From: sylvia@not.at.this.address   
      
   On 29/07/2017 9:10 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?   
   >   
      
   This not answerable as it stands, because you have not specified which   
   frame (Earth or travelling twin) is to be used. Indeed, other observers   
   moving at different relative speeds will get different answers. The   
   problem here is that you're talking about events (flash and time on   
   watch) that are separated in space.   
      
   You might prefer a question that has unique answer, such as what time   
   will the travelling twin's watch show when he sees the flash?   
      
   Sylvia.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)