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More Little Questions

September 12, 2012 Leave a comment Go to comments

I think it’s time to tackle some more of the little questions I discussed a few days back. In the earlier entry I pointed out how the question “why is the Moon round?” can lead to some deeper understanding of how the universe works, so what about another similar type of question. Hey, let’s do a two for one deal here. I’ll tackle the two related questions: “Why is the sky blue during the day?” and “Why is the sky dark at night?”

There’s a simple explanation of why the sky is blue and it’s one you don’t hear very often. The explanation is this: the sky is blue because that’s the colour of air. If the sky was made of chlorine instead of nitrogen and oxygen it would be green because that’s what colour chlorine is.

So the question of why the sky is blue is exactly the same question as why a rose is red or coal is black. There is one complicating factor though: it takes a lot of air to see the colour because the air isn’t very dense. When you look through small amounts of air, such as across a room you don’t see the colour because there just isn’t enough air there.

But saying the sky is blue because it’s made of blue stuff is only the first step. What is it which makes anything a particular colour? It’s because of the way the atoms or molecules the object is composed of interact with the light which reach them. If they absorb certain colours then the colours which remain will continue to the observer’s eye or the camera or whatever is looking at the light. The Sun produces light of many colours but only the blue survives the journey through the air so that’s the colour we see.

We might need to take the explanation one step further though. Why do certain particles absorb light of some colours but not others? It’s because of the energy of the atom’s electrons (amongst other reasons). If the energy of the electron corresponds to that of the light they will interact.

Finally we have to ask why electrons have a certain energy and why is it always the same for the same atom? That’s because of quantum mechanics. Electrons can only “orbit” the atom at certain “distances” because that’s all that quantum theory allows.

So a blue sky (or in fact any colour of anything) tells us a lot about the nature of the the material producing the colour. In fact there is a whole branch of science which uses light to analyse what objects are made of. Astronomers have used spectroscopy to analyse the light from the Sun (and other bodies) for years and by analysing the light in minute detail they can tell a lot about the material in the Sun, not only what’s there but a lot about magnetic fields, temperature, and pressure as well.

In fact in the early days of the spectroscopic analysis of the Sun astronomers found some strange colours (or actually dark lines in the continuous colour for reasons which I won’t go in to here) which could only belong to a new element. That element was helium and it was found in the Sun (hence its name) before it was found in a laboratory on Earth.

Now we know why the sky is blue during the day what about the second question: why is it dark at night?

Obviously it’s dark because there is no light to give it a colour. Black is the absence of light so obviously the night sky should be dark. Or should it?

These are the early assumptions about the nature of the Universe held by most astronomers: the universe is roughly the same everywhere, it is infinite in time, it is infinite in space, and it isn’t expanding or contracting.

If you know anything about cosmology you will know these assumptions haven’t held up well to modern observations but at the time (up to about 100 years ago) they seemed to make a lot of sense. The problem was that if the universe was infinite and the same everywhere there would be an infinite number of stars which should produce an infinite amount of light. So the whole sky should be as bright as day! At that time the fact that the sky was dark was known as “Olbers’ Paradox” and it took a while to resolve it.

There were objections to this idea but they didn’t stand up to scrutiny. For example, you might think that the stars further away would be dimmer so they would contribute less light. But the further away you look the bigger the sphere you are looking at so there would be more stars in that area. The number of stars at any distance would vary with the square of the distance and the brightness would vary with the inverse square. They would cancel each other out making the distance irrelevant.

Another objection might be that dark dust and gas (which there is plenty of in the universe) would block the light. But after blocking this light that dust would get hotter until eventually it re-radiated the light hitting it and would have no effect on the total light.

The only explanation was that one or more of the assumptions were wrong. We still think the universe is roughly the same everywhere so that one is OK. After the Big Bang was discovered it seemed to make sense that the universe wasn’t infinite (in time or space) but even that cannot be assumed to be true according to some ideas. The critical thing was the fact discovered in the early 1900s that the universe is expanding.

There are actually many stars beyond a distance where the universe is expanding away from us at faster that the speed of light. Because the light from those stars only travels at the speed of light it cannot reach us. You might know that nothing can travel faster than light which seems to make this idea illogical. Actually no matter or energy can, but the fabric of the universe can expand at any speed, so relativity is not “broken” by this fact.

So you can see that the colour of the sky is actually a deeply significant observation. The blue sky during the day tells us something about physics at the smallest scale as described by quantum theory and the dark sky at night tells us something about the universe at the largest scale as described by cosmology and relativity.

If you look hard enough the simplest thing can be the most meaningful.

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