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  • Hello. Welcome back to the Cosmic Classroom

  • We'll now talk about Olbers' Paradox

  • which is the question

  • Why is the sky dark at night? Have you ever wondered why is the sky dark at night?

  • So, you probably never stopped to think about why is the sky dark at night

  • and if you did you probably think

  • let me guess

  • Well the stars are so so far away that

  • by the time the light gets here so it's a little light that it doesn't amount to anything.

  • Right? But if you really take a little bit more time to think about it

  • as Olbers did,

  • you realize that even though there is very little light coming from the

  • stars that are very far away,

  • if the universe is infinite. Right?

  • Let's think for a minute that the universe is infinite.

  • There's so many stars out there

  • that the sky should be as bright as the sun

  • and you can't convince yourself without doing the math, really,

  • but it's really not that not that hard to even understand the math. You just think about

  • we're here on Earth

  • at the center of an infinite universe,

  • a universe filled with stars

  • and there are fewer stars close to us

  • and then there are stars further away from us.

  • So even though it's true

  • that

  • you know, those stars here that are close will appear to us brighter,

  • we'll get more light from them

  • There's so many more further from us

  • that it turns out that

  • the amount, the light, the amount of light that we receive from each shell

  • around us is the same.

  • We can do this by remembering that

  • the luminousity,

  • the brightness decreases is a function of one over R square,

  • if you like math and that the volume,

  • the volume will increase as a function of one over R cubed

  • so you have the same amount of light

  • that comes from the inner shell that comes from an outside shell so universe should

  • Ooh? What happened to my...?

  • Hold on.

  • There. The universe should be as bright

  • the surface of a star

  • because, let me go back and

  • show you that again. See if I can show you that again.

  • Hang in there. It just goes a little bit too fast, so let me show you.

  • So what this is gonna show is

  • different stars populating this piece of sky. Alright?

  • The first one is one really

  • close to you so it is really bright

  • and then they are fainter, but then there are more of them.

  • So this is what happens.

  • The sky becomes

  • as bright as the surface of the Sun.

  • Alright? So. Well.

  • So Olbers realized that

  • and wondering why is the sky,

  • the sky dark?

  • So, there are a few options for why the sky is dark at night

  • First of all, maybe the universe is not infinite. Maybe the universe is finite.

  • Or are the universe is too young

  • and we haven't been able to receive the light from most of the

  • stars in the universe yet.

  • Or the universe is expanding, which we know it is.

  • So the lights get red-shifted

  • so instead of receiving, you know, light

  • like the light you see on the surface of the Sun. You see redder and redder and redder light.

  • Or there's too much dust absorbing the light.

  • Alright? So...

  • The simple fact that the sky is dark at night

  • tells you a lot. Right?

  • In the case of our universe, we don't know whether the universe is finite

  • or not.

  • But our universe is very young.

  • So... young enough that we cannot see

  • enough

  • stars to fill it up. Alright.

  • And on top of that the universe is expanding, so the light that we receive from the shells

  • that further away from us

  • are red-shifted. You know, so are redder and redder and redder.

  • So, I think it's pretty cool that the pure fact that the sky is dark at night

  • tell us, tells us that our universe

  • can not be infinitely old and infinite.

  • Or the sky would be bright at

  • night.

  • That's it. I hope it helped

Hello. Welcome back to the Cosmic Classroom

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B1 中級

奧伯斯悖論 (Olbers' Paradox)

  • 511 29
    QAM Chen 發佈於 2021 年 01 月 14 日
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