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The night sky is dark. Obviously. But work out how much starlight should be arriving and the answer comes out blinding — so the darkness is telling you something enormous about the universe, and it took three centuries to hear it.
Why is the sky dark at night?
Because the Sun has gone round the other side. Next question.
Except that is not an answer, it is a dodge. The Sun is one star. There are others — a great many others — and they are all shining at you right now. The real question is why their light doesn't do the job.
And the obvious reply is that they are much too far away to matter. That reply is the thing this episode is going to take apart, because when you actually count it, it is not true.
Imagine the universe filled with stars, spread out roughly evenly, going on and on.
Now picture yourself inside a set of hollow shells, like the layers of an onion with you at the centre. Shell 1 is the stars a certain distance out. Shell 2 is the stars twice that far. Shell 3, three times. And so on, outwards, as far as you like.
Two things happen as you move to a further shell, and they pull in opposite directions:
A quarter as bright each. Four times as many. Hold that pair in your head and count the shells.
Round numbers, chosen to make the pattern visible — the real values are messier but behave identically. Press through and watch the right-hand column.
In a universe that went on forever, had always existed, and never changed, every direction you looked would eventually run into the surface of a star. The whole sky would be as bright as the surface of the Sun — at midnight.
You keep going and reach shell number 100 — a hundred times further out than the first one. Roughly how much light does that shell contribute?
The argument only works if all three things are true: the universe is endless, it has always existed, and it is unchanging.
The sky is dark. So at least one of those is false — and that is a real conclusion drawn from something you can check by walking outside.
The biggest culprit is the middle one. The universe has not always existed. It is about 13.8 billion years old, which means light has only ever had 13.8 billion years to travel. Shells beyond a certain distance are certainly out there, and their light is certainly on its way — it simply has not arrived yet. The outer shells are missing from your night sky the way a letter posted yesterday is missing from your doormat.
Two smaller helpers finish the job. Stars do not burn forever, so no shell has been shining for all of eternity. And space itself is stretching, which drags the light from the most distant things into longer and longer waves until it slides out of the range your eyes can see.
That last one has a beautiful consequence. There is a glow arriving from every direction at once, filling the entire sky, exactly as the paradox demanded. It is the leftover heat of the young universe, now stretched out to microwaves and sitting at about 2.7 degrees above absolute zero. Radio telescopes see it everywhere they point. The sky is not actually dark. It is glowing in a colour you do not have eyes for.
People have been proposing answers to this for four hundred years, and most of them fail. Sort each one, then read what happened to it.
Tap an item, then tap where it belongs
A friend says: 'The night sky being dark is just an ordinary fact. It can't possibly tell you anything about the universe.' What is the best reply?
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The paradox is usually named for Heinrich Wilhelm Olbers, who wrote it up in 1823, though Johannes Kepler raised it in 1610 and Edmond Halley and Jean-Philippe de Cheseaux both worked on it in the 1700s. The shell argument: for stars spread evenly, the number in a shell grows with the square of its distance while each star's apparent brightness falls with the square of its distance, so each shell contributes an equal amount and the total grows without limit. Edgar Allan Poe set out essentially the modern resolution in his 1848 essay 'Eureka' — that light from the most distant regions has not yet had time to reach us. The age of the universe is about 13.8 billion years (Planck mission, 2018 results: 13.787 billion years). The cosmic microwave background fills the sky in every direction at about 2.725 K, discovered by Arno Penzias and Robert Wilson in 1965 and mapped in detail by COBE, WMAP and Planck. 'Tired light' — the proposal that light loses energy over distance — was advanced by Fritz Zwicky in 1929 and is not supported by observation; the observed reddening of distant light is due to the expansion of space.