Eyes are pretty good things to have, and that is going to be true on most planets. Light travels, for practical purposes, in straight lines. Wherever light is available, such as in the vicinity of a star, it is technically easy to use light rays to find your way around, to navigate, to locate objects. Any planet that has life is pretty much bound to be in the vicinity of a star, because a star is the obvious source of the energy that all life needs. So the chances are good that light will be available wherever life is present; and where light is present it is very likely that eyes will evolve because they are so useful. It is no surprise that eyes have evolved on our planet dozens of times independently.
There are only so many ways to make an eye, and I think every one of them has evolved somewhere in our animal kingdom. There’s the camera eye, which, like the camera itself, is a darkened chamber with a small hole at the front letting in light, through a lens, which focuses an upside-down image on a screen – the ‘retina’ – at the back. Even a lens is not essential. A simple hole will do the job if it is small enough, but that means that very little light gets through, so the image is very dim – unless the planet happens to get a lot more light from its star than we get from the sun. This is of course possible, in which case the aliens could indeed have pinhole eyes. Human eyes have a lens, to increase the amount of light that is focused on the retina. The retina at the back is carpeted with cells that are sensitive to light and tell the brain about it via nerves. All vertebrates have this kind of eye, and the camera eye has been independently evolved by lots of other kinds of animals, including octopuses. And invented by human designers too, of course.
Jumping spiders have a weird kind of scanning eye. It is sort of like a camera eye except that the retina, instead of being a broad carpet of light-sensitive cells, is a narrow strip. The strip retina is attached to muscles which move it about so that it ‘scans’ the scene in front of the spider. Interestingly, that is a bit like what a television camera does too, since it has only a single channel to send a whole image along. It scans across and down in lines, but does it so fast that the picture we receive looks like a single image. Jumping spider eyes don’t scan so fast, and they tend to concentrate on ‘interesting’ parts of the scene such as flies, but the principle is the same.
Then there’s the compound eye, which is found in insects, shrimps and various other animal groups. A compound eye consists of hundreds of tubes, radiating out from the centre of a hemisphere, each tube looking in a slightly different direction. Each tube is capped by a little lens, so you could think of it as a miniature eye. But the lens doesn’t form a usable image: it just concentrates the light in the tube. Since each tube accepts light from a different direction, the brain can combine the information from them all to reconstruct an image: rather a crude image, but good enough to let dragonflies, for instance, catch moving prey on the wing.
Our largest telescopes use a curved mirror rather than a lens, and this principle too is used in animal eyes, specifically in scallops. The scallop eye uses a curved mirror to focus an image on a retina, which is in front of the mirror. This inevitably gets in the way of some of the light, as the equivalent does in reflecting telescopes, but it doesn’t matter too much as most of the light gets through to the mirror.
That list pretty much exhausts the ways of making an eye that scientists can imagine, and all of them have evolved in animals on this planet, most of them more than once. It is a good bet that, if there are creatures on other planets that can see, they will be using eyes of a kind that we would find familiar.
Let’s exercise our imaginations a bit more. On the planet of our hypothetical aliens, the radiant energy from their star will probably range from radio waves at the long end to X-rays at the short. Why should the aliens limit themselves to the narrow band of frequencies that we call ‘light’? Maybe they have radio eyes? Or X-ray eyes?
A good image relies on high resolution. What does that mean? The higher the resolution, the closer two points can be to each other while still being distinguished from each other. Not surprisingly, long wavelengths don’t make for good resolution. Light wavelengths are measured in minute fractions of a millimetre and give excellent resolution, but radio wavelengths are measured in metres. So radio waves would be lousy for forming images, although they are very good for communication purposes because they can be modulated. Modulated means changed, extremely rapidly, in a controlled way. So far as is known, no living creature on our planet has evolved a natural system for transmitting, modulating or receiving radio waves: that had to wait for human technology. But perhaps there are aliens on other planets that have evolved radio communication naturally.
What about waves shorter than light waves – X-rays, for example? X-rays are difficult to focus, which is why our X-ray machines form shadows rather than true images, but it is not impossible that some life forms on other planets have X-ray vision.
Vision of any kind depends on rays travelling in straight, or at least predictable, lines. It is no good if they are scattered every which way, as light rays are in fog. A planet that is permanently shrouded in thick fog would not encourage the evolution of eyes. Instead, it might foster the use of some kind of echo ranging system like the ‘sonar’ used by bats, dolphins and man-made submarines. River dolphins are extremely good at using sonar, because their water is full of dirt, which is the watery equivalent of fog. Sonar has evolved at least four times in animals on our planet (in bats, whales, and two separate kinds of cave-dwelling birds). It would not be surprising to find sonar evolving on an alien planet, especially one that is permanently shrouded in fog.
Or, if the aliens have evolved organs that can handle radio waves for communication, they might also evolve true radar to find their way around, and radar does work in fog. On our planet, there are fish that have evolved the ability to find their way about using distortions in an electric field that they themselves create. In fact, this trick has evolved twice independently, in a group of African fish and in a completely separate group of South American fish. Duck-billed platypuses have electric sensors in their bills which pick up the electrical disturbances in water caused by the muscular activity of their prey. It is easy to imagine an alien life form that has evolved electrical sensitivity along the same lines as the fishes and the platypus, but to a more advanced level.
This chapter is rather different from the others in this book because it emphasizes what we don’t know, rather than what we do. Yet even though we have not yet discovered life on other planets (and indeed, may never do so), I hope you have seen and been inspired by how much science can tell us about the universe. Our search for life elsewhere is not haphazard or random: our knowledge of physics and chemistry and biology equips us to seek out meaningful information about stars and planets vast distances away, and to identify planets that are at least possible candidates as hosts for life. There is much that remains deeply mysterious, and it is not likely that we will ever uncover all the secrets of a universe as vast as ours: but, armed with science, we can at least ask sensible, meaningful questions about it and recognize credible answers when we find them. We don’t have to invent wildly implausible stories: we have the joy and excitement of real scientific investigation and discovery to keep our imaginations in line. And in the end that is more exciting than fantasy.
10
WHAT IS AN
EARTHQUAKE?
IMAGINE THAT YOU are sitting quietly in your room, perhaps reading a book or watching television or playing a computer game. Suddenly there is a terrifying rumbling sound, and the whole room starts to shake. The light swings wildly from the ceiling, ornaments clatter off the shelves, furniture is hurled across the floor, you are tipped out of your chair. After two minutes or so everything settles down again and there is a blessed silence, broken only by the crying of a frightened child and the barking of a dog. You pick yourself up and think how lucky you are that the whole house didn’t collapse. In a very severe earthquake, it might well have done.
&
nbsp; While I was beginning to write this book, the Caribbean island of Haiti was hit by a devastating earthquake and the capital city, Port au Prince, was largely destroyed. Two hundred and thirty thousand people are believed to have been killed, and many others, including poor orphaned children, long wandered the streets, homeless, or living in temporary camps.
Later, as I was revising the book, another earthquake, even stronger, occurred under the sea off the north-eastern coast of Japan. It caused a gigantic wave – a ‘tsunami’ – that wrought unimaginable destruction when it swept ashore, carrying whole towns with it, killing thousands of people and leaving millions homeless, and setting off dangerous explosions in a nuclear power plant already damaged by the earthquake.
Earthquakes, and the tsunamis they cause, are common in Japan (the very word ‘tsunami’ was originally Japanese), but the country had experienced nothing like this in living memory. The prime minister described it as the country’s worst experience since the Second World War, when atomic bombs destroyed the Japanese cities of Hiroshima and Nagasaki. Indeed, earthquakes are common all the way around the rim of the Pacific Ocean – the New Zealand city of Christchurch suffered severe damage and loss of life in a quake just one month before that which struck Japan. This so-called ‘ring of fire’ includes much of California and the western United States, where there was a famous earthquake in the city of San Francisco in 1906. The larger city of Los Angeles is also vulnerable, lying as it does on the notorious San Andreas Fault.
In an earthquake, the whole landscape behaves like a sort of liquid. It looks like the sea, with waves passing through it. Solid, dry land, with waves sweeping through it as they do on the sea! That’s an earthquake. If you are down on the ground, you don’t see the waves because you’re too close to them, and too small compared with them. You just feel the ground moving and shaking beneath your feet.
In a moment I’m going to explain what an earthquake really is, and what a ‘fault line’ is – like the San Andreas Fault, and similar ones in other parts of the world. But first, let’s look at some myths.
Earthquake myths
We’ll begin with a pair of myths that may have grown up around particular earthquakes, earthquakes that actually happened at certain moments in history.
A Jewish legend tells how two cities, Sodom and Gomorrah, were destroyed by the Hebrew god because the people who lived there were so wicked. The only good person in either city was a man called Lot. The god sent two angels to warn Lot to get out of Sodom while he still could. Lot and his family headed for the hills, just before the god started to rain fire and brimstone down on Sodom. They had been given strict orders not to look back, but unfortunately Lot’s wife disobeyed the god. She turned around and took a peek. So the god promptly turned her into a pillar of salt – which, some people say, you can see to this day.
Some archaeologists claim to have found evidence that a large earthquake shattered the region where Sodom and Gomorrah are believed to have stood about 4,000 years ago. If this is true, the legend of their destruction might belong in our list of earthquake myths.
Another biblical myth which might have started with a particular earthquake is the story of how Jericho was brought down. Jericho, which lies a little north of the Dead Sea in Israel, is one of the oldest cities in the world. It has suffered from earthquakes right up to recent times: in 1927 it was close to the centre of a severe one which shook the whole region and killed hundreds of people in Jerusalem, some 25 kilometres (15 miles) away.
The old Hebrew story tells of a legendary hero called Joshua, who wanted to conquer the people who lived in Jericho thousands of years ago. Jericho had thick city walls, and the people locked themselves inside so they couldn’t be attacked. Joshua’s men couldn’t break through the walls, so he ordered his priests to blow rams’ horns and all the people to shout at the tops of their voices.
The noise was so great that the walls shook and fell down flat. Joshua’s soldiers then rushed in and slaughtered everybody in the city, including the women and children, and even all the cows, sheep and donkeys. They also burned everything – except the silver and the gold, which they gave to their god, as he instructed them to do. The way the myth is told, this was a good thing: the god of Joshua’s people wanted it to happen so that his people could take over all the land that had previously belonged to the people of Jericho.
Since Jericho is such an earthquake-prone place, people nowadays have suggested that the legend of Joshua and Jericho may have begun with an ancient earthquake, which shook the city so violently that the walls fell down. You can easily imagine how a distant folk memory of a disastrous earthquake could be exaggerated and distorted as it was passed by word of mouth down through generations of people who couldn’t read or write, until eventually it grew into the legend of the great tribal hero Joshua, and all that noisy shouting and horn-blowing.
The two myths just described may have begun with particular earthquakes in history. There are also lots of other myths, from all around the world, that have come into being as people have tried to understand what earthquakes are in general.
Since Japan experiences so many earthquakes, it’s not surprising that Japan has some pretty colourful earthquake myths. According to one of these, the land floated on the back of a gigantic catfish called Namazu. Whenever Namazu flipped his tail, the Earth would shake.
Many thousands of miles south, the Maoris of New Zealand, who arrived by canoe and settled there a few centuries before European sailors arrived, believed that Mother Earth was pregnant with her child, the god Ru. Whenever baby Ru kicked or stretched inside his mother’s womb, there was an earthquake.
Back in the north, some Siberian tribes believed that the Earth sat on a sledge, pulled by dogs and driven by a god called Tull. The poor dogs had fleas, and when they scratched there an earthquake.
In one West African legend, the Earth is a disc, held up on one side by a great mountain and on the other side by a monstrous giant, whose wife holds up the sky. Every so often the giant and his wife hug each other, and then, as you can well imagine, the Earth moves.
Other West African tribes believed that they lived on top of a giant’s head. The forest was his hair, and the people and animals were like fleas wandering around on his head. Earthquakes were what happened when the giant sneezed. At least, that is what they were supposed to believe, though I rather doubt they really did.
Nowadays we know what earthquakes really are, and it is time to put away the myths and look at the truth.
What earthquakes really are
First, we need to hear the remarkable story of plate tectonics.
Everybody knows what a map of the world looks like. We know the shape of Africa and the shape of South America, and we know that the wide Atlantic Ocean separates them. We can all recognize Australia, and we know that New Zealand lies to the south-east of Australia. We know that Italy looks like a boot, about to kick the ‘football’ of Sicily, and some people think New Guinea looks like a bird. We can easily recognize the outline of Europe, even though the borders within it change all the time. Empires come and go; the frontiers between countries are shifted again and again through history. But the outlines of the continents themselves stay fixed. Don’t they? Well, no, they don’t, and that is the big point. They move, although admittedly very slowly, and so do the positions of the mountain ranges: the Alps, the Himalayas, the Andes, the Rockies. To be sure, these great geographical features are fixed on the timescale of human history. But the Earth itself – if it could think – would think that no time at all. Written history goes back only about 5,000 years. Go back a million years (that’s 200 times as far back as written history stretches) and the continents all have pretty much the same shapes they do today, as far as our eyes would notice. But go back 100 million years and what do we see?
The South Atlantic Ocean was a narrow channel by comparison with today, and it looks as though you could almost have swum from Africa to South America. Northern Europe was
nearly touching Greenland, which was nearly touching Canada. And India was not part of Asia at all, but right down by Madagascar, and tilted on its side. Africa lurched over the same way, too, compared with the more upright stance we see today.
Come to think of it, did you ever notice, when looking at a modern map, that the eastern side of South America looks suspiciously like the western side of Africa, as though they ‘wanted’ to fit together, like pieces in a jigsaw puzzle? It turns out that, if we go back a bit further in time (well, about 50 million years further back, but even that is just ‘a bit’ on the vast, slow geological timescale), we find that they actually did fit together.
One hundred and fifty million years ago, Africa and South America were completely joined up, not just to each other but to Madagascar, India and Antarctica too – and to Australia and New Zealand, round the other side of Antarctica. They were all one big land mass called Gondwana, which later split up into pieces, creating one daughter continent after another.
It sounds like a pretty tall story, doesn’t it? I mean, it sounds pretty ridiculous that anything as massive as a continent could move thousands of miles – but we now know that it happened, and what is more, we understand how.
How the Earth moves
We also know that the continents don’t only move away from each other. Sometimes they bump into each other, and when that happens huge mountain ranges get pushed up towards the sky. That’s how the Himalayas were formed: when India collided with Asia. Actually, it isn’t quite true that India collided with Asia. As we shall see soon, what collided with Asia was a much bigger thing, called a ‘plate’, much of it under water, with India sitting on top of it. All continents sit on these ‘plates’. We’ll come to them soon, but first let’s think a bit more about these ‘collisions’, and about the continents moving apart.