I was not certain where to post this but I figured here would be best. I came across this upon a web page:


Question - If light travels through space, why is space dark?
This is a question that has been around for centuries. It is attributed to
an astronomer Heinrich Olbers, and bears the name Olbers' (sometimes called
Olber's) paradox. Your inquiry motivated me to do a web search -- I thought
the answer was simple, I was missing something obvious, and a quick
refresher would supply a profoundly simple answer. I was badly mistaken!

The paradox arises by assuming 1. The universe is an infinite Euclidian
space. 2. The age of the universe is infinite. 3. Matter is uniformly
distributed. 4. The universe is static.

The terms "infinite" and "static" not literally, but sufficiently large to
be approximated by these terms. None of these assumptions is exactly true
and some "explanations" just say the assumptions are wrong therefore there
is no paradox. Sorry, but that is just avoiding the issue. It is clear that
we do not "see" all the radiation in the universe and if we could see
everything from cosmic rays to microwaves, the sky would be uniformly
bright, but that too is only a conjecture because at the present time we
cannot "see" all the radiation in the universe at all wavelengths
(energies). Ronald Koster has proposed a resolution which may be correct,
that says that we are shielded from radiation originating very far away and
gives a simple calculation that he contends resolves the paradox, but I'm
not an astrophysicist, so I am not able to assess the correctness of his
arguments.

I will be interested to see what other Newton BBS responders have to say
about this, too.
I do not think the answer is simple.

Vince Calder
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One day while lying in the cafe at my job after I had finished working it came to me, perhaps the answer is alot more simpler then we like to admit. So I thought why is it that in the warehouse there is light a plenty as well as in the cafe yet the cafe is so much brighter. Could it be cause the cafe is smaller there by less time for the light to travel...lol, ok that was a joke, but seriously could it not be that the reason it is dark in space and say light here on earth simply because of reflection? In space there isn't too much in the way of reflecting the light that is to illuminate an area. I sure if we surrounded the sun in whats that a dison sphere, (I saw that on Star Trex the next Generation) that we would illuminate the solarsystem quite nicely. The smaller that dison sphere would be the brighter it would be and or the more objects placed within that area the brighter it would be, hmmm?? Thoughts....

The one and only Eyajwhynsos

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The limits limitless...."prove it?" is what is said

well take a half of a step of each step taken in life, and be living to you say when....

I think the simplest explanation of Olber's Paradox is in the framing of the question: Infinity.
If the universe is finite in size but infinite in age, then you get the paradox as stated. If the universe isn't inifinite in age, then this "problem" goes away.
The universe, as best we can tell currently, is about 13.7 billion years old, give or take 0.2 billion years.
My simple calculation, being that I'm not very good with math, shows that 13.7 billion +- 0.2 isn't close at all to infinity.

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Feynman
>~~~~<
Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.

Religion is a culture of faith; science is a culture of doubt.

If the universe isn't infinite in age, how exactly does the "problem" go away? I mean now I gotta play my side for a bit, but if there is nothing to reflect light off of, then all you have in the case of outer space (the universe) are but points of lights. Even if these points of light(stars) are very close to each other(galaxies), you still would have and do have a very dark surrounding area. Take for instance a single light bulb hanging in a large room, the larger the room is the less lit it would appear, the smaller the room is the more lit it would appear, yet if there were no walls or any type of surrounding like you have out there in space what would the light reflect off of? Well I really am only theorizing, does anyone have an alternative to either one of these post?

The one and only Eyajwhynsos

__________________
The limits limitless....&quot;prove it?&quot; is what is said

well take a half of a step of each step taken in life, and be living to you say when....

I think the theory goes something like this: If the universe is infinite in size with an infinite number of stars, every point in the night sky you look at will have a star or galaxy in it, although some will be closer and others will be much further away.

But if it’s not infinite, you can look at a lot of points in the night sky and see nothing at all.

Because space is empty.

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I read somewhere that it has something to do with expansion...

Unfortunately, I forgot both the details of why that is, and the name of the book where I read that. -_-

Are you talking about the fact that the space that we can see is filled with a lot of stars, and they are emitting light in all directions, so why is the sky black?

We can’t see light rays from the side. They can pass right past you, in front of you, and you can’t see them. They’ve got to enter your eye directly in a straight line.

It like you can’t see laser light from the side. If you blow smoke at laser light, you will see the beam appear to be “lit up” but that’s because part of the laser light hits the smoke particles and some of that light is reflected by the smoke particles directly into your eyes. You can’t see light from the side. So all those light beams zooming around in space can not be seen unless they travel directly into your eyes. This leaves the background of the stars totally black.

http://en.wikipedia.org/wiki/Olbers%...ed_explanation

Not sure how accurate it is, but I'm sure someone could correct or clarify some points.

Just in case anyone is curious about Dyson spheres.

http://users.rcn.com/jasp.javanet/dyson/

Well of course the other things is that we've found that areas of space that appear "empty" aren't when you have powerful enough optics. Often the number of photons of light are just so spread out that only a few reach us and so we need super optics to see anything there. Take the Hubble deep field for example. Red shifting of light into the infra-red spectrum could also account for some of it. too.

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It must be fun to lead a life completely unburdened by reality. --- JayUtah

You can't reason an irrational person out of an irrational belief. --- Noclevername

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Enter the World of Athran

Halcyon Dayz says: Because space is empty.


My sentiments exactly, not much to reflect off of.

__________________
The limits limitless....&quot;prove it?&quot; is what is said

well take a half of a step of each step taken in life, and be living to you say when....

a lot of material shows up in the infrared...(at least within our galaxy)?

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image centred on draco

draco

240 micron (infrared) sfd
100 micron (infrared) iras
60 micron (infrared) iras

60 * 45 degrees fov
galactic/gnomonic projection
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image centred on m45

m45

240 micron (infrared) sfd
100 micron (infrared) iras
60 micron (infrared) iras

20 * 15 dg fov
galactic/gnomonic projection

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image centred on cygnus-a (invisible in this shot)

cygnus-a

240 micron (infrared) sfd
100 micron (infrared) iras
60 micron (infrared) iras

30 * 22.5 dg fov
galactic/gnomonic projection
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image centred on iris nebula

iris nebula

red = 100 micron (infrared) iras
green = 60 micron (infrared) iras
blue = 25 micron (infrared) iras

30 * 22.5 dg fov
galactic/gnomonic projection
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below lockman's hole

red = 100 micron (infrared dust) iras
green = ?nanometre (ultra violet) ?
blue = 3/4 kev (xray) rosat

60? * 45? dg fov
galactic/gnomonic projection
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images best viewed as desktop wallpaper.

This seems to be correct, and it seems possible to me that the universe could be “infinite”, but there could be a viewing-distance limit where the incoming photons from a distant galaxy are so spread apart by the time they reach us, we can’t see them. This might allow for a dark sky in an infinite universe filled with an infinite number of galaxies, so this phenomenon could overrule Obler’s Paradox.

The universe, in my opinion is most probably infinite, but it's not infinitely old (not an opinion, but based on evidence).
You don't need a viewing limit where the incoming photons "spread out", the viewing limit is that they are farther away in light years then the universe is old, and hasn't reached us yet.

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"Getting lost is part of exploring." Uniqua in "Backyardigans-Heart of the Jungle"

the universe is dark but isn't dark;

of course if we had the capacity, as Humans, to see ALL wave-lengths of the electromagnetic spectrum at the same time, we'd be blinded!!

or could we adapt over thousands of millennium to see it all, as a whole, the Universe i mean, its electromagnetic spectrum. hmmm......

Olber’s paradox assumes a more or less uniform distribution of stars in space. Think of the stars we see as distributed around us in nested thin spherical shells. Although the light flux reaching us from any star in a shell is inversely proportional to the square of the radius of the shell, the number of stars in a shell is directly proportional to the square of the shell radius. Therefore the total light flux from a shell is independent of the shell radius. With a sufficient number of shells the total flux reaching us should produce a night sky as bright as day.

It seems that the fundamental error in Olber’s model is the assumption that stars are uniformly distributed in space. Stars are not uniformly distributed in galaxies. Stars are even less uniformly distributed in galaxy clusters and still less in superclusters. The larger the cosmological object we observe the smaller is its spatial density of stars. The spatial density of stars in a galaxy cluster is orders of magnitude smaller than the spatial density of stars in a galaxy. The voids between galaxies in a galaxy cluster are much larger than the galaxies. Every step up the cosmological structure ladder introduces voids much larger than the objects being separated. The cosmological structure ladder assures a non-uniform distribution of stars in space.

Will this resolution of Olber’s paradox draw fire from Cosmological Principle of Homogeneity supporters?

Why?

I mean, by how much does the total, integrated energy of all EM, received here on Earth, exceed that which our eyes are sensitive to?

First, spatial inhomogeneity, in a static, infinite universe, doesn't necessarily resolve Olbers' paradox - pick a line of sight, any line of sight. Now, for a static, infinite universe, with spatial inhomogeneity, outline how such a line of sight cannot, ever, intersect with the surface of a star (intersecting with a cloud of gas or dust doesn't help; such thing will be, in a static, infinite universe, in thermal equilibrium with the incoming EM ... to them).

Second, the universe's observed spatial inhomogeneity declines as the scales get bigger. IOW, as you take larger and larger 'chunks' of the universe, it gets more and more homogeneous.

Nereid wrote:The issue in Olber’s paradox is not the mere fact that every line of sight in an infinite universe would end on a star. Olber’s paradox requires that the spatial density of stars should not decrease with distance from the observer, otherwise the inverse square law might overwhelm the light flux per shell and result in a finite, small summed series of flux contributions from an infinite number of shells. Such a sum could not produce Olber’s nighttime sky as bright as the sun.

Heinrich Olber wrote about the paradox in 1826. At that time the local galaxy was thought to be the universe. Every improvement in telescopes revealed more stars in previously dark spaces. The spatial distribution of stars in the local galaxy was not known. Hence the assumption that on a large scale stars might be uniformly distributed in space. After Hubble proved that the Andromeda galaxy was a great collection of stars very distant from the local galaxy, the universe was thought to be a great sea of galaxies. It was then thought that on a large enough scale galaxies would be uniformly distributed in space. Later it was discovered that galaxies clumped together as clusters and that clusters clumped together as superclusters.

Whether the inverse square law wins or not depends on the spatial distribution of stars as a function of distance from the observer. We are about 2/3 out on one of the spiral arms of our galaxy. The nearest star in the night sky is Alpha Centauri at a distance of 4-1/3 lightyears (LY). All the bright stars of the night sky are in our neighborhood of the local galaxy. Since our galaxy is about 100,000 LY in diameter the inverse square law reduces most of its light to the diffuse, dim light of the Milky Way. The light contributions of the few dwarf galaxy satellites of the local galaxy can be neglected.

The nearest member of the local group of galaxies is the Magellanic Cloud (MC) at a distance of 179,000 LY. The local group has a diameter of about 10,000,000 LY. The Andromeda galaxy (M31), a large neighbor in the local group, is at a distance of 2,300,000 LY. M31 appears to the naked eye as a patch of very dim light. (How about the recent stunning Spitzer picture of M31 and its dust!)_ Other neighbors are at comparable or greater distances. The farthest member of the group is Sextans A, at a distance of about 4,730,999 LY.

Let us examine the implications of the data in the preceding two paragraphs. The densely star populated region of space nearest to us is the local galaxy. The light it contributes ranges from bright for close stars to very faint over a distance of 100,000 LY. The next dense collection of stars, MC, is 179,000 LY away. Its light, using the inverse square law, is about 8/100 as intense as the faint Milky Way. Keep in mind that the local galaxy and MC are not shells of stars surrounding us; they occupy only a small portion of the sphere of space out to their distance. The light from Sextans A, at the far side of the local group, is about 1 /10,000 as intense as the faint Milky Way. The local group, consisting of some 30 galaxies, also is not a shell of stars surrounding us. The first 4,730,999 LY radius volume of space shows an enormous decrease in star density of space with distance. If we take the light of the local galaxy as our unit candle (1 candle), then it can be readily shown that light from the remaining members of the local group contributes only 0.135 candle.

In order to produce a night sky as bright as the sun, the spatial density of stars would have to start to increase enormously further out to compensate for the density decreases so far, and for the next inverse square law reductions. At what distance might that occur? It will not occur at the next step up the structure ladder. That next step, the local supercluster, introduces voids larger than the clusters. Star density drops again, and that brings us to distances where the inverse square law really gets important.. As for the other superclusters, enormous voids are between them. Distances get into the billion LY range with inverse square law reductions that really take a toll on the light we receive from out there. It is not likely that superclusters are ultimate structures, or building blocks of the universe. As Bruce medalist astronomer C. V. L. Charlier first suggested, matter is clumped in space at all scales. Clumping introduces voids.

Nereid also wrote: Even if such increasing homogeneity were true (which it may not be), it is irrelevant to the Olber’s problem. To say that the further out you look the more homogeneous it seems does not mean that most of the light received on earth comes from those distant homogeneous seeming regions. The light from the nearest regions, which is least diminished by the inverse square law, comes from a very heterogeneous distribution of stars or galaxies. Even if the most distant regions seem more homogeneous, that does not preclude the possibility that densities might decrease beyond those regions.

In The Density field of the local Universe by Will Saunders, Carlos Frenk, Michael Rowan-Robinson, George Efstathiou, Andy Lawrence, Nick Kaiser, Richard Ellis, John Crawford, Xiao-Yang Xia, & Ian Parry, Nature, vol 349, 3 January 1991, page 32 data from the Infrared Astronomical Satellite were used to map a part of the Universe on a whole sky basis. In addition to the previously well known 19 superclusters and 11 voids, 8 new superclusters and 4 new voids were identified. The following parenthesized phrase appears in the abstract of the article: “(the voids are larger than the superclusters but depart less from the mean density).”

In #16 I wrote That is true for an atom, for the solar system, for a galaxy, for a galaxy cluster and for a supercluster. That introduction of voids means that density drops dramatically at every step up the ladder. The local galaxy is much less dense than the star systems comprising it. A galaxy cluster is much less dense than the galaxies comprising it. A supercluster is much less dense than the clusters comprising it. An ultracluster, if there were such a thing, would be much less dense than the superclusters comprising it. As you take larger and larger ‘chunks’ of the universe the spatial density of stars decreases ever more dramatically.

The ‘Density field …’ article gives the mass densities of the identified superclusters and voids. The 27 supercluster densities range from 0.13 to 5.49. The 15 void densities range from 0.00 to 0.41. Even at that scale things look far from homogeneous.

Another problem for the Cosmological Principle of Homogeneity is that with every step up the cosmological ladder the membership is less than that at the preceding step. There are fewer stars than atoms, fewer clusters than galaxies, fewer superclusters than clusters, and so on. Considering the sample size, how can we know that the range of superclusters presently identified is representative of those that are still unknown? And how about the density at the next step up the ladder, or is there some reason that either superclusters or the next higher structures are ultimate structures, i.e. ‘building blocks of the universe’?