百科知识视频之煎蛋小学堂01 夜空为什么是黑的?

Why is the sky dark at night?

夜空为什么是黑的?

You might think the answer is obvious - the sun isn't up! But the only reason the sky looks blue during the day is that sunlight scatters off of the atmosphere - if we didn’t have an atmosphere (like on the moon), the sky would always be dark, even when the sun is shining. So let’s rephrase the question - why is space dark?

你可能认为答案很简单——因为没有太阳嘛!但白天的天空之所以是蓝色的，完全因为大气层把阳光散射了。而如果没有大气层，就比如月球，那么天空就一直都是黑的，即便有太阳照射，那就让我们换种问法——为什么太空是黑色的?

I mean, space is full of stars - countless stars which are all about as bright as the sun, and in an infinite eternal universe, no matter what direction you picked, if you looked far enough in that direction, you would see a star or galaxy. So the whole sky should be as bright as the sun, night and day!

太空里充满了各种各样的恒星，每一颗的亮度都和太阳一样，而在无穷无尽的宇宙中，不管你选择任何一个方向，只要你走得足够远，你都能遇到一颗恒星，或者一个星系，于是天空就应该是亮如白昼的，不论白天还是黑夜。

And since it's not, does the darkness of the night sky mean that there's some distance away from us when stars and galaxies just… stop? A boundary between something and nothing?

但是事实却并非如此，那么，黑色的夜空是否告诉了我们，在一定的距离以外，就没有恒星或者星系了吗?是不是存在一个有星星和没有星星的边界?

An "edge" to the universe?

宇宙有边际吗?

Not exactly - all of our evidence seems to indicate that space has no edge. But the universe itself does - not a spatial edge, but a temporal one: as far as we know, the universe had a beginning. Or at least, a time about 13.7 billion years ago when the universe was so small and crumpled-up with itself that our standard notion of space and time breaks down.

并非如此。几乎所有的现有证据都表明，宇宙空间是没有边界的。但宇宙本身却有边界——并非是空间上的边界，而是时间上的。 宇宙是有一个起源的，至少137亿年前的宇宙是很小，高度压缩的。以至于我们现在对时间和空间的概念都不适用了。

And since only a finite amount of time has passed since this so-called beginning, that means that some of the stars necessary to fill up the "brightness in every direction" are so far away that light from them plain hasn't had time to reach us yet… it's as if the universe were a big thunderstorm and we're still waiting to hear the thunder from the really distant stars.

而从这个所谓的起源开始，仅仅经过了有限的时间，因此要造就一个明亮的夜空所需要的有些恒星距离我们实在太远，他们发出的光亮还没有到达地球呢。如果把宇宙比作一个巨大的雷阵雨，那么我们还在等待着那些遥远恒星的雷声，它们还没达到地球呢。

But wait, it’s better than that - since light takes time to travel across the universe, when we point our telescopes at something really far away, we're actually seeing that part of universe as it was when the light was emitted. So when we look at 13.5 billion-year-old light, it's not that we don't see stars just because light from them hasn't gotten to us yet - we don't see any stars because we’re getting a peek at the universe before any

stars had formed! A star-less universe! Now that sounds to me like a pretty good reason why we look up and see a dark night sky.

但是稍等，似乎还有更好的解释，既然光穿越宇宙需要时间。当我们把望远镜对着一些很遥远的星星时，我们看到的宇宙，其实还是在光发出时候的样子，因此当我们看到135亿年前发出的光，并不是由于恒星本身不存在，而只是它发出的光还没有达到地球而已。我们看不到任何星星，那是因为我们窥探到的宇宙，那些星星都还没有形成呢。一个没有星星的宇宙!这看似是一个很好的解释，解释了为什么夜空是黑色的。

But… it’s not. I mean, it is true that we can find points in the sky where there aren’t any stars by looking past the earliest stars and thus farther back in time. But even when we point our telescopes past the earliest stars, we still see light. Not starlight, but the light left over from the big bang. And we detect this “cosmic background radiation” coming more or less evenly from all directions, forming a background beyond the stars.

但是当我们把望远镜对着这些古老的星星时，我们却的确能看到光，不是星光而是大爆炸时留下的光，并且我们可以观察到宇宙的背景辐射。从各种方向几乎完全均匀地发射，形成了所有星星的背景。

So, I guess the night sky ISN’T actually dark to begin with. Right... so if our telescopes tell us that the night sky isn’t dark, then why does it look dark?

因此我觉得，夜空从一开始就不是黑的，而既然望远镜告诉我们，夜空不是黑的，那它为什么看上去是黑的呢?

Here’s a clue to the real answer: when the Hubble telescope photographed the distant stars of the astoundingly beautiful Hubble extreme deep field, it took the picture using an infrared camera. Why? Well, distant stars and galaxies are moving away from us because the universe is expanding. So the same way a record slowing down lowers the pitch of my voice, the doppler effect causes stars moving away from us to become redder, and the farther away they are, the faster they move away from us and the redder they become, until they become… infrared. And then we can't see them any more. At least not with our human eyes - and that’s why the night sky appears dark!

这里有个提示：当哈勃望远镜拍下那些最古老的恒星时，当它拍下那些极其壮美的极深场图片时，它使用的是红外线照相机，为什么呢?我们知道，星星都在向远离我们的方向移动着。因为宇宙是在膨胀的。就如同如果慢放，我的声音就会变低沉，多普勒效应使得那些远离我们的恒星变得更红离我们越远的恒星，移动速度越快，因此也就越红。知道它们发出的光都变成了红外线，因此我们看不见它们。至少用肉眼是看不到的，因此夜空是黑的!

In summary: If we lived in an infinite, unchanging universe, the entire sky would be as bright as the sun. But the sky is dark at night, both because the universe had a beginning so there aren't stars in every direction and more importantly because the light from super distant stars (and the even-more-distant cosmic background radiation) gets red-shifted away from the visible spectrum by the expansion of the universe, so we just plain can't see it.

总结一下，如果我们生活在一个无限的，不变的宇宙里面，那么夜空就一定是完全亮的。但夜空是黑的，因此我们推断，宇宙有一个起源。所以并不是所有方向上都有星光。而更重要的是，那些来自遥远恒星——甚至更遥远的宇宙的背景辐射，由于宇宙的膨胀，都在可见光谱上发生了红移，因此我们看不见这些光。

Finally! We’ve shed some light on why the night sky is dark. And why it isn’t.

至此，我们简单解释了为何夜空看上去是黑的，但它却其实却并不是黑的。喵!