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Layman question about distance and speed of light

  1. Apr 21, 2012 #1
    Hello, this has been bugging me for a bit, so some clarification would be greatly appreciated. Say I'm located on a planet 30x10^8 m away. And assuming light to be 3x10^8 m/s for easy calculation, it would take light 10s to travel to Earth, where you'd be able to see me, as I was 10 seconds ago, via an incredible telescope. Is this true (regardless of redshift, which I'm still struggling to comprehend)?

    I apologize if this question has been asked before, but most of the questions raised are if light always travels at a constant speed, or if anything can travel faster than light. I'm not asking that, just wondering if my feeble theory holds, imagine a planet located 70 light years away with their own Hubble telescope. Would they be seeing us as we were 70 years ago, engaged in WWII? This is truly mind boggling!
  2. jcsd
  3. Apr 21, 2012 #2
    to answer you simply, yes. yes to the first paragraph, and yes they would see us as we were 70 years ago. its mind boggling but true.
  4. Apr 22, 2012 #3


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    It is a necessary condition to preserve causality. Without a 'speed limit' on conveyance of 'information' everything in the universe would happen at the same time. If you deeply consider this, it would result in logical paradoxes. Everything would influence its own history. In statistics, this is called a bootstrap paradox.
  5. Apr 22, 2012 #4


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    Why is it so mind-boggling? Think of the finite speed of sound. When you hear a crash of thunder, you are not hearing the lightning bolt happening 'now', you are hearing it a few seconds after it happened. When you are at a baseball stadium sitting a long way from home plate, you hear the crack of the bat a fraction of a second after the bat hits the ball. This is really no different. In the above scenarios, since light travels so much faster than sound, you can see what is happening more or less immediately, so you get information from the light that arrives much sooner than the information that arrives with the sound. But if you close your eyes, then you have a situation like astronomy.
  6. May 5, 2012 #5
    Yes, like when a bullet hits you and then you hear the gunshot. That was poignantly shown in the film Quigley Down Under where the protagonist kept picking his enemies off with his long-range rifle and they heard the report only after getting hit. That is to say they experienced the sound moments after it had already happened just like we visually experience the light approx four years after it has been emitted from the Alpha Centauri system and visually experience sunlight approx eight minutes after it is emitted from the sun toward earth. Light from the moon takes about 2.556 seconds. to reach us. So we are seeing these celestial bodies not as they are now but as they were.
    Last edited: May 5, 2012
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