Why is the Sky Dark at Night?

After writing the last post, “Afraid for the Dark”, I was reminded of a seemingly simple question that surprisingly few people know the answer to.   The question is the title of this article: “Why is the Sky Dark at Night?”    It’s a question that concerns something that we can observe on a daily, or rather nightly, basis (although perhaps not as well as we could at one time), just like the effects of gravity.  Gravity, like night-time darkness, has been around for a long time.  It was only in the time of Isaac Newton, and his Principia of 1687, that we started to understand something of how gravity worked.  In the 20th century, Einstein showed, in the theory of General Relativity, that Newton’s understanding was not complete.

In the case of night-time darkness, it was the 19th century before the question was asked — and the answer was not obvious:

In 1826, the astronomer Heinrich Olbers asked, “Why is the sky dark at night?”   By his time, physicists had learned enough to realize that, in a stable, infinite universe with an even distribution of stars, the entire universe should gradually heat up.

In a very informal ongoing experiment, I’ve asked the question of a number of people I know; and only one, out of a few dozen, has come up with the currently accepted answer.  I’m not going to give you the answer here, because Paul Lutus, a modern Renaissance man, who has designed spacecraft components for NASA, written the Apple Writer word processing program, and sailed solo around the world, has written a really first rate explanation.    Before reading it, you should try to spend a bit of time trying to work out the answer for yourself, but be forewarned that your intuition probably won’t help, and in fact will probably be a hindrance.

Scientists and physicists had to learn quite a lot about the behavior of energy before they were even prepared to ask Olbers’ question. In fact, for millennia the dark night sky provided an answer to a question no one thought to ask.

I hope you enjoy reading the article.  I think this simple question, and its answer, is another great example of how science can reveal wonderful complexity that is beyond our ordinary imaginations.