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Authors: Richard Dawkins

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Later cotton mills were driven by coal-fired steam engines – again using energy ultimately from the sun. But before they switched to steam entirely the factories went through an intermediate stage. They kept the great water wheel to drive the looms and shuttles, but used a steam engine to pump water up into a tank, from which it flowed down over the water wheel, only to be pumped back up again. So, whether the water is raised by the sun into the clouds, or whether it is raised by a coal-fired steam engine into a tank, the energy still comes from the sun in the first place. The difference is that the steam engine is driven by sunlight collected by plants millions of years ago and stored underground in coal, whereas the water wheel on a river is driven by sunlight from only a few weeks ago and stored in the form of the water up at the top of the hills. This kind of ‘stored sunlight’ is called potential energy, because the water has the potential – the power within it – to do work as it flows downhill.

This gives us a nice way to understand how life is powered by the sun. When plants use sunlight to make sugar, it is like pumping water uphill, or into a tank on a factory roof. When plants (or the herbivores that eat the plants, or the carnivores that eat the herbivores) use the sugar (or the starch that’s made from the sugar, or the meat that’s made from the starch), we can think of the sugar as being burned: slow-burned
to
drive muscles, for instance, just as coal is fast-burned to make steam to propel a drive shaft in a factory.

It wouldn’t do us any good if we literally burned our sugar and other food fuels by setting fire to them! Burning is a wasteful and destructive way to recover the sun’s stored energy. What happens in our cells is so slow and carefully regulated that it is like water trickling down a hill and driving a series of water wheels. The sun-powered chemical reaction that goes on in green leaves to make sugar is doing the equivalent of pumping water uphill. The chemical reactions in animal and plant cells that use energy – to drive muscles, for example – get the energy in carefully controlled stages, step by step. The high-energy fuels, sugars or whatever they are, are coaxed into releasing their energy in stages, down through a cascade of chemical reactions, each one feeding into the next, like a stream tumbling down a series of small waterfalls, turning one small water wheel after another.

Whatever the details, all the water wheels and cogs and drive shafts of life are ultimately powered by the sun. Perhaps those ancient peoples would have worshipped the sun even more devotedly if they had realized just how much all life depended on it. What I now wonder is how many other stars drive engines of life on their own orbiting planets. But that must wait for a later chapter.

7

W
HAT IS
A RAINBOW?

 

THE EPIC OF
Gilgamesh is one of the oldest stories ever written. Older than the legends of the Greeks or the Jews, it is the ancient heroic myth of the Sumerian civilization, which flourished in Mesopotamia (now Iraq) between 5,000 and 6,000 years ago. Gilgamesh was the great hero king of Sumerian myth – a bit like King Arthur in British legends, in that nobody knows whether he actually existed, but lots of stories were told about him. Like the Greek hero Odysseus (Ulysses) and the Arabian hero Sinbad the Sailor, Gilgamesh went on epic travels, and he met many strange things and people on his journeys. One of them was an old man (a very, very old man, centuries old) called Utnapashtim, who told Gilgamesh a strange story about himself. Well, it seemed strange to Gilgamesh, but it may not seem so strange to you because you have probably heard a similar story … about another old man with a different name.

Utnapashtim told Gilgamesh of an occasion, many centuries earlier, when the gods were angry with humankind because we made so much noise they couldn’t sleep.

The chief god, Enlil, suggested that they should send a great flood to destroy everybody, so the gods could get a good
night’s
rest. But the water god, Ea, decided to warn Utnapashtim. Ea told Utnapashtim to tear down his house and build a boat. It would have to be a very big boat, because Utnapashtim was to take into it ‘the seed of all living creatures’. Utnapashtim built the boat just in time, before it rained for six days and six nights without stopping. The flood that followed drowned everybody and everything that was not safely inside the boat. On the seventh day the wind dropped and the waters grew calm and flat.

Utnapashtim opened a hatch in the tightly sealed boat and released a dove. The dove flew away, looking for land, but failed to find any and returned. Then Utnapashtim released a swallow, but the same thing happened. Finally Utnapashtim released a raven. The raven didn’t come back, which suggested to Utnapashtim that there was dry land somewhere and the raven had found it.

Eventually the boat came to rest on a mountaintop poking out of the water. Another god, Ishtar, created the first rainbow, as a token of the gods’ promise to send no more terrible floods. So that is how the rainbow came into being, according to the ancient legend of the Sumerians.

Well, I said the story would be familiar. All children reared in Christian, Jewish or Islamic countries will immediately recognize that it is the same as the more recent story of Noah’s Ark, with one or two minor differences. The name of the boat-builder changes from Utnapashtim to Noah. The many gods of the older legend turn into the one god of the Jewish story. The ‘seed of all living creatures’ comes to be spelled out as ‘every living thing of all flesh, two of every sort’

or, as the song has it, ‘the animals went in two by two’ – and the Epic of Gilgamesh surely meant something similar. In fact, it is obvious that the Jewish story of Noah is nothing more than a retelling of the older legend of Utnapashtim. It was a folk tale that got passed around, and it travelled down the centuries. We often find that seemingly ancient legends have come from even older legends, usually with some names or other details changed. And this one, in both versions, ends with the rainbow.

In both the Epic of Gilgamesh and the Book of Genesis, the rainbow is an important part of the myth. Genesis specifies that it was actually God’s bow, which he put up in the sky as a token of his promise to Noah and his descendants.

There is one more difference between the Noah story and the earlier Sumerian tale of Utnapashtim. In the Noah version, the reason for God’s discontent with humans was that we were all incurably wicked. In the Sumerian story, humanity’s crime was, you might think, less serious. We simply made so much noise the gods couldn’t get to sleep! I think it’s funny. And the theme of noisy humans keeping the gods awake crops up, quite independently, in the legend of the Chumash people of Santa Cruz Island, off the coast of California.

The Chumash people believed that they were created on their island (it obviously wasn’t called Santa Cruz then, because that is a Spanish name) from the seeds of a magic plant by the Earth goddess Hutash, who was married to the Sky Snake (what we know as the Milky Way, which you can
see
on a really dark night in the country, but not if you live in a town where there is too much light pollution). The people of the island became very numerous, and, just as in the Epic of Gilgamesh, too noisy for the goddess Hutash’s comfort. The racket kept her awake at night. But instead of killing them all, like the Sumerian and Jewish gods, Hutash was kinder. She decided that some of them must move off Santa Cruz, onto the mainland where she wouldn’t be able to hear them. So she made a bridge for them to cross by. And the bridge was … yes, the rainbow!

This myth has a strange ending. As the people were crossing over the rainbow bridge, some of the noisy ones looked down – and they were so frightened by the drop that they got dizzy. They fell off the rainbow into the sea, where they turned into dolphins.

The idea of the rainbow as a bridge crops up in other mythologies, too. In old Norse (Viking) myths, rainbows were seen as fragile bridges used by the gods to travel from the sky world to Earth. Many peoples, for example in Persia, west Africa, Malaysia, Australia and the Americas, have seen the rainbow as a large snake which soars out of the ground to drink the rain.

How do all these legends start, I wonder? Who makes them up, and why do some people eventually come to believe these things really happened? These questions are fascinating and not easy to answer. But there’s one question we can answer: what is a rainbow
really
?

The real magic of the rainbow

When I was about ten, I was taken to London to see a children’s play called
Where the Rainbow Ends
. You almost certainly won’t have seen it because it is too unfashionably patriotic for modern theatres to perform. It is all about how exceptionally special it is to be English, and at the climax of the adventure the children are rescued by St George, the patron saint of England (not Britain, for Scotland, Wales and Ireland have their own patron saints). But what I most vividly remember is not St George but the rainbow itself. The children actually went to the place where the rainbow planted its foot, and we saw them walking about in the middle of the rainbow where it hit the ground. It was cleverly staged, with coloured spotlights beaming down through swirling mist, and the children stumbled about in a spellbound daze. I think it was at about this moment that the shining-armoured, silver-helmeted St George appeared, and we children gasped at the scene as the children on the stage shouted: ‘St George! St George! St George!’

But it was the rainbow itself that seized my imagination. Never mind St George: how wonderful it must be to stand right in the foot of a giant rainbow!

You can see where the author of the play got the idea. A rainbow really does look like a proper object, hanging out there, perhaps a few miles away. It seems to have its left foot planted, say, in a wheat field and its right foot (if you are lucky enough to see a complete rainbow) on a hilltop. You feel you ought to be able to go straight to it and stand right where the rainbow steps on the ground, like the children in the play. All
the
myths I have described to you have the same idea. The rainbow is seen as a definite thing, in a definite place, a definite distance away.

Well, you’ll probably have worked out that it isn’t really like that! First, if you try to approach the rainbow, no matter how fast you run, you’ll never get there: the rainbow will run away from you until it fades away altogether. You can’t catch it. But it isn’t really running away because it isn’t really in a particular place at all, ever. It’s an illusion – but a fascinating illusion, and understanding it leads on to all sorts of interesting things, some of which we’ll come to in the next chapter.

What light is made of

First, we need to understand about something called the spectrum. It was discovered in the time of King Charles II – that’s about 350 years ago – by Isaac Newton, who may well have been the greatest scientist ever (he discovered lots of other things besides the spectrum, as we saw in the chapter on night and day). Newton discovered that white light is really a mixture of all the different colours. To a scientist, that’s what white
means
.

How did Newton find this out? He set up an experiment. First he blacked out his room so that no light could get in, and then he opened a narrow chink in the curtain, so that a pencil-thin beam of white sunlight came in. He then let the beam of light pass through a prism, which is a sort of triangular chunk of glass.

What a prism does is splay the narrow white beam out;
but
the splayed-out beam that emerges from the prism is no longer white. It is multicoloured like a rainbow, and Newton gave a name to the rainbow he made: the spectrum. Here’s how it works.

When a beam of light travels through air and hits glass, it gets bent. The bending is called refraction. Refraction doesn’t have to be caused by glass: water does the trick too, and that will be important when we come back to the rainbow. It is refraction that makes an oar look bent when you stick it in the river. But now here’s the point. The
angle
at which light bends is slightly different depending on what colour the light is. Red light bends at a shallower angle than blue light. So, if white light really is a mixture of coloured lights, as Newton guessed, what’s going to happen when you bend white light through a prism? The blue light is going to bend further than the red light, so they will be separated from each other when they emerge from the other side of the prism. And the yellow and green lights will come out in between. The result is Newton’s spectrum: all the colours of the rainbow, arranged in the correct rainbow order – red, orange, yellow, green, blue, violet.

Newton wasn’t the first person to make a rainbow with a prism. Other people had already got the same result. But many of them thought the prism somehow ‘coloured’ the white light, like adding a dye. Newton’s idea was quite different. He thought that white light was a mixture of all the colours, and the prism was just separating them from each other. He was right, and he proved it with a pair of neat experiments. First, he took his prism, as before, and stuck a
narrow
slit in the way of the coloured beams coming out of it, so that only one of them, say the red beam, passed through the slit. Then he put another prism in the path of this narrow beam of red light. The second prism bent the light, as usual. But what came out of it was only red light. No extra colours were added, as they would have been if what prisms did was add colour like a dye. The result Newton got was exactly what he expected, supporting his theory that white light is a mixture of light of all colours.

The second experiment was more ingenious still, using three prisms. It was called Newton’s Experimentum Crucis, which is Latin for ‘critical experiment’ – or, as we might say, ‘experiment that really clinches the argument’.

White light passed through a slit in Newton’s curtain and through the first prism, which spread it out into all the colours of the rainbow. The spread-out rainbow colours then passed through a lens, which brought them all together before they passed through the second of Newton’s prisms. This second prism had the effect of merging the rainbow colours back into white light again. That already neatly proved Newton’s point. But just to make quite sure, he then passed the beam of white light through a third prism, which splayed the colours out into a rainbow again! As neat a demonstration as you could wish for, proving that white light is indeed a mixture of all the colours.

How raindrops make rainbows

Prisms are all very well, but when you see a rainbow in the sky, there isn’t a great big prism hanging up there. No, but there
are
millions of raindrops. So, does each raindrop act as a tiny prism? It is a bit like that, but not quite.

If you want to see a rainbow you have to have the sun
behind
you when you look at a rainstorm. Each raindrop is more like a little ball than a prism, and light behaves differently when it hits a ball from how it behaves when it hits a prism. The difference is that the far side of a raindrop acts as a tiny mirror. And that is why you need the sun behind you if you want to see a rainbow. The light from the sun turns a somersault inside every raindrop and is reflected backwards and downwards, where it hits your eyes.

Here’s how it works. You are standing with the sun
behind
and above you, looking at a distant shower of rain. The sunlight hits a single raindrop (of course it hits lots of other raindrops too, but wait, we’re coming to that). Let’s call our one particular raindrop
A
. The beam of white light hits
A
on its upper near surface, where it is bent, just as it was on the near surface of Newton’s prism. And of course the red light bends less than the blue, so the spectrum is already sorting itself out. Now all the coloured beams travel through the raindrop until they hit the far side. Instead of passing through into the air, they are reflected back towards the near side of the raindrop, this time the lower part of the near side. And as they pass through the near side of the raindrop, they are again bent. Again the red light bends less than the blue.

So, as the sunbeam leaves the raindrop, it has been splayed out into a proper little spectrum. The separated coloured beams, having doubled back around the inside of the raindrop, are now hurtling back in the general direction of
where
you are standing. If your eye happens to be in the path of one of those beams, say the green one, you’ll see pure green light. Somebody shorter than you might see the red beam coming from
A
. And somebody taller than you might see the blue beam from
A
.

Nobody sees the full spectrum from any one raindrop. Each of you sees only one pure colour. Yet all of you say you see a rainbow, with all the colours. How come? Well, so far, we have only been talking about one raindrop, called
A
. There are millions of other raindrops, and they are all behaving in the same kind of way. While you are looking at
A
’s red beam, there is another raindrop called
B
, which is lower than
A
. You don’t see
B
’s red beam because it hits you in the stomach. But
B
’s blue beam is in exactly the right place to hit you in the eye. And there are other raindrops lower than
A
but higher than
B
, whose red and blue beams miss your eye but whose yellow or green beams hit your eye. So lots of raindrops together add up to a complete spectrum, in a line, up and down.

BOOK: The Magic of Reality
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