Olbers Paradox

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jamesbird18
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#1
Report Thread starter 7 years ago
#1
Can someone please explain this to me, i really don't get it.
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mik1a
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#2
Report 7 years ago
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If the universe is infinite and homogeneous (=same wherever you go) then you should have an infinite number of stars, because homogeneity would mean that the same density of stars must exist everywhere as exists in our immediate vicinity. If there is an infinite number of stars, then wherever you look, you'll be looking directly at one of them. Another way of putting it is there exists no line of sight through space that avoids all of the infinitely many stars.

So far so good.

The problem is we don't actually see infinitely many stars. We see tiny white dots and huge areas of darkness.

We can explain this away if the universe is not infinitely old, because some of the very distant stars might be so far away that their emitted light hasn't yet had enough time to travel all the way to our eyes. This is an argument based on the finite speed of light. So an infinitely large homogeneous universe is okay as long as it is young.

However, what people thought back then was that the universe had been around forever (infinitely old). Then we can't use the speed of light argument because it doesn't matter how slow light travels, if it's been travelling for infinitely long, it should have reached us by now.

Thus in an infinitely large, homogeneous, and infinitely old universe, we should see a star everywhere we look. The night sky should be stupidly bright. The paradox is that this is not what we see.



A good analogy is a forest. If the forest is small, you can see a few tree trunks and then see the clear land beyond it. If the forest is not homogeneous, you might see a dense patch of trees and then the spacing increases, so you can still see the clearing. However, if the forest is infinitely large and homogeneous, then no matter where you look, you're going to see a tree trunk some distance away.
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jamesbird18
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#3
Report Thread starter 7 years ago
#3
(Original post by mik1a)
If the universe is infinite and homogeneous (=same wherever you go) then you should have an infinite number of stars, because homogeneity would mean that the same density of stars must exist everywhere as exists in our immediate vicinity. If there is an infinite number of stars, then wherever you look, you'll be looking directly at one of them. Another way of putting it is there exists no line of sight through space that avoids all of the infinitely many stars.

So far so good.

The problem is we don't actually see infinitely many stars. We see tiny white dots and huge areas of darkness.

We can explain this away if the universe is not infinitely old, because some of the very distant stars might be so far away that their emitted light hasn't yet had enough time to travel all the way to our eyes. This is an argument based on the finite speed of light. So an infinitely large homogeneous universe is okay as long as it is young.

However, what people thought back then was that the universe had been around forever (infinitely old). Then we can't use the speed of light argument because it doesn't matter how slow light travels, if it's been travelling for infinitely long, it should have reached us by now.

Thus in an infinitely large, homogeneous, and infinitely old universe, we should see a star everywhere we look. The night sky should be stupidly bright. The paradox is that this is not what we see.



A good analogy is a forest. If the forest is small, you can see a few tree trunks and then see the clear land beyond it. If the forest is not homogeneous, you might see a dense patch of trees and then the spacing increases, so you can still see the clearing. However, if the forest is infinitely large and homogeneous, then no matter where you look, you're going to see a tree trunk some distance away.

Perfect answer, thanks!!
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