Slight change of pace this week folks, my back is still playing up, and I haven’t had much of a chance to do a lot of research this week. I’m also feeling a bit guilty following on from last week’s article, which got far fewer obsessive rewrites than I would usually dedicate to my musings.
So, today, I have a simple question for you. Have you ever looked at something which is everyday and innocuous, like a flower or a feather, or the night sky, and wondered, “Why is that the way it is?” Of course you have, we all have at one time or another, often without realising we are doing it. Well, it turns out that some of the greatest discoveries in science began with people asking surprisingly simple questions. Such as, “Why do apples fall?” or “Why do planets move?” or, “Why does time seem to flow in only the one direction?” Questions I have attempted to answer in previous articles.
One such question, as I alluded to in the title, is, “Why is the night sky dark?” And this very question came to be known as Olbers’ Paradox, and it may even be one of the most deceptively simple questions ever asked.
Why is it called Olbers’ Paradox? I hear you ask. Well, it is named after the German astronomer Heinrich Wilhelm Olbers, who first wrote about it in 1823. Olbers wasn’t the first one to ask this question, others such as Johannes Kepler, Edmund Halley and Jean-Philippe de Chéseaux had asked the same question over the preceding couple of centuries, and I’m sure it is a question that has been asked going way back to the Stone Age.
When you think about it, like many deceptively simple questions, it hardly seems worth discussing. The sky is dark because it is night, isn’t it? Case closed. Everyone can go home now that we have answered that one. Hang on a minute though, let’s not forget that the universe has a habit of making fools of us whenever we think we have an easy answer such as that one.
To actually find the answer, we have to look back into the dim and distant past and try to imagine what ancient astronomers thought the universe was like. For centuries, many believed the universe was infinitely large, filled with stars, and had existed forever, which, when you think about it, sounds like another perfectly reasonable answer. If the universe had no beginning and stretched endlessly in every direction, then it stands to reason there would be stars everywhere you looked. And that, my faithful readers, is where things start to go wrong, and leads me to analogy time!
Now then, imagine you are standing in an endless forest and no matter which direction you look, your line of sight always and inevitably falls upon a tree. There may very well be lots of gaps between the trees, but with an infinite number of trees spread across an infinite landscape in our endless forest, every view eventually ends with bark, twigs and leaves. Now, swap all the trees for stars.
If the universe is infinite and has existed forever, then it now stands to reason that every line of sight should eventually end on the surface of a star. Therefore, the night sky should not be mostly black. It should be one giant wall of starlight or, put another way, the entire sky ought to be bright. Very bright. So bright, it should potentially be as bright as the surface of an average star, and it is at this point that nature gently clears its throat and points out that the night sky is, in fact, not blindingly bright. It is actually very dark with little pinpricks of light scattered all over it. So, why is that? What has gone wrong?
The first thing to come to mind might be to say that distant stars are simply too faint to see, which again, sounds completely reasonable until you dig into the subject I dread the most. The maths. OK, while distant stars are dimmer, there are also loads more of them, and every time you look deeper into space, the number of stars increases dramatically. These extra stars effectively make up for the loss of brightness, and an infinite universe would therefore provide an infinite number of contributions to the brightness of the sky. Yeah? Well, no. And this is one of those annoying situations where maths refuses to cooperate with our common sense.
Maybe, scientists began to wonder, clouds of dust are blocking the light, and again, this feels like a perfectly reasonable solution. Unfortunately, as soon as we bring physics into the picture, that perfectly reasonable solution gets binned very, very quickly.
This is because physics tells us that dust absorbs light, which means it also absorbs energy, and absorbing energy makes things warm up. Given an infinite amount of time, all that dust would eventually become hot enough to glow, and rather than obscuring and hiding the starlight from our view, it would simply absorb it and reradiate it. The sky would still end up bright, and once again, the universe has outsmarted us.
For a surprisingly long time, nobody had a satisfactory answer to this and the paradox sat there quietly pointing out that something about our picture of the cosmos was wrong.
The breakthrough finally came when astronomers realised that the universe has not existed forever, a realisation that turned out to actually be quite important.
Modern measurements show that the universe is about 13.8 billion years old, and as unimaginably ancient as that may seem, it is not eternal. As you and I know, from my previous articles as well as from science, light travels at 299,792,458 metres per second, which is a finite speed. Therefore, we can only see objects whose light has had enough time to reach us, which in turn means that a lot of the universe simply lies beyond our current cosmic horizon. We can’t see everything.
Think of it like being in a vast city at night during a power cut. If someone starts switching on lights across the city, you only see the ones whose light has already reached you. The fact that more lights exist farther away doesn’t matter.
The universe works in a similar way as there has only been a finite amount of time for light to travel to us, so we do not receive light from every possible star. In fact, this finite age does most of the work.
So, is that the problem solved? Well, not quite. Another part comes from the discovery that the universe is expanding.
When astronomers like Edwin Hubble began studying distant galaxies, they found something remarkable. Space itself is stretching, and as it stretches, light waves stretch with it. Their wavelengths become longer, their energy drops, and the light is shifted towards the red end of the spectrum. Push this process far enough and visible light can become infrared radiation, microwaves, or radio waves. In essence, the universe has been turning down the brightness knob a bit more.
Although a huge amount of ancient light still exists, much of it has been stretched beyond the range that our human eyes can detect. If our eyes were designed to see microwaves instead of visible light, the sky would actually look rather different. In fact, it would be glowing, but not with starlight.
This glow is something you have very likely heard of and is known as the Cosmic Microwave Background, and was discovered by astronomers over 60 years ago, way back in 1964. What’s more, it fills the entire sky. It is the earliest light we can observe, released about 380,000 years after the Big Bang, and before any stars were formed. It has been travelling through expanding space for billions of years, and over those billions of years it has cooled to just 2.7 degrees above absolute zero.
So, thinking about it, the night sky is not dark at all. We are simply looking at it with the wrong kind of eyes.
What makes Olbers’ Paradox so fascinating is that it starts with an observation so ordinary that most people never think twice about it. Every child has looked at a dark night sky, every human civilisation has seen the same darkness between the stars, yet hidden within that darkness was a clue that the universe had a beginning.
Long before anyone discovered the Big Bang, long before anyone measured the expansion of space, and long before anyone detected the Cosmic Microwave Background, the night sky was already giving the game away. It was quietly telling us that the universe could not be infinitely old and unchanging. How about that for a large expanse of darkness!
It makes you think though, doesn’t it? Well, it does me. Sometimes the most profound discoveries do not come from finding something new. They come from finally noticing something that has been staring us in the face all along. In the case of Olbers’ Paradox, the clue was not a bright star, a spectacular galaxy, or an exotic black hole. It was all the blackness in between!