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How far the light can be shine?

  1. Apr 11, 2008 #1
    light can be speed faster about 300,000 km/s,
    but how long the distance that light can be arrive?
     
  2. jcsd
  3. Apr 11, 2008 #2

    pam

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    You can see stars, can't you?
     
  4. Apr 11, 2008 #3

    dst

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    Any distance until it interacts with something.
     
  5. Apr 11, 2008 #4
    We've observed objects as distant as about 13 billion light years. So, we know of light that has traveled that far. (For reference, 13 billion light years is a little more than 1.2*10^26 meters. That's 120 trillion trillion meters, which is about 75 billion trillion miles.)
     
  6. Apr 11, 2008 #5
    How do the scientists conclude that the particular light we received is x years old?
     
  7. Apr 11, 2008 #6
    light year rings...

    i would imagine triangulation and knowing the speed of light would be an unrealistically simple way of doing it...
     
  8. Apr 11, 2008 #7

    dst

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  9. Apr 11, 2008 #8
    Inertia still applies for EM radition, so i belive it depends entirely upon the environment into which you are shining the radiation. Since the majority of space is nearly a perfect vaccum EM radiation will countinue along its original vector untill acted upon by an outside force ie... black hole, nebula, telescope ect. Someone please correct me if im wrong.
     
  10. Apr 11, 2008 #9

    russ_watters

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    Staff: Mentor

    Most of those methods are highly accurate, especially since stars tend to fit a very tight band of types.

    Note, though, that those methods are used for stars in our galaxy. For more distant stars/galaxies, they use redshift, type 1a supernovas, and cephid variable stars (for really nearby galaxies).
     
  11. Apr 11, 2008 #10

    russ_watters

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    Inertia is a property of mass, so it doesn't apply to light, but otherwise yes, it is similar to Newton's first law - if nothing gets in the way, it'll keep going essentially forever. The Hubble has taken pictures of objects 13 billion light years away.

    http://hubblesite.org/newscenter/archive/releases/1997/25
     
  12. Apr 12, 2008 #11

    dst

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    Oh right, well I wouldn't know :grumpy:
     
  13. Apr 12, 2008 #12
    they use the supernovae as a reference because of how much light it emits, right?
     
  14. Apr 16, 2008 #13
    And eyeballs: if a photon manages to travel anything like a billion trillion miles I'd rather catch it with my eye than a ccd.
     
  15. Apr 16, 2008 #14

    Andy Resnick

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    Indirect methods. One way is to measure the redshift- how far a particular emission line has been skewed. The age of the universe is determined, in part, by the redshifted background radiation (the 4 K noise).

    Another way, as russ mentioned, is using "standard candles". These are sources that we "know" emit a certain amount of light:

    http://csep10.phys.utk.edu/astr162/lect/cosmology/cosmicd.html

    Parallax measurements are good only for very close objects.
     
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