 years, it is in the moment when   
   the twin changes direction (very suddenly) and in   
   his frame of reference the Earth-bound clock moves   
   from 0.625 years to 79.375 years, and the Earth sends   
   7 flashes of light in one single moment. Actually the   
   twin changes his reference frame infinitely fast at   
   that point, which is rather unphysical.   
      
   You should better take a scenario with a smooth   
   reversal of direction at the turn-around. But that   
   wouldn't change the basic fact: the answer depends   
   on the reference frame if you ask "what was the   
   situation *on the ship* when a certain event happened   
   *on Earth*".   
      
   This is best analyzed by imagining two very long   
   trains, one moving with high velocity past the other.   
   Then there is a point of the first train adjacent to   
   a point of the other train for all the questions of   
   the kind you are asking here. As long as you *do not*   
   try to change direction of the moving train it is   
   relatively simple in that case.   
      
   --   
   Jos   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)