pariah

nononono, you misunderstand. i will give you a clear question that will answer my questions! Similiar to other one, but different in a way:

Two events occur, A and B.

They occur at the same time in a "cosmic sense", as in, not when they are seen by humans.

Event A occurs 100 billion miles away. Event B occurs 200 billion miles away. Do we see event A in half the time it takes us to see event B?

Thanks! You confused me, b/c registering with the brain is infinitesimally small but light speed implies varying distance (to me), so...

jayh

That does not solve all the puzzle, because the effective scanned area within any patch of sky increases at the same square rate... i.e. twice as far away yields 1/4 the light per star but packs 4 times as many stars into the view. Theoretically in a nearly infinite universe, a star 'should' be at every direction, making the night sky incredibly bright. That does not happen.

Interstellar dust is a major reason (far more matter is in dark dust than in active stars), as well as the expansion of the universe. Perhaps there are other factors as well.

Shadowy Man

The Magellanic Clouds are closer.

And yes, dust obscuration plays a very important role in our Galaxy. If you've ever seen a picture, or taken a good look in a dark place, at the Galactic center you can tell how much starlight is blocked out from view. You are essentially looking through the disk of the Galaxy and thus are maximizing the dust attenuation.

At least at visible wavelengths. As you move toward infrared wavelengths the dust becomes more transparent (until you get far enough into the infrared where the dust actually emits light).

Asha'man

The “speed of sight” is exactly the speed of light, minus a little bit for neurons and processing inside the brain.

When you see a nearby galaxy, 3 million light-years away, you are actually seeing the light that left 3 million years ago. The more distant things you look at, the further back into history you are looking. If you look at a more distant galaxy, 100 million light years away, you can only see it as it looked 100 million years ago. For all we know, aliens bulldozed it to make an interstellar bypass, and we wouldn’t see the change for millions of years.

That is one of the neat things about astronomy: we can get an idea of what the distant past looked like by trying to look at things that are very far away. We can directly observe the past.

So, to answer your question, Event A becomes visible to us sooner than Event B, simply because the light only has to travel half as far.

Asha'man

Dark Matter is matter that has a gravitational effect, but we aren't able to detect it using visual methods. We don't know for sure that Dark Matter exists, or what it is, but it seems like the most reasonable explanation for what we do see.

We know there is matter out there by it's gravitational interaction with galaxies. We can examine the orbits of stars around the galactic core and estimate the amount and distribution of mass in the galaxy. We can also look at the visual size of the galaxy and estimate the number of stars and the amount of dust. (We can "see" dust, since it actually blocks light from things behind it, producing a shadow.)

These two estimates just don't match each other, and the difference is huge. Somehow, there is more matter out there that we just can't see. Therefore, we call it "Dark Matter," and speculate about what it is and where it came from. Dark Matter is more a name for an observation than it is a name for something specific.

Quantum Ninja

Well, if you want to get technical, the Sagittarius Dwarf Elliptical Galaxy is our closest neighbor. Ok, so I slipped. Still, relatively speaking, Andromeda is one of the closer galaxies to us.

EDIT: Removed a reply to a quote because I felt it was unnecessary.

HeatherD

Apparently the interstellar dust theory has been discounted: Why is the night sky dark? Look at the bottom of the article for additional links that are a bit more detailed.

Do a search for Olbers' Paradox for more articles on it, although this one, on the Wolfram website, summarises it well: Olbers' Paradox

RoddyM

I thought that the reason we do not see a totally lit sky is that most of the stars are very distant. And that very distant stars are travelling very fast away from us (and any other point of space). The wavelength of their light is no longer visible. Maybe the most distant stars from earth are just about lightspeed relative to earth and about to vanish from "our bubble" ie their radiation would have infinite wavelength and zero amplitude.

Something like that.

Gawdawful

Besides dark matter and interstellar dust, there is the possibility that space (the void) is infinite but the universe is finite, expanding like it is now and contracting sometime in the future and perhaps in the distant past. Common question: If the universe is finite what is outside the universe? The most common answer I've heard is that we don't know. How about nothing (infinite void or space)? Sure would explain why we don't have infinite amounts of light from the stars.


Warren in Oklahoma

triplew00t

Simple answer, pariah. If one event occurs half as far away in space than another, then we see if twice as soon.

Event A occurs at 8 million light years away from us, Event B occurs at 16 million light years away from us. They occur simultaneously in cosmic time. Say, NOW. At the same moment. If will be 8 million years before we, from our original observation point can see event A, and 16 million years before we can see event B. The reason we can see the stars now is simply because they have been casting light for so many millions of years that the light is reaching us from millions of years ago now. If one of them went supernova, we would not know it until millions of years later. Its very likely that many of the stars we see now are already dead and gone, and many new stars are casting light that has not yet reached us, so we know nothing of their existance. In short, the further away an object is from us in space, the further into the past we are seeing.

Nero