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Stargazing Working of a telescope ( help please)

  1. Jul 3, 2011 #1
    i just had an argument with a friend regarding the working of a telescope.Could you please help me out with it.

    ok the thing is that i say that the light of an object reaches the telescope then we r able to see that object which means that if we r lookin to an object which is 100 light years away might not be there now, it was there 100 light years ago.

    now my frend says that this is not the case. he is of the view that the modern telescopes actually covers the distance, like they show us what is happening right now, they give the real time images.

    Please help me out with this..please tell me which one is correct..
    thankyou very much
  2. jcsd
  3. Jul 3, 2011 #2


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    Your friend needs some guidance. It takes time for light to get here from distant objects, and the farther they are, the more time is involved. There are no real-time images available for objects at cosmological distances.
  4. Jul 3, 2011 #3
    It depends on your chosen frame of reference. In the conventional, earth-based frame, viewing an object 100 light-years away will show you what happened there 100 years ago.* In the incoming photons' frame of reference, no time has passed since emission, so the view is real-time.

    Since we can only measure the two-way speed of light (there and back again), it is quite possible within the Theory of Relativity to devise a consistent coordinate system in which the speed of light is infinity in one direction and 1/2 c in the other. So despite what others have said on this post, your friend isn't crazy, just different :biggrin:.

    *The earth-based frame is usually the most convenient conceptually and mathematically, which is why it is conventional.
  5. Jul 3, 2011 #4
  6. Jul 3, 2011 #5


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    Good catch. No.
  7. Jul 3, 2011 #6


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    No, his friend is incorrect. Plain and simple. He wasn't even asking about frames of reference, but the only plausible one here is the frame from Earth anyways. His friend doesn't understand that light takes time to travel and will not arrive quicker simply because you use a telescope.
  8. Jul 3, 2011 #7


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    The best I can come up with to defend his friend is that...

    Nope. Can't do it. You are correct; your friend is wrong*.

    When we see Jupiter eclipse its moons, we see it at least 37 minutes after it happened. This lag must be factored in when calculating ephemera or they will get the wrong answers.

    * However, I wonder if you need to listen to your friend more carefully. I wonder if there is more to his viewpoint than you're getting.
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