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With infinite stars, why is the night sky dark? [1]
['Ellsworth Toohey']
Date: 2025-08-26
Why is the night sky dark? This simple question, known as Olbers' paradox, has perplexed astronomers for centuries. In an infinite, eternal universe filled with stars, every line of sight should end on a star, bathing the night in constant light. Yet we see darkness punctuated by points of brilliance.
As reported in the Futility Closet blog, this conundrum is named after German astronomer Heinrich Wilhelm Olbers. Edgar Allan Poe touched on this puzzle in his 40,000-word 1848 essay "Eureka," writing,
No astronomical fallacy is more untenable, and none has been more pertinaciously adhered to, than that of the absolute illimitation of the Universe of Stars. The reasons for limitation, as I have already assigned them, à priori, seem to me unanswerable; but, not to speak of these, observation assures us that there is, in numerous directions around us, certainly, if not in all, a positive limit — or, at the very least, affords us no basis whatever for thinking otherwise. Were the succession of stars endless, then the background of the sky would present us an uniform luminosity, like that displayed by the Galaxy — since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all. That this may be so, who shall venture to deny? I maintain, simply, that we have not even the shadow of a reason for believing that it is so.
The resolution to this paradox reveals fundamental truths about our cosmos. The darkness we observe is evidence that our universe has a finite age and is expanding. As the Futility Closet explains, "We now know that the sky is dark because the universe is expanding, which increases the wavelength of visible light until it appears dark to our eyes." This expansion stretches light from distant stars beyond the visible spectrum, while the universe's finite age means light from the farthest reaches hasn't had time to reach us.
"The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all," Poe presciently suggested, hinting at the finite age of the universe.
Previously:
• The Introvert's Paradox: Brave Dave's TikTok explores the love-hate relationship with socializing
• Podcast episode about creativity, paradox, and what if questions
• The paradox of The Bottle Imp
• Copyright's Paradox: brilliantly argued scholarly book tackles free speech vs. copyright
• The coin paradox
• The absent-minded driver's paradox
• The Ellsberg Paradox
• The Paradox of Tolerance: should intolerance be tolerated?
[END]
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