From: fortunati.luigi@gmail.com   
      
   Jos Bergervoet alle ore 09:00:51 del 30/07/2017 ha scritto:   
   >> 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.   
      
   Of course I had done my calculations before I started the discussion and   
   I find myself perfectly with all your numbers.   
      
   In the travel twin reference, at the end of the forward trip (when his   
   watch marks the 5-year time), the simultaneous earth time is 0.625 years   
   and therefore there is still no simultaneous emission with the Earth   
   twin flash.   
      
   But even on the return trip there is no simultaneousness with earth   
   events from 1 to 7 because just after the turn the time on Earth has   
   shrunk from 0.625 to 79.375 by skipping all events, including the "flash   
   7 emission" Which has already happened more than 9 years earlier.   
      
   The simultaneity of Earth events E1..E7 (land light emitted 1...7) does   
   not occur at the start and even when the return occurs, all occur during   
   the reversal.   
      
   It is there, in that short time, that the Earth's twin ages almost 79   
   years, while the watch and aging of the traveling twin are almost firm.   
      
   A landmark sequence of earth events lasting 60 years on Earth are   
   simultaneous events that take place in the short turnaround but without   
   the exact times of any simultaneity on the twin traveler clock.   
      
   --   
   Luigi Fortunati   
      
   Credere e' piu' facile che pensare   
   Believing is easier than thinking   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)