Can you solve the famous Olbers' Paradox?
Why is the night sky dark?
To the mind that is still, the whole universe surrenders. - Lao Tzu
Greetings, fellow Bohron
Today, I got a problem for you. One with a very interesting solution. Just tell me
Why is the night sky dark?You may say that you are too old to answer this foolish question. But I bet this isn’t just a child’s play. You may answer that the reason is simple: the Sun isn’t up. So what? A trillion Suns are up at night staring at us from the farthest of galaxies.
Why is the sky bright during the daytime?
Before we solve this problem, let me clear your misconception. Just because the Sun is up doesn't necessarily mean that the sky should be bright. Think of the Moon. Although sunrise and sunset occur at the moon, it is always dark there.
Here on Earth, during the daytime, the sky appears blue to us. This is because particles in the Earth’s atmosphere scatter the sunlight. Since blue light has the shortest wavelength, it is scattered the most. If Earth didn’t have its atmosphere, the sky would always be dark, just like on the Moon.
The Paradox
The universe is full of stars that are as bright as the Sun and in an infinite static universe, any line of sight from Earth must end at the surface of a star and hence the sky should be completely illuminated both at night and daytime. But this is never true, the night sky is always dark.
Olbers’ paradox or the “dark night sky paradox”, given by Heinrich Wilhelm Olbers, is the argument that the observed darkness of the night sky conflicts with the assumption of an infinite and static universe.
This problem continued to puzzle scientists beginning from the 6th century AD before its explanation was proposed in the 19th century and confirmed in the 20th century by Edwin Hubble’s observations.
One idea is that the intervening clouds between us and the stars absorb the light and prevent it from reaching us. However, this isn't completely true. As the clouds absorb energy, they get heated up. After they become hot enough, they radiate light again.
Well, let’s find out the real answer.
The Solution
As we all know, the universe is not infinite and eternal. The universe has a temporal edge. It had a beginning(the big bang) and is 13.7 billion years old. Also, the speed of light isn’t infinite, contrary to what was earlier believed.
Since only a finite time has passed since the big bang, the light from stars far away from us outside of our observable universe hasn’t reached us yet. It’s like the universe is a big thunderstorm and we are still waiting to hear the thunder.
Thanks to our powerful telescopes, we now know that it takes light a billion years to reach us from the farthest stars. It means that when we look at the sky, we look at a very distant past. The more powerful the telescopes get, the further back in time we can see.
But that’s not the end of the story. We can’t even see some stars which are not so far away. What if I tell you that space is not dark? It is only that the human eyes just can’t see the colour of the space.
Hubble Telescope photographs the stars using infrared cameras. Since the universe is undergoing accelerated expansion, stars and galaxies are also moving away from us at ever-increasing speeds. The farther away a star is, the faster it moves away from us. Now, due to the Doppler effect, the light from such distant stars get redshifted and eventually stretches to the level of infrared and microwave radiation, which we can’t see with our naked eye.
So, in conclusion, there are two main reasons that why the space is dark:
Our Universe doesn’t consist of an infinite number of stars and also that it had a beginning. So the light from very distant stars which are not part of our observable universe has not reached us yet.
As our universe is not static but expanding, so the light from distant stars gets red-shifted, loses energy and becomes infrared.
A simple yet thought-provoking question, Olbers’ paradox proves that even the most basic of our problems can have deep answers hidden in them.
Well explained 👏
Never thought about it.