Why is the sky black?

Much of the knowledge and understanding about the cosmos prove time and again that humanity’s cognitive capabilities are extremely limited. Some of the most elementary things that we see and hear are basically a testament to our mental frailty. To start, the term Big Bang was a derogatory term given to the idea that the universe started out as a humongous explosion. Einstein’s Theory of Relativity was a name given to his theory, which had a boring name originally. He did not get his Nobel Peace Prize for the General or the Special Theory, but rather for his work that is in conflict with his relativity theory.

I was once talking to a friend of mine while driving somewhere in northern Texas. It was a bright sunny day with a very clear blue sky. He is an engineer and I just wanted to indulge in some science discussion. I asked him why the colour of the sky was blue. He quickly said because of the oceans. I asked him to explain his answer. He said that every time the earth is shown from space, we see that it is blue, which is because of the oceans. He was right about this. Then he said that the oceans being blue reflect on the sky making the sky look blue. In his mind, the sky was some kind of a mirror. Well, in short, he was totally wrong.

The sky is blue during the daytime because there is a phenomenon called refraction. I remember reading it in class 8 but I don’t trust the caliber of Pakistani schools nowadays. I am not sure if students of class 8, or as they now call it 8^th grade, have any clue about any of this. Refraction is basically the phenomenon of the splitting of the light rays into its 7 colours. When sunlight reaches the earth’s atmosphere, it is scattered in all directions. The blue is scattered more than all other colour components of the light because blue has a short wavelength. And so we see a blue sky.

But the night sky being black is more mysterious and thought-provoking than X-Files. There are hundreds of billions of galaxies, each with hundreds of billions of stars like the sun and bigger than the sun. And that is only part of the observable universe. With so many bright stars and galaxies, why does space appear black? It should be supremely bright.

Well, for one, space is so vast that light with its finite speed from distant stars and galaxies still has not reached us. An extremely important thing to understand — and this is something over which even Einstein had made a mistake — is that the universe is rapidly expanding. The speed of that expansion is the only thing in the universe that is faster than the speed of light. Because of this expansion, light from distant galaxies does what is called red-shifting, which is not visible to the naked eye. It was this red-shifting, which convinced Edwin Hubble that the universe was actually expanding. And since the expansion is faster than the speed of light, light from distant stars and galaxies will never reach us. We will never see them, let alone observe, that light lit up our night sky. The enormously large number of black holes along the path of these light rays do not help the situation either.

There is enough light in the universe to make the sky completely white but the distance for the light to travel is not only vast but ever increasing and at a speed more than the light can keep up with. Perhaps someday, the night sky would be lit up with a lot of light rays from billions of other stars from our own Milky Way galaxy. Those light rays haven’t reached here yet. They have been traveling long before human beings existed, long before the earth existed. And they would be traveling long after we are gone.