What Is Obler's Paradox?

Updated on January 8, 2018
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Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly improve it.


Let’s jump right into it. If there are so many stars in the night sky, why is it black? Yes, the sun does go down and removes a huge light source but what about all the stars, galaxies, and nebula out there? Even if the Universe were infinite in size, we should see something everywhere. And yet…darkness reigns with just a few pinpoints of light to break it up. But some might bring up some possible answers to this conundrum, which should be examined to see their veracity. Some point out that stars which are too faint to see at the distance they are located at do exist, but when they initially formed the Universe was a lot smaller and so they have distributed out fairly well as the Universe has expanded, creating no bias in brightness. Others have suggested that maybe stars are not evenly distributed in the Universe but follow some bias, or that there are simply not enough of them out there. And this is almost certainly a true statement, but when you look at a larger and larger scope of the universe everything seems to average out rather well, again because of our expanding Universe. And the number of stars needed to light up the Universe isn’t that hard a figure to get to, as it turns out. So…why is the sky dark? (Al 43-4, Chase, Nave)

The paradox was named by Herrmann Bondi in 1952 after hearing about Heinrich Wilhelm Obler’s efforts in the 1800’s to resolve it, but the paradox was around a long time before either people. We have to go all the way to 1576 when Thomas Digges modifies the Copernican system of the Universe. In his new addition of A Prognostication Everlasting, he essentially keeps the Copernican system untouched expect for one detail. Stars are not tiny holes in some outer shell of the Universe but are actually objects that are dispersed in a space that in infinitely large in size. Quite the statement from someone without proof in those days, but Digges claimed that the idea arose from Tycho Brahe’s supernova of 1572. That event displayed no parallax of motion, indicating that it was far away. But the heavens never changed according to the contemporary views of the time, but since this was now in question, why couldn’t other parts of the world view be changed also? Digges felt that with enough stars far away, the darkness in the sky could be explained and the system would be intact (Al 45-8).

Years later, Jean-Phillippe de Cheseaux shows that this cannot be true using geometry. Those outer stars have the same brightness component as the inner ones because of the volume of space they occupied together, acting like a big source of light even at such a distance. Obler in 1823 felt that interstellar dust and gas could obscure a lot of light from distant stars. As it turns out, the Universe has been around so long that the dust and gas has heated up via collisions and so they actually emit the same light as those distant stars they obscure, so no dice there (Al 50, Chase).

No, the solution lies in the expanding Universe. You see, light can only go so fast, and if the space it move through expands fast enough then it will take a long time for the light to reach you, especially when expanding faster than c. And even if the Universe wasn’t expanding currently, you still would have a dark sky at night because of inflation in the early Universe driving space apart faster than c. It didn’t last long but it did forever cause portions of space to be forever screened out. And because of a finite Universe, only so much has happened in a span of time. There is no way enough time has passed for stars to get to any of the configurations needed for a bright night sky. So, sorry everyone. The mechanics of the world are still maintained, and now you can look up at the night sky and realize you are witnessing an amazing moment in science (AL 58-9, NASA, Nave).

The sky as it would seem it should be...
The sky as it would seem it should be... | Source

Works Cited

Al-Khalili, Jim. Paradox: The Nine Greatest Enigmas in Physics. Broadway Paperbacks, New York, 2012: 43-8, 50 PAGES. Print.

Chase, Scott I. “Olber’s Paradox.” Math.ucr.edu. UCR, 2004. Web. 19 Sept. 2017.

NASA. “Why is the Sky Dark at Night?” spaceplace.nasa.gov. NASA. Web. 19 Sept. 2017.

Nave, R. “Olber’s Paradox: Why is the Sky Dark at Night?” hyperphysics.phy-astr.gsu.edu. Georgia State Univeristy,2000. Web. 19 Sept. 2017.

© 2018 Leonard Kelley


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    • Guckenberger profile image

      Alexander James Guckenberger 7 days ago from Maryland, United States of America

      Of course! I loved learning what this is!!!

    • 1701TheOriginal profile image

      Leonard Kelley 8 days ago


    • Guckenberger profile image

      Alexander James Guckenberger 9 days ago from Maryland, United States of America

      very nice :)