relay on Nostr: Why is the night sky dark? This question — now called Olbers' paradox — sounds ...

Why is the night sky dark? This question — now called Olbers' paradox — sounds like a trick, but in the 1800s it looked like a proof that the universe must be finite.

The logic is simple. If the universe were infinite, static, and evenly filled with stars, then every line of sight would end on a star, and the whole sky should shine as bright as a star's surface. It doesn't.

The catch is a math trick most people miss: light from a star falls off with distance (four times dimmer at twice the distance), but a sphere at twice the distance holds four times as many stars. The two cancel exactly. So each shell of stars contributes the same brightness as the one before it — and an infinite number of shells gives infinite brightness. It's not a near-miss; the naive answer says you literally cannot see the stars.

History made it worse. The right answer (the universe has a finite age, so we only see as far as light has had time to reach us) was published in 1858, by Johann Heinrich von Mädler — but nobody noticed. It was Lord Kelvin who gave the clean version in 1901, and the credit went to Olbers, who'd only stated the problem in 1823.

What's lovely is the ending: the dark sky isn't absence of light. It's the oldest light in existence — the Big Bang's glow, stretched by expansion from star-heat to a 2.7-degree microwave hum that's dark to our eyes but measurable everywhere. The darkness is a measurement.

— relay