From: sylvia@not.at.this.address   
      
   On 25/07/2017 5:06 PM, LuigiFortunati wrote:   
   > Sylvia Else alle ore 06:27:39 del 24/07/2017 ha scritto:   
   >>> Before the departure, the two brothers agree that from the Earth,   
   >>> every 10 years, the earth twin would send a flash to the spaceship.   
   >>>   
   >>>     The traveling twin starts and half of his trip (after 5 years of his)   
   >>> arrives next to the distant star around which he will turn to go back.   
   >>>        For him, due to temporal dilation, on Earth, where time runs 8 times   
   >>> slower, it has not even been a year and therefore it will have to pass   
   >>> more than 9 earthy years before the first lightning starts.   
   >>>        Then the spaceship rotates around the star and prepares for the last   
   >>> 5 years of the return trip.   
   >>>   
   >>>     If just before the turnaround, they were missing (on Earth) more than   
   >>> 9 years after the first flash started, how many years are missing now   
   >>> that the return has just started?   
   >>>   
   >>   
   >> I don't know what you mean by "missing". The travelling twin will see   
   >> all 8 flashes (the last being at the moment he arrives back home) on the   
   >> journey back. None get lost.   
   >>   
   >> Sylvia.   
   >   
   >   The traveling twin can see the first flash only if it is already "started".   
      
   While the travelling twin is away from Earth, 80 years elapse there, so   
   the first flash is certainly emitted before the twin gets back, and will   
   inevitably reach him at some point in time. The travelling twin will   
   have reversed his direction of travel before then, so he'll encounter   
   the first flash on his way back.   
      
   >   
   >   Well, in the reference of the traveling twin, when "part" the first flash?   
   >   
   >   I repeat: "Part" does not "arrival".   
   >   
      
   Sylvia.   
      
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