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Name: Omar J.
Status: student
Age: 7
Location: N/A
Country: N/A
Date: 8/27/2004

Why is space dark?

- Hi Omar,

This is a good question, but, unfortunately, not very easy to answer.

To begin with, we should first ask: "Should we expect space *not* to be dark?" A guy named Olber thought so. He thought that if space was huge, and there were lots and lots of stars in it, then, given time, the light from all those stars should make space bright. --But we know it is not, so what is going on?

The answer is in the "if" part of Olber's statement. He assumed that space was huge with lots of stars (both true), but he also assumed that the light from all those stars would reach us here on Earth given time.

But the universe is not infinitely old, it is expanding, and stars do die out.

Because the universe is not infinitely old, that puts a limit to how long the light from all those stars can have travelled. So some of the light has not gotten here yet. Because stars do die out, that means that their light dies out too - which means that we may get the light from some stars at one time, and the light from some other stars at a different time, but not all the light of all the stars that everywhere put together. Finally, because the universe is expanding, the light gets weaker before it gets to us. All in all, this means that there is not enough light from all those stars to make any point in space bright.

Does that answer your question?

-Roberto Gregorious

Hi Omar!

A very good question you have asked.

We are able to see with our eyes because light enters our eyes. This light comes from something which makes light like the sun in our sky, or a flashlight.

Now, think of yourself inside a closet with a flashlight. If the flashlight is off, the closet is dark because there is no light reaching your eyes. If you turn the flashlight on and look directly into it, you see the light directly from the flashlight. If you turn the flashlight away from you, you no longer see the light directly, but you see it as it bounces off the closet walls and other objects in the closet (including you!) and reaches your eyes. If there was nothing for the light to bounce off of, you would not know if the flashlight was on or off!

When you look up into the night sky, you see the light from the stars (like looking into the flashlight). You can also see light which has bounced off of other objects such as our moon, or the other planets in our solar system. Unfortunately, the rest of space is pretty empty and although light is traveling through it, if it does not reach our eye we will see darkness.

Bob Hartwell

This is a very good question. But it does not have a simple answer, but here goes. The question you raise has a long history. It is called Obler's paradox. He was the first one to ask the question. He said that if the Universe is infinite the sky should always be bright. That obviously is not the case, so why? First, remember we "see" only a small sliver of the radiation coming to us from outer space. There are radio waves, infrared light (heat), visible light (that we see), ultraviolet light, and x-rays -- none of which we can see with our eyes. If all that "light" were added in the night sky would seem a lot brighter. In addition, even for visible light, our eyes have limited sensitivity. That is why astronomers make telescopes. If you look at the night sky with even with low power binoculars, a whole lot of stars will fill you field of view, and the sky looks a lot brighter. It is this combination of limited visible light according to its wavelength or frequency, and the limited sensitivity of our unaided eyes that makes the night sky look so dark.

Vince Calder

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