Why Is It Dark At Night - Revisited

ftg appears to be remembering the gist, but not the details of Edward Harrison’s treatment of the Paradox. He wrote various papers on the subject, but the best place to start is his Darkness at Light (Harvard, 1987), which is usually taken to be standard on the subject.

As Achernar notes, the canonical form of the paradox only deduces that the night’s sky should be as bright as the Sun. Given that, you can indeed do the refutation by calculating the energy density you would get if all the matter in the universe were converted to light. In our universe this is far too low to get the required brightness. But, equally, you can easily imagine universes where the ambient radiation would be comparable to sunlight.

If you neglect the effect of nearer stars blocking the farther ones, you get infinity. In any direction, you’re summing not just the light from the nearest star, but from any ones behind it.

Thanks for the post bonzer. I found a page (in French) that summarized Harrison’s 1977 paper. I used Google to translate it so any bad grammar is likely due to the mechanical translation:

The “seeing stars everywhere” version also runs into the issue of dust clouds immediately, so you have to get into re-radiating arguments which are just plain easier to deal with using a more mathematical version. I didn’t think that anyone still hopelessly believing in Olbers’ mistake still did the surface of stars version given its obvious deficits.

bonzer wrote “But, equally, you can easily imagine universes where the ambient radiation would be comparable to sunlight.”

Actually, our universe use to qualify, a long time ago. (As stated above.)

As to Olber not knowing about e=mc^2, we can safely assume he was aware of the laws of thermodynamics and that whatever source of energy the stars were using, it would be finite.

The Universe may be infinite in size. If it is it is safe to assume that there are an infinite number of stars. If there are an infintie number of stars you will sooner or later run into one going in any direction.

However, the Universe is not infinite in age. At roughly 10-15 billion years old our Universe (the one you and I are aware of) is a sphere with a 20-30 billion light year diameter. Anything beyond that, for all practical purposes, doesn’t exist as far as we are concerned. Even if there is an infinity of stars beyond our Universe Sphere it can have no effect upon us. Not gravitationally, electromagnetically or otherwise to the point that you may consider it as if it isn’t even there.

In short, the Universe simply hasn’t been around long enough to brighten up and I would wager the Universe will die a heat death long before our night sky would grow as bright as day.

Wouldn’t it be fair to Olbers to have the courtesy to use his argument as a basis for what you are critiquing?

If I read it correctly (and the translated diction is a bit wonky) the Edward Harrison quote that ftg supplies says that the sky would not be equally luminous even in an infinite universe because of its expansion.

But that is a completely different problem than the one that Olbers proposes. His infinite universe is static. And his answer is completely correct. In a static universe the sky would indeed glow bright.

You can hardly critique someone for not knowing a fact that would not be discovered for an additional 150 years. But, and here’s the real kicker, you also can’t arbitrarily change a person’s argument and then use that change to accuse him of not knowing what he’s talking about.

If you want to talk about Olbers’ Paradox, talk about the one he propounded, ftg, not some variant that you are arbitrarily dragging into the argument.

[hijack]
A lighting engineer? Is that kind of like a guy who pulls a handle all day gets a title of Lever Engineer?

:confused:

[/hijack]

I agree this thread seems to be going a bit hard on a man making his suppositions 150+ years ago. What may seem obvious to us today was by no means obvious back then.

However, if Olbers assumed an infinite universe you still wouldn’t get a glowing sky even if it was static per the reasons I laid out in my previous post. Additionally, stars have finite lifetimes and are at all sorts of different distances from us. If all the stars had their light reach earth all at the same time you’d get a glowing sky. However, in reality while we may eventually hit a star in any direction we choose Star A might be 1 billion light years from us while Star B, just beside it in our line of sight, might be 15 billion light years from us. Star A will likely stop shining on us before the light from Star B even makes it to earth. As a result you aren’t ever likely to get your infinite number of stars shining sufficiently long enough to brighten our sky.

You’re getting it at least half right, Whack-a-Mole, by trying to get a sense of what the assumptions were that Olbers brought to the table.

Yes, that stars have finite lifetimes is one reason why the sky is not bright. But did anyone of his day really have a sense that stars were not infinite in time? He was working under the assumption that the universe had always existed and would always exist.

Remember what I said earlier about his radical proposal that there must have been a time before the stars had started to shine. This is obviously contrary to an assumption that the stars had and would always be there.

This is what makes the paradox so powerful as to last and be remembered today. It takes something everybody knows not to be true, yet hardly ever thinks about, and forces people to examine their hidden assumptions about the way the world works to try to explain it. Yes, today we can. But at the time, this question had been rattling around the best brains of the day for 100 or 200 years. There must have been something to it.

Knowing that the paradox is wrong, that the sky is dark at night, is as trivial as trivial can be. Knowing that this is an important question to ask is genius.

Doing a little more searching I ran across Olbers’ Paradox: A Review of Resolutions to This Paradox, by David Newton which talks about the assumptions that Olbers and others were working under, and why they are important. It even cites Edward Harrison, so maybe it will even satisfy ftg.

I don’t understand what is going on with post such as those by Exapno Mapcase and those I referred to earlier. Why do people tell me that my posts contain the exact opposite of what I am clearly stating?

In my very first post in this thread I explicitly said:
“You in no way shape or form have to make any assumptions about age, size, expansion, or curvature of the universe.”

Is this clear enough for you Exapno Mapcase? I never said anything about assuming expansion. The Harrison quote I gave only refers to expansion to point out how the conditions now are different than the conditions billions of years ago (when the sky was bright). Note that this discussion of exspanion in the quote obviously has nothing to do with Olbers’ flaw. I cannot understand (again) how someone can misread such direct statements. Olbers just flat out didn’t know the laws of thermodynamics.

You do not have to assume an age limit on the universe. (There is the direct observation that a star can’t shine forever due to thermodynamics, but that has nothing to do with age of the universe.)

Remember the Basic Statement: Olbers screwed up. Therefore there is no need to invoke anything fancy to resolve it. There is nothing to resolve.

ftg, I’m going to have to ask you for a cite for this statement from your first post to this thread (which seems to be the basis for your statement that “Olbers screwed up”):

When I do a search for Olbers’ paradox, every site I find describes the statement as saying that the sky should be as bright as a star or as the sun, not that the night sky would be infinitely bright. For example:
Encyclopædia Britannica
Eric Weisstein’s World of Astronomy
Scientific American
Encyclopedia.com

You have to make assumptions. You can’t take away the assumptions. The entire point of what Olbers said is based on the assumptions he made. If you take away the assumptions you have a different problem.

Did you read the Newton article at all?