Olbers’ Paradox

April 19, 2012 — Leave a comment

Wendy Sadler (of Science Made Simple fame) was asking, on Facebook, what explanations people usually gave for Olbers’ Paradox. The slew of answers from several people revealed that the canonical answer is not the only one people think of.

The paradox is named for a 19th Century astronomer, Heinrich Olbers, who remarked that if the universe is infinite, and there are stars throughout it, then in whatever direction you look you would be looking at an infinite number of stars and the night sky would therefore be as a bright as the surface of the Sun. This is not what we see, so why is the night sky dark?

I have always thought that Olber’s paradox is resolved by two ideas. Firstly, realising that the Universe has not existed forever, so you do not look infinitely far into the distance. Then there is also the fact that the Universe is expanding, and stars move faster from us the further you go back, and their light is redshifted beyond our vision.

Another common answer is that the Universe may be full of stars, but it is also full of dust and gas. This blocks some of the light. I don’t think this really does help solve the paradox. If there were infinite stars illuminating the dust, the dust would end up glowing itself. Also if the sky were as bright as the surface of the Sun, you’d need an awful lot of dust – by comparison, you’d need to put a huge amount of dust between us and the Sun to block it out.

In looking into this further, I discovered that Edgar Allan Poe was one of the first to suggest that the resolution to the problem was that the Universe is not infinitely old. In 1848 he wrote Eureka, which contained the following:

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.

Lord Kelvin is attributed to the best, correct response in 1901. More curiously it turns out that it is not necessarily Olbers’ paradox at all. The problem was first noted 300 years earlier by Thomas Digges and was also expounded by Kepler.

On a final corrective note, it was only when I Googled a bit for this blog post that I realised Heinrich’s surname is Olbers and not Olber, thus moving the apostrophe in an already misnamed paradox.

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