• Question: Why is the sky dark at night?(If light is reaching us from other suns surely the night would be light)

    Asked by mirza to Joel, Kristian, Tim, Venus, Zachary on 20 Nov 2013.
    • Photo: Tim Hollowood

      Tim Hollowood answered on 20 Nov 2013:

      Good question…yes it’s true that at night we have to light from billions of stars reaching us. But they are so far away that they don’t come close to matching our sun. In fact the intensity of light from an objects goes down like the square of the distance away and other stars are so so so far away…even the closest one Alpha Centauri is 4 light years away compared with the sun which is 8 light minutes away. So the sun is (4 x 365 x 24 x 60 / 8 )^2 = 7000000000 brighter! And that’s just the closest star.

    • Photo: Kristian Harder

      Kristian Harder answered on 20 Nov 2013:

      That is actually a very important question, the answer to which contributed to a major revolution in science! For a while, people assumed that the Universe would be of infinite size, and that it would have existed forever and in all eternity. But then indeed, no matter which direction you look, you should be looking at a star, and the entire sky would be bright.
      The fact that the sky is dark thus means the universe is either not infinite, or not eternal, or neither. Either there really are gaps between stars because the size of the universe and the number of objects in it is limited, or light from very distant objects had no time to reach us yet because the universe isn’t old enough. This was quite a dramatic thought at the time.
      We know now that both seem to be true. Our universe has only existed for a bit more than 13 billion years, and it seems to have finite size too.

    • Photo: Zachary Williamson

      Zachary Williamson answered on 21 Nov 2013:

      Hi mirza,

      You’re right about light from other stars reaching us, but the amount of light is absolutely miniscule compared to our Sun.

      There’s a great thought experiment that revolves around this though: if the universe is infinite in size, then there are an infinite amount of stars. Even if each star contributes a tiny amount of light, there are an infinite number of them! So the logical conclusion is that the night sky should be dazzlingly bright.

      The problem is solved because of the limits on the speed of light. Light from distant stars takes time to reach us. The universe is 12 billion years old. That means that we can’t see any star that’s more than 12 billion light-years away from us because its light hasn’t yet reached us. Even if the universe is infinite, we can only ever see a finite amount of it because of this constraint.

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