Why isn't the night sky completely lit up with all-white light?

Originally posted by NewAgeMan


Why does light, in the vacuum of space follow the inverse square law - what's stopping or limiting the photons according to that law, why don't the photons just continue on..?

The inverse square law is about the spreading out of energy as it propagates. Take a look at this diagram, it should answer your question:

Originally posted by NewAgeMan
Ok forget about the light - there are 10,000 galaxies on average in every square millimeter of the sky, all around the sphere of the earth, for which there are something like 10,000,000 or more.. such points, that's what we're really in the middle of, aside from our own galaxy, just think about it - anyone's who's not awed by that doesn't have their head screwed on right.

Forget about the light? That's the topic of this thread, isn't it?

Of course I'm in awe of how much stuff is in the universe, but there's something else we can also be in awe of.

Despite the fact that there are billions and billions of galaxies each with billions and billions of stars, the size of the universe is so vast that the cosmos is still relatively empty.

So while we should be awed by the vast numbers of stars, perhaps we should be even more in awe of the vast distances between them, which brings us back to the light, or the lack of it as seen by the human eye. As vast as the distances between stars are (the nearest star to us is roughly 5 light years away), the distances to galaxies are even more vast (Andromeda, our neighboring galaxy, is 2 million light years away.) And the distances between superclusters of galaxies are even more vast.

reply to post by Arbitrageur


That's true, and you were right about the amount of blackness in the Hubble Deep field.

Originally posted by Arbitrageur
reply to post by NewAgeMan

 

Even with a long exposure like the Hubble Deep field, you can see there is black between the light parts.

Also, for Olber's paradox, an inquiry occuring as recently as the late 1970's did not have access to the Hubble Ultra Deep Field image.

Yes, the space between stars, and between galxies, even within galactic clusters is truly mind numbing.

My I'm just glad that there's something and not nothing at all. It's better this way imho.

reply to post by john_bmth


Thanks John, that makes sense.

reply to post by NewAgeMan


Thanks for the reply. Yeah I already knew about this, but for some reason it didn't come to my mind when I posted.

Originally posted by NewAgeMan

Originally posted by elfie
Olber's paradox

A technical explanation.

The inverse square law, as stated above, is also a good explanation.

Thanks for this, at least someone understood the nature of the question posted in the OP.

Even still, you would think more starlight than that eminating from the closest stars would also be visible.. but instead all we have between the closest stars is the blackness of space.

If you stair in one spot for long enough you'll start to see more and more stars that had been "invisible" and "black" at a glance. I think it's very simply of the closes stars "over shadowing" the other stars with their immense brightness. The other stars "pale in comparison" to the star.

It's just like the light of a flashlight becoming invisible in a very bright room, or out doors during daylight. The dim light of the flash light is whitewashed by all the noise from the much more bright sun.

The Wikipedia article explains it pretty well.

Olbers described the paradox nearly 200 years ago. He assumed the universe was infinitely large and infinitely old and populated uniformly by a constant density of stars at the largest scale. It did not occur to him that space might be expanding. Simple high school math demonstrates that those assumptions lead to a paradox; the night sky should be as bright as the average surface of a star. His conclusion was that the universe could not be infinitely large.

Olbers did take into account the inverse square dimming of light. So that is not the solution.


If the universe is infinitely old and not expanding, you can get the right brightness by plugging in a small enough size. (Similarly, you could assume that the density of stars per unit volume decreases with distance; we happen to be in the only pocket of densely populated space.)

If the universe is infinitely large and not expanding, you can get the right brightness by plugging in a small enough age. Light from beyond a certain distance hasn't had time to reach us yet.

If the universe is finite in both size and age, you can get the right brightness by plugging in a large enough expansion factor (Hubble parameter). The greater the expansion rate, the smaller the observable universe. At the present measured rate (about 2.5 x 10^-18/s), the observable universe is about 13.7 billion ly across. Even if the universe were infinitely old, light from beyond that distance could never reach us. Light originated near the edge of the observable universe is also extremely redshifted.

The correct solution is a combination of finite size, finite age and expansion of space.

if you stand on mount everest and look up.
you will not belive how many stars you can see. at night!

in the city the sky is Full of all kinds of dust and pollution.
what makes it wors is the Light pollution from the ground.
the light reflects of the ground in the sky.

if you ware in space you could see even more stars.
thats why big telescopes are high up and far from citys.

For what it's worth, I have never been convinced that there is a paradox at all. Most of the objects in the night sky are far too distant to be detected with the naked eye. You do the maths.

Originally posted by NewAgeMan
Why does light, in the vacuum of space follow the inverse square law - what's stopping or limiting the photons according to that law, why don't the photons just continue on..?

They do continue on. There's just not enough of them reaching a particular point in space at that distance to be detectable. The number of photons given off by a star is enormous, but its still finite and they're traveling in all directions, not just at your eyeball.

Consider a star just 100 light years away. Think of space at that distance as a sphere around the star (a sphere, since any given photon can be emitted in any direction from the star). What percentage of the surface area of that sphere does the side of the Earth facing that star consist of? And what about your eyeball? Its ridiculously tiny. That star has to emit an absolutely insane number of photons just to hit the Earth with one of them, and far, far more to actually hit your eye. What are the odds of that happening for a star or even a galaxy a billion light years away?

There's the problem. Those photons are still there, but since they can be emitted in any direction from the star, they're spread out over an enormous area, and the odds of them hitting your eye are ridiculously small. There's not enough of them that make it to your eye to activate the light receptors in your retina.

reply to post by NewAgeMan


heck yeah but i like his later stuff more