The Mechanism
The paradox is older than its name. In 1577 the English astronomer Thomas Digges, writing the preface to a translation of Copernicus, pointed out that an infinite universe of stars should fill every line of sight. Just as a person standing in the middle of an infinite forest sees a tree-trunk in every direction, an observer in an infinite static universe of stars should see a star in every direction. The whole sky should be uniformly as bright as the Sun. The sharpened version that gave the problem its modern name came in 1823, from a German amateur astronomer named Heinrich Wilhelm Olbers - a physician in Bremen who observed comets and asteroids from a converted attic between patients. Olbers showed that you could not save the puzzle by saying dust absorbs the light. Dust would, over enough time, heat up to the same temperature as the stars and re-radiate the same energy back out. Absorption could not save you. The first person to give the right answer, in essentially modern form, was the American writer Edgar Allan Poe, in his 1848 prose-poem "Eureka." Poe argued - without the calculus to formalize it - that the sky was dark because the universe was not infinite in time. The starlight from sufficiently distant stars had simply not yet reached us. Light travels at a finite speed; the universe has a finite age; the volume of stars from which photons have had time to arrive is therefore finite. Outside that bubble, however many stars there are, their light is still in transit. Lord Kelvin formalized the same answer mathematically in 1901. That answer was correct, but it was one step short of the modern picture. The other piece arrived in 1929: the universe is not only finite, it is expanding. Light from distant galaxies is redshifted as the space between them and us stretches. A galaxy at the edge of the observable universe has its visible light shifted out of the visible band entirely - into the infrared, and then the microwave. We do, in fact, see a faint glow from every direction in the night sky. Most of it is at a wavelength of 1.9 millimeters, in the microwave band, with a brightness temperature of 2.7 kelvin. It is the cosmic microwave background: the redshifted afterglow of the early hot universe, the closest the real sky gets to fulfilling the 1823 bright-sky prediction. Our eyes cannot see it. A microwave receiver can.
Why It Matters
The blackness between the stars is not actually blackness. It is a glow from the Big Bang itself, stretched until it is invisible to us. The dark patches between the stars are not dark - they are redshifted to a wavelength outside our sight. Two of the deepest facts in cosmology are encoded in the most ordinary observation a person can make: stand outside at night and notice that, mostly, the sky is dark. The fact that you can see stars at all - that there are dark gaps between them - tells you, mechanically, that the universe is not eternal. The fact that those gaps appear black rather than dimly orange tells you the universe has been expanding for billions of years.
Wait — That's Not Quite Right
A lot of people assume the dark sky is just empty space - that the gaps between stars are nothing, where the light has nowhere to come from. That is not quite right. In an infinite eternal universe of stars there would BE light coming from every direction. The gaps are dark for a reason: the universe has not been around long enough for the light from every direction to arrive, and the light that has arrived from the most distant places has been stretched, by the expansion of space itself, into wavelengths your eyes cannot detect. The darkness is not absence. It is a kind of evidence.
Vocabulary
- Olbers paradox
- line of sight
- finite light-travel time
- observable universe
- redshift
- cosmic microwave background
- CMB
- brightness temperature
- 2.7 kelvin
- expansion of the universe
Quick Quiz
5 questions · For classroom or kitchen table
The Experiment
Draw the Sky We Do Not See
On a clear night, go outside with a sheet of dark paper and a white pencil or piece of chalk. Stand somewhere away from streetlights and let your eyes adjust for about 5 minutes. Look up. Draw what you actually see - just the bright stars, with most of the page left dark. Now flip the paper over and draw what the sky would look like if Olbers were right: every spot on the page filled with light, no gaps at all, like the whole sheet was the surface of the Sun. Compare the two drawings. The difference between them is the answer to one of the oldest questions in astronomy - and the dark side of your paper is, secretly, the cosmic microwave background. It really is glowing in every direction. You just cannot see it. (Bonus: a microwave receiver tuned to 1.9 millimeters - the kind radio astronomers use - can see it. Your eyes cannot, because the universe has been expanding for too long.)
A sheet of dark-colored paper, a white pencil or piece of chalk, a clear night, and a spot away from streetlights. About 15 minutes.
Where this came from
- Olbers, H.W.M. "Ueber die Durchsichtigkeit des Weltraums," Astronomisches Jahrbuch fuer das Jahr 1826 (Berlin: Spaeten, 1823), pp. 110-121. The original paper that named the paradox and ruled out dust as the resolution.
- Harrison, E.R. Darkness at Night: A Riddle of the Universe (Harvard University Press, 1987). The standard scholarly history of the paradox from Digges through the cosmic microwave background.
- Poe, E.A. Eureka: A Prose Poem (New York: G.P. Putnam, 1848), pp. 100-106. The 1848 prose-poem that first gave the right answer in modern form: the universe is finite in time.
- Penzias, A.A. & Wilson, R.W. "A Measurement of Excess Antenna Temperature at 4080 Mc/s," Astrophysical Journal 142: 419-421 (1965). The 1964 detection of the cosmic microwave background.
- The Conversation - Why is the sky dark at night? (the 200-year history)
- Olbers paradox (Wikipedia)
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