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Will light eventually fade away given enough distance?

  1. Jul 4, 2011 #1
    I was watching a physics lecture. The professor stated that there is no light in intergalactic space because it is so vast, no matter what you do, you cannot see the light of any stars. I know that light gets dimmer by a factor of 1/4πr^2 from the source. I thought in theory you can detect electromagnetic radiation from any distance, even visible light. Is this with all electromagnetic radiation of just light? I also know that the farther you go out, the lesser amount of photons will hit your detector. It seems that all electromagnetic radiation dims out as r goes to infinity.
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  3. Jul 4, 2011 #2
    This is obviously wrong as we can see light from other galaxies using telescopes. I think Andromeda can be even seen with the naked eye.
  4. Jul 4, 2011 #3
    Light appears dimmer becuase it's not focused, so you lose intensity, but you can still see it if you look hard enough. It's used as method for calculating the distance of stars.

    The question is ambiguous though. For really distant stuff we can't see individual star light becuase telescopes do not have the resolution, so all the stars blur into one blob of light.
    Last edited: Jul 4, 2011
  5. Jul 4, 2011 #4


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    The professor is either wrong, or he is talking in a context that refers to something specific. The latter is still hard to believe though. Photons do not lose energy except over extreme distances (millions upon millions of parsecs) due to the universe expanding. You could definitely see galaxies from intergalactic space unless you were in an enormous void, and even then a simple camera would easily be able to see things with just a second or two of exposure.
  6. Jul 4, 2011 #5
    He said that a typical place in the universe is completely dark. He said its so dark that if you were to look at a star that went supernova in front of you, you still won't be able to even see a glimmer of it.
  7. Jul 4, 2011 #6


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    I'm not sure about the effect of directly looking at sources and their attenuation over distances, but one thing to keep in mind is that in space, there is no medium to reflect off of. So light doesn't scatter like it does on Earth. We have a complex photon traffic pattern on Earth because of all the different scattering. In space, I imagine the photon traffic would be much reduced in a density comparison.
  8. Jul 4, 2011 #7
    If he really said this I am afraid he is a little confused. It seems (from what you say) that he thinks that darkness is some sort of intrinsic property, and the intergalactic space have a very high value (or amount?) of it.

    Darkness is simply the absence of light. If the environment is dark it makes it only easier to detect any light that happens to appear at a later moment. The darkness does not cancel light or anything.

    You can have a very dark room (in terms of visible light) quite easy here on Earth. Probably darker than the intergalactic space (again, in terms of visible light).
  9. Jul 5, 2011 #8
    There are stars between the stars. Why don't we see one bright light in the sky? I have heard it is because there is not enough light to fill the universe. I guess it could be explained like this: Two photons next to each other set out on their journey. Over millions of light years of travel, one just barely misses your eye on one side while the other just barely misses your eye on the other side.
  10. Jul 5, 2011 #9
    Yeah that's just weird. It's kind of like saying "the room was so empty that even if we filled it with people it would still be empty".
  11. Jul 5, 2011 #10
    I supose light would eventually red shift to the point that its wavelength would be longer then the diameter of the observable universe but the universe has not existed anywhere near long enough for that to happen yet.
  12. Jul 5, 2011 #11
    Great analogy.
  13. Jul 5, 2011 #12


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    Damn I'm glad I havent been taught by that professor's seriously misleading teaching.
    That is just so wrong!!!

  14. Jul 6, 2011 #13
    What he probably meant was that light can't be seen because it hasn't had the time to travel far enough to be observed. Since the universe has only been around for x blank-years (not sure how many) and since light can only go as fast as 3*10^8m/s, then light may only have traveled as far as x _blank-years *(conversion)*3810^8m/s = limit.
  15. Jul 6, 2011 #14


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    That is incorrect jsr9119. There is no point in the universe, that we know of, that is so far away from any light emitting sources that light hasn't had a chance to get there yet. In fact, the very existence of the Cosmic Microwave Background shows that the universe is filled with EM radiation left over from the Big Bang. There shouldn't be anywhere that this doesn't exist.
  16. Aug 2, 2011 #15
    I think he means to the naked eye. I asked a question very similar to this. If you are WAAAAAY out there in the voids between galactic clusters would it be pitch black to the naked eye. i think it would be..here on earth the andromeda galaxy is only 2 million light years away and you can barely see that with your naked eye! so figure if you're in a void between the clusters with nothing for 50 million light years in any direction I think it would be pretty damn dark!Imagine floating along in a space suit!!
  17. Aug 3, 2011 #16
    JLA727, that would be failure of the optic nerve. Not due to the lack of light.
    I can stand in Missouri and see stars that are untold light years away.
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