10 November 2012
(Edit: I have added definition of light year and other stuff which I think may be of benefit for non-geeks.)
Have you noticed that during the day it is light out side and during the night (for most of us) it is dark?. You may have noticed that in all of the pictures and videos you see of space, it is also dark, regardless of time of 'day'? Why is that?. That is pretty strange.
Surely with a galaxy of stars, billions upon billions of them, there should be light in space, after all we get light all of the time. Although it is dark at night we get some light from our sun reflected by the moon (that is why you can see the moon at night, it does not glow in the dark).
If we take a look at an earth day. The sky is blue. Please go ahead and check this for scientific accuracy. Welcome back. This is caused when light from our sun hits molecules in our atmosphere. This causes the light to scatter everywhere and we see blue as a result. At night when we are not facing our sun it is dark or black because there is not enough light to scatter in our atmosphere to cause the blue effect. Understood?. Good. The moon has no atmosphere, so the light from the sun can not scatter. Hence the black you see in those famous pictures from Apollo landings, which were not faked.
So what about space, why is it black?. If you think about it, every bit of space is facing a light source, ie: stars, so space should be light, right?.
This is where it gets a bit complicated. Consider this, for light to get from our sun to earth it takes roughly eight minutes. Our sun is about 93 MILLION miles away. So the light we are getting now is eight minutes old. The universe was created about 14 billion years ago according to widely accepted theory and the furthest star from us is about 13 BILLION light years away from us, so consider how long it would take for light from the furthest star to get to us now.
A light year is a measurement of distance, not time. The speed of light is about 186,000 miles per second. A light year is about 6 TRILLION miles. Mind boggling vastness I am sure you will agree.
See, I am mixing time and distance, I told you it was complicated. Sorry. Stick with me for a few more moments.
So, it takes a heck of a long time for light to get to us. In theory, pretty much all of it has not reached us yet. Therefore we can not see it. It is not here so it can not illuminate space and our atmosphere at night. There is more. Space is expanding, moving away from us. When a source of light moves away, its wavelength alters. It is made longer. Due to this, what we see as visible light is lost, there is still light, there is still energy. The light shifts to red. So for example, infrared, the light that makes it possible to change channels on your TV. You can not see it. So then, we have a lot of 'light' that we can not see and a lot of light that hasn't arrived yet.
That is why space is dark... Or something like that.
For anyone that is interested.
What A Load Of Olbers • Opuss № I