Why Is The Night Sky Dark If There Are So Many Stars?
If you’ve ever found yourself asking this question while staring at the night sky, then you’re in good company. The question can be traced back to Johannes Kepler in 1610 (the planetary motion guy), and was rediscussed by prominent astronomers like Edmond Halley (the comet guy) before being written about by the awesomely named Heinrich Wilhelm Matthias Olbers (somewhat mistakenly, as his thinking on the question wasn’t very valuable).
As a post by Cornell University reads:
A more detailed description of Olbers’ paradox allows you to conclude that if the universe (a) were big enough so that every line of sight ended in a star, (b) were infinitely old, (c) were static and not expanding and (d) if several other simple assumptions were satisfied, then the entire night sky would be roughly as bright as the surface of our sun!
While the first satisfactory scientific explanation to the problem was (probably) given by Lord Kelvin is 1901, someone else had a surprisingly accurate crack at it earlier, in 1848.
In his essay Eureka, poet Edgar Allan Poe provided the framework for what would ultimately be the correct answer to the paradox:
Were the succession of stars endless, then the background of the sky would present us a 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
Poe’s explanation, summarized, is that because the age of the universe and the speed of light are both finite, only finitely many stars can be observed within a certain volume of space visible from Earth. Basically, there is a horizon of sorts at every point in space, extending as many light-years as the universe has existed. Beyond that horizon, light from that area simply hasn’t had enough time to travel to the other point yet. When considering the incredible vastness of space, light from most of the stars just hasn’t had enough time to reach us yet.
Sources: Cornell, NYTimes
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