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The Galaxy Paradox 2

  1. Apr 18, 2006 #1
    Some time ago i post this topic, but i think my explanation was not very clear , i hope now could be better understood

    My new argument is this: suppose that much billions years ago existed in what now is our Solar System an alien civilization, suppose they have developed the videorecording technique, then they attach it to a telescope and point to a galaxy that i call PGalaxy that was just forming, and they let the video recorder run for billion years till our time
    (One important point here is how they measure the time, it could be, for example, by heart beeps, or any other oscillatory phenomena)

    Now in our present time theres an astronomer that is photographing with his telescope the PGalaxy, and he deduct from measuring t=x/c , distance versus light velocity , the age of the Galaxy in the photo he is taking
    (note the diference in the measuring of time, because aliens dont need to know distance to the galaxy, nor the lights velocity)

    Then the contradiction arises if you ask the Alien how was the appearance of the PGalaxy billions of years ago, then he surely take his videorecorder
    and run it backwards until the first time he setup his recorder and pointed to the Galaxy.
    But when you ask the astronomer the same thing , he simply answer: just take a look at my telescope and see.

    Please can give a hint about this, or how Relativity deals with this kind of problems?
  2. jcsd
  3. Apr 18, 2006 #2


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    Staff: Mentor

    That's called relativity of sumultenaity. The astronomer most certainly knows about the signal delay and will make sure the question and answer are unambiguous about who'se time they are talking about and if necessary, explain it to the alien historian, who may or may not know about it.
  4. Apr 25, 2006 #3
    no paradox with length contraction?

    suposse that the galaxy is moving at velocity c/2 from us and that certain event happens T years after galaxy birth (a supernova explosion , for example)

    if the explosion is being seeing right now, then i want to know how the Alien and astronomer calculate the age of this explosion

    i think the Alien takes into account only the time dilation in the galaxy and making the calculations conclude that the explosion ocurs

    {2/(sqrt 3)*T} - T years ago (in his reference frame , the same astronomer's frame )

    the astronomer calculates first the actual distance to the galaxy and then divide it by c

    { (c/2)T } + { (c/2) [(2/(sqrt3) *T - T] } and dividing by c

    T/(sqrt3) not = as 2/(sqrt3)*T - T (Alien time)

    but i think that introducing the length contraction the times should be the same, im right?
    but is this a real "length contraction" or is better that the galaxy grow in mass and is not really moving at c/2?
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