• Question: why is it dark in space if there are so many stars giving off light?

    Asked by shrike to Leo on 13 Jun 2010 in Categories: .
    • Photo: Leo Garcia

      Leo Garcia answered on 13 Jun 2010:

      Now that IS an amazing question. And one which confused some very clever people for a very long time. There are billions upon billions of stars surrounding us in our galaxy and in galaxies far, far away (cheap Star Wars reference) with nothing between us and them except a transparent vacuum. So why aren’t we blinded by bright, constant star-light all the time – and why is the night sky (when the sun isn’t shining on us) actually very dark? This question actually has a name: Olbers’ paradox, although Heinrich Olber apparently was not the first person to describe the problem (http://en.wikipedia.org/wiki/Olbers%27_paradox).

      Let’s think of it like this: If this universe in infinitely old, and there were an infinite number of stars within it, then we could assume that everywhere you looked into the sky, your line of sight would eventually fall on a star somewhere. Under these assumptions, the sky would be infinitely bright. Of course, though, it isn’t. Why?

      First, the universe isn’t infinitely old – it is, according to our best calculations, around 14 billion years old. And so we can only see objects that are 14 billion (or less) light years away – because the speed of light is the fastest that anything in the universe (including light from stars) can travel. This is smaller than the known universe – which is currently thought to be at least 93 billion light years in diameter (http://en.wikipedia.org/wiki/Universe). Remember that a light year is the DISTANCE (not time) that light travels in a year. And it travels at 3 x 10^8 metres per second – so a light year is a long way!

      So – we can only see a portion of the total light of all the stars in the universe. That’s that dealt with.

      Add to this that the universe isn’t standing still – it is expanding! We know this from observing a phenomenon known as Doppler shift (http://www.astro.ucla.edu/~wright/doppler.htm) in starlight. This means that, over time, the amount of light energy reaching the earth is decreasing. These two effects, of a universe which isn’t infinitely old, and is expanding away from us in all directions, resolve the paradox. The story doesn’t end there, though. In fact, there are many other proposed solutions to the paradox (http://www.mathpages.com/home/kmath141/kmath141.htm) but, I think, the two I stated above are the most common ways of resolving it.

      But that’s the kind of question which really drives scientific thought! And isn’t it nice to know that some of the most amazing minds in the history of mankind also came up with that question? AND they didn’t know the answer!

      In science, the most simple questions which we can’t answer are the ones which reveal the most about the current state of knowledge. And being a scientist means confronting those questions head on and finding out their answers. It sounds like you are already well on the way to becoming a great scientist!