# Why Is It Dark At Night - Revisited **URL:** \ **Category:** Factual Questions\ **Created:** [May 12, 2002, 4:20am UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592 "2002-05-12T04:20:43Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![bonzer](https://avatars.discourse-cdn.com/v4/letter/b/45deac/32.png) [@bonzer](https://boards.straightdope.com/u/bonzer)\ **Post date:** [May 13, 2002, 9:03pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/21 "2002-05-13T21:03:05Z")
**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. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [May 13, 2002, 9:34pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/22 "2002-05-13T21:34:39Z")
> [@](#): > > It goes on to say that “the Olbers paradox” shows the universe to finite in age, rather than finite in size. Where do you get the idea of infinite energy out of this, ftg? 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. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [May 14, 2002, 8:53pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/23 "2002-05-14T20:53:58Z")
Thanks for the post **bonzer**. I found a [page](http://nabla1.chez.tiscali.fr/7_s_e_i.htm) (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: > [@](#): > > In 1977, Edward Harrison shows that makes the Universe of them does not contain sufficient energy per unit of volume to generate a brilliant sky . According to him, one would need thousands of billion times more stars than there are currently some, or many stars having lifespans much longer, to obtain the “gilded ramparts” of Poe . This solution gathers into only one several exact solutions suggested before. Missing energy necessary, our Universe could not in no case to generate a sky illuminated by stars, even if it were of infinite extension. Subjected to the expansion which increases the deficit in energy, the galaxies, and the stars which compose them, became in fact much too distant from/to each other at the time current. However, little after the Big-bang , the energy density was still sufficiently significant so that the sky was then brilliant. 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. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [May 14, 2002, 9:49pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/24 "2002-05-14T21:49:06Z")
> [@](#): > > \*Originally posted by scr4 \* > **But the apparent brightness of the sky should be comparable to the surface brightness of a star, because surface brightness is independent of distance, and in an infinite universe (in space and time) there should be a star in every direction. If you put an absorptive material in between, it will just absorb radiation till it reaches the same temperature as other stars.** 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. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 14, 2002, 10:23pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/25 "2002-05-14T22:23:35Z")
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. ---
**Author:** ![bernse](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@bernse](https://boards.straightdope.com/u/bernse)\ **Post date:** [May 14, 2002, 10:33pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/26 "2002-05-14T22:33:51Z")
> [@](#): > > \*Originally posted by ftg \* > \*\*Talk to a lighting engineer about how intensity decreases \*\* [hijack] A _lighting_ engineer? Is that kind of like a guy who pulls a handle all day gets a title of _Lever Engineer_? 😕 [/hijack] ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [May 14, 2002, 10:46pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/27 "2002-05-14T22:46:45Z")
> [@](#): > > \*Originally posted by Exapno Mapcase \* > **[Olbers’] infinite universe is _static_. And his answer is completely correct. In a static universe the sky would indeed glow bright.** 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. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 14, 2002, 11:21pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/28 "2002-05-14T23:21:39Z")
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. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 14, 2002, 11:31pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/29 "2002-05-14T23:31:19Z")
Doing a little more searching I ran across [Olbers’ Paradox: A Review of Resolutions to This Paradox, by David Newton](http://ast.leeds.ac.uk/~knapp/olbers.pdf) 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**. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [May 15, 2002, 9:05pm UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/30 "2002-05-15T21:05:25Z")
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. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [May 16, 2002, 2:15am UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/31 "2002-05-16T02:15:44Z")
**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”): > [@](#): > > Olber, the anti-genius, made the foolish argument that our cube should be receiving an infinite amount of energy. 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](http://www.britannica.com/eb/article?eu=58381) [Eric Weisstein’s World of Astronomy](http://scienceworld.wolfram.com/astronomy/OlbersParadox.html) [Scientific American](http://www.sciam.com/askexpert/astronomy/astronomy33/astronomy33.html) [Encyclopedia.com](http://www.encyclopedia.com/html/O/Olbers-H.asp) ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 16, 2002, 3:27am UTC](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592/32 "2002-05-16T03:27:36Z")
> [@](#): > > \*Originally posted by 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.” > > \*\* 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? [Previous page](https://boards.straightdope.com/t/why-is-it-dark-at-night-revisited/108592.md?page=1)