From: ram@zedat.fu-berlin.de   
      
   Luigi Fortunati  writes:   
   >If in one minute of the spaceship the time on Earth changes by 24 hours,   
   >it means that in that minute of the spaceship the Earth becomes a crazed   
   >top that makes a complete revolution on itself in one minute.   
   ...   
   >But since during the journey the terrestrial twin ages by only 48 hours   
   >(24 hours during the outward journey and 24 during the return journey),   
   >all the remaining aging (6 days and 6 almost instantaneous rotations!)   
   >should take place during the U-turn.   
   >Is this what you two think is happening?   
      
     From the traveler's perspective, it looks like this.   
     But an observer on Earth does not observe anything special.   
      
     And the traveler would /not/ observe a jump in the Earth's   
     rotation during his acceleration if he used a telescope to   
     observe the Earth, because this simultaneous moment on the   
     Earth only results after the traveler's calculations, /in which   
     he takes the effect of signal travel times into account/.   
      
     For example, if I see a flash coming from a lamp one light-year away,   
     I know that the flash there was not produced simultaneously with my   
     observation, but one year before it, while I will not be able to see   
     what is happening there simultaneously with me for another year.   
      
     If the traveler used a telescope to observe the Earth, he would   
     find that the Earth rotates slower in the first half of his   
     journey and faster in the second half (without a jump in the   
     middle). But because of the time it takes light to travel, he would   
     /not/ observe moments that are simultaneous for him in his frame.   
      
     Therefore, what is seen in the telescope should not be equated   
     with what is happening simultaneously!   
      
     One can understand the whole situation with special relativity   
     alone. But one can also wonder how the traveler interprets the   
     passage of time if the acceleration at the reversal point does   
     not happen instantaneously but takes a finite amount of time.   
      
     Albert Einstein addressed this in an article in 1918, "Dialog ueber   
     Einwaende gegen die Relativitaetstheorie" ("Dialogue on Objections to   
     the Theory of Relativity"). According to him, for the traveler during   
     his (constant) acceleration during the reversal, the acceleration   
     would be equivalent to a homogeneous gravitational field that   
     fills the entire space.   
      
     In this case, the traveler considers himself to be at rest,   
     since the propulsion of his rocket prevents him from falling   
     within this gravity field. The Earth and the spacecraft are at   
     completely different altitudes in this field (and the difference   
     is the greater the further away the spacecraft is from the Earth).   
     These different gravitational potentials explain to the traveller   
     why time passes faster on Earth during his acceleration.   
      
     In this sense, the Earth would rotate quite fast for the   
     traveler during his turn, but he would not see this in a   
     telescope, and nothing special would be noticed on Earth.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)