Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light dying while traveling in space?

  1. Sep 14, 2010 #1
    I was wondering if light from distance stars actually get weaker the more they travel through space? I'm not really sure I understood the article I read, but I think they were using this explanation to explain why the stars of the universe do not light up the night (because we are surrounded by an infinite number of stars, which means night should be as bright as day).
  2. jcsd
  3. Sep 14, 2010 #2
    That is due to the fact that intensity of light is inversely proportional to the distance.
    u can imagine ur room increased to the size of the city but with the same bulb before.u can see the light u previously saw on the walls were much greater than now.so it is just the intensity being smaller.sun is nearer to earth,so its light has more intensity on earth but the stars are farther,so their intensity is much smaller on earth.
  4. Sep 14, 2010 #3
    Also, the extreme Doppler shift from faraway stars would make them invisible, even if their intensity didn't fall off as distance squared.
  5. Sep 14, 2010 #4
    100% correct.
  6. Sep 14, 2010 #5
    thank u for that.
  7. Sep 14, 2010 #6
    Thank you for your answer. But I do not really understand why the intensity diminishes as the distances increases (such as the walls of the gigantic room). Here is the definition I found of the intensity of light: the number of photons being emitted from a light source. If the intensity diminishes, this means that there are fewer photons traveling. Does this means photons just disperse more so it seems like there is less photons, or do they "die"??
  8. Sep 14, 2010 #7
    no. the same number of photons are spread over a larger area.
  9. Sep 14, 2010 #8
    Ok, so why does stars look like dots?? if the photons are spread over a large area, then we should just see a bunch a specs of light in the night sky, but obviously this is not the case. Why is it that the light of the stars we see are so concentrated? Is this because they are close to us?? Then is it true to what I say about distance stars?
  10. Sep 14, 2010 #9
    Think about a sphere. A light bulb emits a certain amount of light energy. which radiates away in all directions. if this light struck the inside surface of a large sphere the total energy hitting the inside surface would be the same as the energy emitted by the bulb no matter how big the sphere is. The surface area of a sphere is 4*pi*R^2 so a if the sphere were 1 meter in radius the surface area would be about 12.57 square meters. If it was a 100 watt bulb then the intensity of the light hitting the inside surface of the sphere would be 7.96 watts per square meter. If the sphere were 2 meters in radius then the inside surface area would be 50.27 square meters and the intensity would be. 1.99 watts per square meter. Your eye captures light falling on a certain surface area, the area of your pupil, the further away the source the smaller fraction of the total energy strikes this area. That's why distant lights seem not to be as bright.

    Now think about it the other way around. If you are looking at an object 1 meter wide and only 1 meter away then looking from the right side of the object to the left you would have to scan an angle of about 60 degrees. Someone with good peripheral vision can see about 180 degrees to this object would occupy about 1/3 of your visual field (side to side). If the object were 2 meters away then your left to right scan would only be about 30 degrees. The object would only occupy 1/6 of your visual field. That's why distant objects seem smaller.

    In short. Distant lights seem dimmer because their light is spread out. Distant objects seem smaller because they occupy a smaller part of your field of view.
  11. Sep 15, 2010 #10


    User Avatar
    Gold Member

    The article should be about http://en.wikipedia.org/wiki/Olbers%27_paradox" [Broken].
    And yes observations show that wavelength of light increases as distance to star increases so light gets weaker as longer wavelength of light means less energy.

    The paradox can not be resolved if you only take into account that light disperses with distance because number of stars increases by the same proportion as intensity of light from single object decreases. Of course we have to take scale where distribution of stars can be regarded as homogeneous.
    Last edited by a moderator: May 4, 2017
  12. Sep 18, 2010 #11
    the wavelength increase is not actual,it just seems so.And this is the Doppler's effect.I would like u to study Doppler's effect.:smile:
    Last edited by a moderator: May 4, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook