This old question is based on the argument that the stars have been around for soooo long, and the universe has no heat sink (a place to dump the heat - like your A/C can dump heat outside your home), so (a) why hasn’t the universe reached thermal equilibrium already - and (b) the equilibrium temp would be ubiquious, of course by its very definition, and hot enough to glow in a homogeneous fashion to fill the night sky to be as bright as daylight.
However, isn’t this age-old question premised on the assumption (and belief at that time) that the universe is filled with ether? This is the only way in which the universe could reach thermal equilibirum - with some medium to to put everything in physical contact with each other - thus allowing conduction to facilitate thermal equilibrium of the universe?
Maybe I’m missing something here…the only other possible way this might happen is if every body in the heavens were a perfect black body, but not all heavenly bodies are…like earth, for one!
This classical question is treated as a sign of genius, but did the genius who first posed the question have all the facts?
It’s called Olber’s paradox. The paradox means that one or more of the assumptions to the argument is false.
The assumptions are:
The Universe is infinite in extent.
The Universe is fairly uniform in its distribution of matter.
The Universe is old.
Given these assumptions the amount of radiation from any equally sized shell of space surrounding the Earth is the same. Since the number of these equally sized shells is infinite (by (1)) the amount of radiation from any piece of the sky should be infinite.
The obvious failure of Olber’s paradox just means that at least one of those assumptions is false. The Big Whoosh (aka Big Bang) pretty much does away with numbers 1 and 2.
The Big Bang does not imply that the universe is finite in extent. Current thinking is that the universe is flat. A flat or negatively curved universe may be infinite. Olber’s paradox depends on a universe that has existed forever (or long enough to have reached thermal equilibrium). The Big Bang does away with this assumption.
I don’t see how the Big Bang does away with number 2. I believe the current thinking is that the Universe is fairly uniform—that is, there’s no discernable structure—on the largest scales.
How does the Big Bang theory not imply that the universe is finite? I thought the whole point of the BB was that the universe is expanding at a rapid, but finite, speed from a finite size.
“The Big Bang does not imply that the universe is finite in extent. Current thinking is that the universe is flat. A flat or negatively curved universe may be infinite.” – DrMatrix
There is a difference between infinite and open. A negatively curved universe is definitely the dominant view now, but that doesn’t mean that it is believed that there are an infinite number of stars.
Everything in the universe emits and absorbs radiation to some extent. If it does, it will exchange heat with the surroundings through radiative heat transfer. If an object is completely enclosed by a wall with uniform temperature, the object will reach the same temperature as the wall. Even if it’s not a perfect blackbody, the equilibrium temperature is the same.
Does that answer your question, or any part of it? I’m not entirely sure if I understood the question.
Olber’s paradox is bad science. Hardly the work of genius. It has been thoroughly debunked so many times, but like the little trick math puzzles, it keeps coming back. If one could do a practical search of the SDMB for this you’d find a power engineer who points out the specific error in Olber’s math.
To simply things: (ignore expansion, age, etc.) take a cube of universe maybe 100 million light years across. This cube is around 2.7 degrees Kelvin. Ditto its neighboring cubes, the neighbors’ neighbors, etc. By simple laws of thermodynamics each cube on average receives the same amount of energy from its neighbors as it gives off to its neighbors. So let’s line our cube with perfectly reflective foil and ignore the neighbors. For our cube to get much warmer than 2.7K, let alone daylight conditions, there would have to be something within our cube that would raise the temp. Stars do produce energy, but not nearly enough, even assuming perfect matter to fusion to make much a difference. (Keep in mind that space is quite nearly empty. Our little corner is a fluke.) Ergo, our cube will never get much hotter. (Throw in expansion and in fact it gets colder.)
Olber, the anti-genius, made the foolish argument that our cube should be receiving an infinite amount of energy. But that would mean we are giving off an infinite amount as well. A truly obvious flaw. You in no way shape or form have to make any assumptions about age, size, expansion, or curvature of the universe.
The point here is that Olber (and earlier others: he did not invent this paradox) was living in a time when the basic assumption was that the universe was infinite in expanse. It is hardly a foolish assumption that an infinite number of stars resided in such a universe. You can even make his argument simpler by saying that for each spot on the sky, an infinite number of stars must be contained in it. No matter their distance, they must add up to an infinite amount of light.
Olber’s paradox was therefore a way of telling people in his time that there must be something wrong in their assumptions, because they lead to conclusions that were demonstrably wrong. It is not much different from Zeno’s paradox. Both state that the cozy – and unexamined – notions of the world of their time must have flaws, and that no understanding could procede until those flaws were found.
In fact, Olber had a radical solution for the paradox. He thought that there must have been a time in the past when the stars had not yet started to shine.
That’s pretty sophisticated thinking, if you ask me, and not foolish at all.
It’s a paradox! You don’t “debunk” a paradox. The point of a paradox is to point out that a certain set of assumptions lead to an obviously incorrect conclusion. This indicates that there is an error in the assumptions. This particular paradox is still immensely useful in illustrating the difficulties of an infinite universe - it clarifies what theories needs to be able to explain, or a way to test new theories. Obviously the assumptions used in the Olbers paradox is wrong because it’s a paradox. But the interesting question is, which part of it is wrong??
Olber was just plain flat out wrong in his physics! Talk to a lighting engineer about how intensity decreases cubically not quadratically.
Achernar, all you need to know is that there is a finite amount of energy and mass that can be converted into energy in a cube to disprove that it can’t be infinite. You don’t need to know the exact population of the earth to know it’s not infinite.
Olber’s Bad Math is like the “taste map of the tongue”. Debunked over a hundred years ago but still in the textbooks.
Exapno Mapcase’s "You can even make his argument simpler by saying that for each spot on the sky, an infinite number of stars must be contained in it. No matter their distance, they must add up to an infinite amount of light. " is an incorrect simplification of an already incorrect argument. It’s doubly wrong. The thermodynamics explanation (which appears in the literature, it’s not mine) claerly points out how wrong this line of thinking is. You can use Exapno Mapcase’s argument to prove that the ocean must be self warming (just not to infinite temp.s). Take a cubic cm deep in the middle of the Pacific, it’s receiving radiation from nearby water molecules, so much from ones further away, so more from ones still further away, add it all up (wrongly, like Olber) and you get boiling water. But the Pacific doesn’t boil.
Not necesssarily finite. The following quote is by Victor F. Weisskopf from “The Origin of the Universe”, American Scientist September-Octover 1983 vol 71:
Yes, I know. I made the mistake of saying that flat implies infinite in another thread and was called on it by FriendRob. I should also mention that in an older thread Chronos noted that there are finite negatively curved topologies. A flat or negatively curved universe may be infinte or finite. In my defense, Edwin Hubble made the same mistake in “Our Sample of the Universe”, Scientific Monthly December 1937:
If the universe is infinite, I’m pretty sure it does follow that there are an infinite number of stars.
How was Olber expected to know this in 1826? If the stars were lamps, plugged into God’s own outlets, there would be no reason to expect they would have to go out.
I’d be interested in seeing a cite for this. Perhaps lighting engineers have some other effects to take into account when designing light for rooms. From Gravitation and Cosmology by Steven Weinberg, “the apparent luminosity of a star of absolute luminosity L at a distance r in a naive cosmological model will be L/4[symbol]p[/symbol]r[sup]2[/sup]” where naive model means “the universe is supposed to be infinite, eternal, and Euclidean, and the stars are more or less at rest, with constant average luminosity per unit volume”. i.e., it decreases quadratically.
Huh? Light intensity is inversely proportional to the square of distance. How can it be anything else? If you have a point source enclosed by a spherical shell, all the light from that point source falls on the spherical shell. The area of the spherical shell is proportional to the square of radius, so intensity (amount of energy per unit area) must fall as inverse square, if energy is to be conserved.
I’ve asked before, and I ask again: please give me the reference for this paper you keep referring to. I promise I will read it and report on it.
That’s the simplified textbook version. We know stars have finite size, so clearly stars block each other’s radiation and the universe will not reach infinite temperatures. 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.
By the way it’s Olbers’ Paradox, not Olber’s. Named after Heinrich Wilhelm Olbers.
Just about everything I know about science I learned from Issac Asimov. He said that the reason that the sky is not bright with light is because the universe is expanding.
I think maybe we’re having a disagreement on just what Olber’s Paradox is. Here’s a quote from Fundamental Astronomy (Kartunnen et al., 1993):
The paradox is the following: Let us suppose the Universe is infinite and that the stars are uniformly distributed in space. No matter in what direction one looks, sooner or later the line of sight will encounter the surface of a star. Since the surface brightness does not depend on distance, each point in the sky should appear to be as bright as the surface of the Sun.
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?
I’d be interested in seeing a cite for this. Perhaps lighting engineers have some other effects to take into account when designing light for rooms. From Gravitation and Cosmology by Steven Weinberg, “the apparent luminosity of a star of absolute luminosity L at a distance r in a naive cosmological model will be L/4[symbol]p[/symbol]r[sup]2[/sup]” where naive model means “the universe is supposed to be infinite, eternal, and Euclidean, and the stars are more or less at rest, with constant average luminosity per unit volume”. i.e., it decreases quadratically.