• Registered: 2005-06-26

Topic: Olbers' Paradox

Has anyone here ever pondered Olbers' Paradox or come up with a plausible explanation for why the night sky is dark? Just curious. To be honest, I never thought about it before, but saw it on Ellie Crystal's site today and now I'm wondering about it.
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Olber's Paradox: Why is the Night Sky Dark?
Suppose that the universe is (1) static, (2) infinite, (3) eternal and (4) uniformly filled with stars (or galaxies, which are made of stars). If we look in any direction, our line of sight must eventually run into a star (galaxy), just as a frictionless arrow shot in the middle of an infinite forest will eventually stick itself into a tree. Therefore, the night sky should be as bright as the average star (galaxy) and certainly should not be dark.

Of course, most distant stars will appear to be fainter than nearby ones; their brightness will fall off as 1/d2, where d is their distance. But, at the same time, there will be many more stars at larger distances than smaller ones, and the number at the larger distance will increase with d2 so that the two effects should cancel.


Basically, the flux arising from all the stars in a given shell, is equal to the product of the flux arising from an average star at that distance times the number of stars per shell. If the density of stars in constant (assumption 4 above), then the number of stars in a shell is just the number of stars in the shell times the volume of the shell:

http://astrosun2.astro.cornell.edu/academics/courses/astro201/images/olber_eq.gif

where t is the thickness of successive shells, and n is the number density of stars per unit volume. Therefore the flux arising from each shell is constant, that is, stars at any given distance from us contribute the same amount of flux as stars at any other distance. In every direction our line of sight should intercept the surface of a star and thus the sky should be bright (and the universe should be warm!).

http://astrosun2.astro.cornell.edu/acad … aradox.htm

"The most important decision you have to make is whether you live in a hostile or friendly universe."
~ Albert Einstein

The real voyage of discovery consists not in seeking new landscapes, but in having new eyes. ~Marcel Proust

The evolution of humanity is an evolution of the heart. The path is through the heart.
  • Registered: 2005-06-26

Re: Olbers' Paradox

I guess my first trouble is why do we assume the universe is "static"? I must not be understanding the definition of that term properly, I'm thinking.

I'm no scientist, by any means, but I've seen some amazing work by a few here, so I thought I'd ask about this. smile

"The most important decision you have to make is whether you live in a hostile or friendly universe."
~ Albert Einstein

The real voyage of discovery consists not in seeking new landscapes, but in having new eyes. ~Marcel Proust

The evolution of humanity is an evolution of the heart. The path is through the heart.
  • Registered: 2005-06-17

Re: Olbers' Paradox

Because the moon is a Death Star sucking all the light into it to charge up it's Star Destroyer weapon by using an artificial black hole.

I'd explain it in more detail, but since you say you're no scientist, all the complexities will sail over your head.

  • Registered: 2005-06-26

Re: Olbers' Paradox

neutral....

What?

big_smile

"The most important decision you have to make is whether you live in a hostile or friendly universe."
~ Albert Einstein

The real voyage of discovery consists not in seeking new landscapes, but in having new eyes. ~Marcel Proust

The evolution of humanity is an evolution of the heart. The path is through the heart.

5 Reply by impatiens (edited by impatiens 2005-07-20 12:30:10)

  • impatiens
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Re: Olbers' Paradox

In the middle of the day when the sky is blue,
the stars are still there,
just hidden from view.

But, at night when darkness falls,
they reappear,
we can see them all.

Myopic during the light of day?
But in darkness we can see light-years away?
It's an optical "illusion", that's all I can say!

-----

Ergo it's (sun)light which obscures our view? Thought it was the opposite. Light refraction and all that jazz. But, hey what do I know. Nada.

The Moon as a Death Star, eh? Sure, I'd buy that. Why not! wink

  • lyra
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Re: Olbers' Paradox

All I know is that regarding this very topic, about why is the sky black at night, I was told by somebody that space isn't what they're telling us it is.  That if you built a spaceship and went up there yourself, and got far enough from Earth, things would look different than you could ever imagine.

Don't know if it's true or not, but considering the other things this person has told me, most of which have proven to be true in some form or another as the years wear on, I do have to wonder.  The conviction with which it was said....I don't know, it still gets me when I think back on it.

I'm left with something the C's once said when Laura asked them how big the universe really is.  They said, "If we answered that it would be like telling a 4 year old child that there is no Santa Claus."

Read between the lines on that one, and add in the stuff I mentioned before.   It makes you really wonder about the nature of outer space.

"Life's journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways, totally worn out, shouting "Holy shit ... what a ride!"  - Anonymous
-----
"I get by with a little help from my (higher density) friends."
-----

lyra's Website

  • From: NOTHING
  • Registered: 2004-09-03

Re: Olbers' Paradox

lyra wrote:

I was told by somebody that space isn't what they're telling us it is.  That if you built a spaceship and went up there yourself, and got far enough from Earth, things would look different than you could ever imagine.

BINGO! Now we are getting somewhere. Of course they're lying about space! Just like they lie about everything else! Was it really that much of a suprise? big_smile

Ever see Contact? I had wished that representation of the vast beauty of space was accurate somehow.

Here's one - How is it threre are no stars in space when seen from video from the lunar landings? DX Too advanced for that level of special effects? I'm guessing they will say that the 'exposure' of the camera didn't allow it, etc. . . .

I am as is Void.
  • montalk
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Re: Olbers' Paradox

After sketching out the paradox and pondering for a while, a possible solution came to me.

The paradox is correct that the amount of photons reaching us does not change with the shell radius. Whether we consider stars ten light years or a million light years away, each shell sends us photons in the amount of 4*PI*I photons, where I is the average number of photons the average star radiates.

However, there is more to eyesight than just the density of photons passing through our pupils. What also matters is the angular spread or focus of these light rays.  The more diffuse the rays, the more a given quantity of photons is spread out over the retina and the fewer photons each cone or rod receives. Beneath a certain threshold the cones or rods do not fire and we see only darkness.

A star in the sky sends very little light our way. But the only reason we can see it is because the source of light is a pinpoint, meaning its angular spread is very small. So despite the low number of photons passing through the pupil, they hit a small enough area of the retina that the rods or cones located there receive sufficient concentration of photons to trigger.

But if we spread the pinpoint of light into a uniformly glowing shell, which is what the paradox assumes through its calculations, then the number of photons hitting each rod or cone drops to beneath the detection threshold. Thus we see darkness.

So the universe does glow, but even with 4*PI*I photons the light is so spread out that our eyes do not pick up this glow.  It's like a magnifying glass focusing the sun's rays -- the same amount of light always passes through it, but depending on the diameter the focal point can be either diffuse or too bright to look at.

Can't find the link right now, but I once read that frog or toad eyes are so sensitive that they can see an infinite distance into the night sky which to them would look like a glowing field.

Acquiring fringe knowledge is like digging for diamonds in a mine field.

montalk's Website

9 Reply by Sowelu (edited by Sowelu 2005-07-20 17:50:22)

  • Registered: 2005-06-26

Re: Olbers' Paradox

Oh man, right on! I understood that, montalk! Awesome! smile (I'm consistently amazed at your ability to translate complex ideas into a form of language mainstream enough so that laymen can easily grasp at least the gist.)

Soooo.... what you've placed on the mirror staring me back in the face is that it's all about our perception - our ability, or lack thereof, to perceive What IS.

This reminds me (for some strange reason) of an experience I had about 5 years ago where I was meditating and suddenly had 360 degree vision. Everything was where it should be, no weirdness in the physical stuff around me (meaning the altered state was not all that altered, and I was still in my body), yet I could see all 360 degrees at once. Obviously this was not done with my physical eyes. But the point is, there was light when viewing this way, whereas if I were looking through my physical eyes, there would not have been any, because there was no physical source for the light.

So this whole dark night sky thing,  like seeing the day sky as blue instead of purple, may simply be due to limitations in our physical form's ability to process information, eh? Right on, montalk. You da man!

Thanks!!
Sowelu

"The most important decision you have to make is whether you live in a hostile or friendly universe."
~ Albert Einstein

The real voyage of discovery consists not in seeking new landscapes, but in having new eyes. ~Marcel Proust

The evolution of humanity is an evolution of the heart. The path is through the heart.

10 Reply by montalk (edited by montalk 2005-07-20 20:14:41)

  • montalk
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Re: Olbers' Paradox

Yeah perception is a funny thing, and the paradox shows that reality is more surprising and expansive than can be boxed in by mathematical equations. My explanation could be wrong, but it's the only one that I could come up with. At least it made sense, haha.

impatiens wrote:

In the middle of the day when the sky is blue,
the stars are still there,
just hidden from view.

That's a great analogy for so many things. "In the middle of trouble when you're feeling blue, hope is still there just hidden from view." Is it a matter of perception? If so, does that mean joy and sorrow, fortune and misfortune, are illusions because they are subjective?

Acquiring fringe knowledge is like digging for diamonds in a mine field.

montalk's Website

11 Reply by Human Being

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Re: Olbers' Paradox

Poe's cosmic insight predates Big Bang theory
Although known for his poetry, Edgar Allen Poe also proposed an early model of the universe

Sreejit Nair, Cavalier Daily Senior Writer, wrote:

Why is the sky dark at night? The obvious answer is that the sun lights up the sky during the day but is on the opposite side of the planet at night, leaving half of the world in darkness. Would this still be true if the universe were static, infinite and evenly filled with stars?

To answer this question, first place an observer somewhere on Earth.Though some stars would be more distant than others and correspondingly dimmer, there also would be more stars at these greater distances resulting in constant brightness regardless of where the observer is located. Not only would the person standing on the Earth see stars along any given line of sight, but the sky would be bright even at night!

This theory does not match reality, however, because the universe cannot meet the above description.This problem, known as Olber's paradox, led former University student Edgar Allen Poe to foresee some important developments in modern cosmology. Although Poe most often is recognized for his gift with the pen, he also was able to predict correctly that the universe was not static, but actually was increasing every second.

{snip}

The whole article is quite an entertaining read, and Poe's "Eureka" can be read here:

http://eserver.org/books/poe/eureka.html