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Can you see the back of your head

  1. May 27, 2009 #1
    if you stood on a mountain would you be able to look straight ahead with a powerful telescope and see the back of your head? or would you just look out into space?
     
  2. jcsd
  3. May 27, 2009 #2
    I just use a mirror.
     
  4. May 27, 2009 #3

    russ_watters

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    You would just look out into space - light doesn't curve around the earth like that.
     
  5. May 27, 2009 #4
    Or two?
     
  6. May 27, 2009 #5

    Borek

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    Depends on where the mountain is. Assuming your eyes lie on the events horizon you should be able to see your back.
     
  7. May 27, 2009 #6

    sylas

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    Quite so... except that you don't need to be quite that close.

    Orbits around a black hole are ... different. The "photon sphere" is the distance from a point mass at which light moving tangentially will follow a perfect circle around the mass. For a non-rotating black hole, this is 1.5 times the Schwartzchild radius (event horizon).

    Cheers -- sylas
     
    Last edited: May 27, 2009
  8. May 27, 2009 #7

    George Jones

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    This doesn't work, as the light will just fall into the black hole. You have to be at the photon sphere, which is a bit farther from the black hole. See

    https://www.physicsforums.com/showthread.php?t=246751.

    Maybe the original poster is thinking about closed universes.

    [edit]Didn't notice that sylas posted while I was searching.[/edit]
     
  9. May 27, 2009 #8

    atyy

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  10. May 27, 2009 #9

    Borek

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    Photon sphere... one learns all his life and dies stupid :wink:
     
  11. May 27, 2009 #10

    vanesch

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    Especially if you see your back :rofl:
     
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