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Can we see the earth in the past ?

  1. Feb 10, 2010 #1
    Hi everyone, I am not very knowledgeable in cosmology but I hope you would be patient enough to clarify this issue for me.

    I understand that nothing can move through space with speeds higher than the speed of light, however space itself can expand with higher speed. Now I have this, probably silly, idea that i am asking clarification for. If we assume that the current expansion of the universe exceeds the speed of light, that will mean that we are moving from our current place faster than light, right ? If so, then if for any reason the expansion speed decelerates so that light from our previous place reaches us, would we be able to look at the earth in the past ?

    In other words, would the expansion of the universe mean that we are moving relative to any observable frame of reference ? or are our coordinates fixed during this expansion (neglecting the movement of earth through space) ?
  2. jcsd
  3. Feb 10, 2010 #2


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    Expansion of the universe does not imply we will catch up to the light already emitted from earth: even if the expansion turns to contraction, that light will always be moving away from us.
  4. Feb 10, 2010 #3
    If I understand you correctly, then this will mean that the expansion of the universe does not result in any movement from our previous location (relative to any sensible reference frame) so that we will always look stationary to the emitted beam of light. am I getting this correctly ?
  5. Feb 10, 2010 #4


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    Staff: Mentor

  6. Feb 10, 2010 #5
  7. Feb 10, 2010 #6


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    Wait. That's it??

    The OP asks a question, it gets answered, and the OP is enlightened???

    There's got to be a trick here. If this is allowed to continue, PF may just explode.
  8. Feb 10, 2010 #7
    LOL. I can see why you get the best humor award :)

    The thing is, the answer showed me clearly that I had the wrong understanding of the universe expansion, which I already felt. That's why I was "enlightened" so quickly.

    Don't worry, that won't happen again :)
  9. Feb 17, 2010 #8
    You would be absolutely amazed how rarely that happens here.
  10. Feb 17, 2010 #9


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    Ya know, if you had a really big mirror way out there, oriented just right, with a little concave parabolic curvature, you really COULD see Earth in the past.

    Ya think it would make us look fat?
  11. Feb 17, 2010 #10


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    It would take a very, very, very long time to view our 'reflection'.
  12. Feb 17, 2010 #11
    Indeed, if we make such a mirror, let's say, in Andromeda to see the earth two million years ago, we will have to wait another two million years to see our reflection.
    Also if we make it much closer so that we don't wait so much, let's say ten light years away from us, we will only see the earth ten years ago. Which is not interesting at all !!
  13. Feb 20, 2010 #12
    Okay, so we are moving at (faster than light) that means that we will never see ourselves because there is no force to slow us down?
  14. Feb 21, 2010 #13
    Well - it may not be necessary to make a mirror and place it in space - if we assume our future technology might be able to collect and process scattered photos reflected from an earthlike planet with a large water reflecting surface. If anyone is interested in investing in the development I would be willing to sell a 90% interst in the project for about 20 cents.
  15. Feb 26, 2010 #14
    Here is a thought that I had similar to yours. If the Earth moves through space-time does it leave a wake behind it? If it does can an instrument detect that modulated space-time and reproduce whatever modulated it?
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