I know why it’s dark at night. Do you?

11 10 2008

Today is the 250th anniversary of the birth of the greatest of paradox proposers, Heinrich Wilhelm Olbers.  Olbers did a number of things in his life to advance the study of astronomy, but what he is most famous for is the question that bears his name:

If the universe is infinite and stars are distributed evenly throughout, then why isn’t the sky infinitely bright?

Think of it this way, if you were standing in a large forest you wouldn’t be able to see your way out of it.  Near you there wouldn’t be many trees and you could see past them.  Farther away the trees would appear smaller yet more numerous.  This balances out and explains why you can’t look in any horizontal direction and not see a tree.  Now imagine those trees as stars.

This question may not be as well known today, but 100 years ago this was the BIG question.  Last century people wondered about the nature of light, whether matter was continuous or quantized, and what was the source of gravity.  Today we have big questions like “Did we accelerate global warming?”, “What is dark matter?”, and “Do gravitons create the gravitational force?” (I guess gravity may baffle us for centuries to come).  But in social circles people talked about Olbers’ Paradox to stretch their imaginations and to show off for the ladies!  Just like today — the ladies love a man who can discuss the implications of the discovery of the Higgs particle on multi-dimensional physics.  Ummm Hmmmm!

Olbers’ Paradox was originally proposed by my hero, Johannes Kepler, 200 years before Olbers described the argument in 1823.  This argument arises from the idea that the universe is steady-state, meaning that it has always looked like it does today and always will.  Einstein held this belief so strongly that he refused to accept the Big Bang Theory, even though his own equations pointed to that reality.

Here’s the idea in a nutshell.  There were a few generally accepted beliefs among scientists and the majority of the educated population.

1.  The universe is steady-state

2.  Stars are evenly distributed throughout

3.  The universe is infinitely big

If all three of these is true then there would be no line of sight that would not end in a star.  If everywhere you looked there was light coming toward you, then the sky would be infinitely bright.  The only way you would know where the sun was would be to look for the slightly darker area of the sky.  A seemingly ridiculous hypothesis, yet completely logical.

This means, of course, that one of the accepted beliefs was most likely wrong.  Or two.  Or all.  But what is the truth?  There were many ideas — good ideas — suggested by some very smart people to explain our reality.  But many fell victim to Occam’s Razor; they were just too convoluted to be viable.

Strange as it may sound, the first person to publish a correct solution was Edgar Allen Poe.

“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.”  —Eureka: A Prose Poem

What he proposes here (if you can read through all his commas!  Sheesh!  He writes run-on sentences like I write sentence fragments.  –you do read my blog, right?) is that light has a speed limit and the universe is not infinitely old, thus this infinite amount of star light hasn’t reached the Earth yet.  He did not contradict any of the three suppositions, yet his idea of the universe having a beginning gives us a good starting point for the explanation.  He wrote this in 1848, 80 years before Hubble would show conclusive evidence for a beginning to everything.  Odd thing is, no one paid attention to Poe just like no one paid attention to Hubble at first because folks were so convinced that the universe had always been here.

So that’s only part of the explanation.  The rest of the story is that the universe is expanding.  In a not-so-readily-apparent sense, because the universe was smaller in the past it was also brighter.  As the expanding space-time continuum spreads things out, the overall energy density decreases.  (Wow!  That sentence made me feel smart!)  There is a ‘horizon’ to the universe — a boundary which we cannot see beyond.  The expansion is faster than light, which means anything beyond that boundary will never be visible to us because the light from those objects can’t outrun the space-time expansion.  Thereby preventing us from ever knowing how far the universe actually extends.  And keeping the visible universe of limited size.

Also, because of the expansion the stars aren’t uniformly spaced.  They occur in clumps.  These clumps were hypothesized way back in the 1700s by philosophers, but not confirmed by astronomers until the 1920s!  The clumps were given the name galaxies after the Greek word for milk.  Yeah, makes total sense to me too.  Still a cool word, though.

And there you have it! All three postulates failed and the paradox is now moot.  The scientists who proposed the problem knew that something was wrong with popular opinion, they just weren’t sure where the answer lied.  It took a lot of looking, a lot of thinking, and a lot of technological advances to put the pieces together, but today we have a pretty clear view of our place in the universe.

So, Happy Birthday, Heinrich!  Of all the science you brought us, you showed that sometimes it’s more important to ask the right questions.

[BTW, while double-checking all my facts on the intertubes I found countless articles that were just plain wrong or else written so poorly that I could barely read them.  Honestly, I’m going to have to side with Wil Wheaton on this and say that these days the ability to spell is a superpower.  And do they just let any idiot with a library card have a blog?  <checks library card>  Oh, yeah… guess they do.]

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2 responses

12 10 2008
Nick Jones

Its a good thing that my head has paradox absorbing crumple zones, or that post would have really hurt. It is a really cool questions though. Great post to get me thinking on a weekend when I was shooting for being as brain dead as possible. Good Job.

20 11 2008
Terry Johnson

Thanks a ton, Nick! I love talking about that stuff, about the historical connections to scientific discovery, but I get the opportunity to get this deep into it so rarely. Don’t know who would ever benefit from reading this, but that’s alright. I’m pleased with it anyway. :o)

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