Why it gets dark at night and the answer is more complicated than you probably think

The Milky Way in the night sky. (Image source: Pixabay/theartofsounds2001)

It is completely normal for us humans that it gets dark at night. And if we now ask you what the reason for this is, many of you will probably immediately think of it: Well, quite simply because the sun sets regularly!

While the answer is not wrong, it is at least incomplete. Rather, it has to do with the siblings of the sun, the countless stars in the sky. But back to the beginning:

200 years ago, in 1823, a certain Heinrich Wilhelm Olbers thought about a problem that we know today as Olbers’ paradox.

The modern view of Olber’s problem

Formulated in modern terms, it says that in an infinitely extended and infinitely old universe, the sky must always be bright, because then there would also be an infinite number of stars whose light would fall on earth. But that’s obviously not the case (via Terra X Lesch & Co).

However, one could argue that dust between the stars may be intercepting the light, preventing it from reaching Earth. Or that the luminosity decreases with the square of the distance and distant stars cannot be seen at all or only very faintly.

However, both are insufficient explanations. Dust would eventually become so hot from the radiation that it itself shone as brightly as a sun. And just as the luminosity of distant stars decreases, the number of stars increases with the square of the distance – so it balances out.

In the end there is only one explanation left that resolves Olbers’ paradox: the universe cannot be infinitely old and at the same time infinitely large. If only it were infinitely large, the speed of light is the upper limit and the light from all the stars could not hit the earth at the same time. On the other hand, if the universe were infinitely old, the night sky would always shine brightly.

Did you already know this, or is this completely new to you? Write it to us in the comments!