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.
But a line up and down is not a rainbow. Where does the rest of the rainbow come from? Don’t forget that there are other raindrops, stretching from one side of the rain shower to the other and at all heights. And of course they fill in the rest of the rainbow for you. Every rainbow you see, by the way, is trying to be a complete circle, with your eye at the centre of it – like the complete circular rainbow you sometimes see when you water the garden with a hose and the sun shines through the spray. The only reason we don’t usually see the whole circle is that the ground gets in the way.
So that’s why you see a rainbow at any one split second. But in the next split second, all the raindrops have fallen to a lower position. A has now fallen to where B was, so you now see A’s blue beam instead of its green one. And you can’t see any of B’s beams (although the dog at your feet can). And a new raindrop (C, whose beams you couldn’t see at all before) has now fallen into the place where A was, and you now see its red beam.
That’s why a rainbow seems to stay still, although the raindrops that make it are constantly falling through it.
On the right wavelength?
Let’s now look at what the spectrum – the ordered range of colours from red through orange, yellow, green and blue to violet – really is. What is it about red light that makes it bend at a shallower angle than blue light?
Light can be thought of as vibrations: waves. Just as sound is vibrations in the air, light consists of what are called electromagnetic vibrations. I won’t try to explain what electromagnetic vibrations are because it takes too long (and I’m not sure that I entirely understand it myself). The point here is that although light is very different from sound, we can talk about high-frequency (short-wavelength) and low-frequency (long-wavelength) vibrations in light, just as we can for sound. High-pitched sound – treble or soprano – means high-frequency, or short-wavelength, vibrations. Low-frequency, or long-wavelength, sounds are deep, bass sounds. The equivalent for light is that red (long wavelength) is the bass, yellow the baritone, green the tenor, blue the alto and violet (short wavelength) the treble.
There are sounds that are too high-pitched for us to hear. They are called ultrasound; bats can hear them and use the echoes for finding their way around. There are also sounds that are too low for us to hear. They are called infrasound; elephants, whales and some other animals use these deep rumbles for keeping in touch with each other. The deepest bass notes on a big cathedral organ are almost too low to hear: you seem to ‘feel’ them fluttering your whole body. The range of sounds that we humans can hear is a band of frequencies in the middle, between ultrasound, which is too high for us (but not bats) to hear, and infrasound, which is too low for us (but not elephants) to hear.
And the same is true of light. The colour equivalent of ultrasound bat squeaks is ultraviolet, which means ‘beyond violet’. Although we can’t see ultraviolet light, insects can. There are some flowers that have stripes or other patterns for luring insects in to pollinate them, patterns that can only be seen in the ultraviolet range of wavelengths. Insect eyes can see them, but we need instruments to ‘translate’ the patterns into the visible part of the spectrum. For example, the evening primrose flower looks yellow to us, with no pattern, no stripes. But if you photograph it in ultraviolet light you suddenly see a starburst of stripes.
The spectrum goes into higher and higher frequencies, far beyond ultraviolet, far beyond what even insects can see. X-rays could be thought of as ‘light’ of even higher ‘pitch’ than ultraviolet. And gamma rays are even higher still.
At the other end of the spectrum, insects can’t see red, but we can. Beyond red is ‘infrared’, which we can’t see, although we can feel it as heat (and some snakes are especially sensitive to it, using it to detect their prey). A bee might call red ‘infra-orange’. Deeper ‘bass notes’ than infrared are microwaves, which you use to cook things. And even deeper bass (longer wavelength) are radio waves.
What is a bit surprising is that the light we humans can actually see – the spectrum or ‘rainbow’ of visible colours between the slightly ‘higher-pitched’ violet and the slightly ‘lower-pitched’ red – is a very tiny band in the middle of a huge spectrum ranging from gamma rays at the high-pitched end to radio waves at the low-pitched end. Almost the whole of the spectrum is invisible to our eyes.
The sun and the stars are pumping out electromagnetic rays at a full range of frequencies or ‘pitches’, all the way from radio waves at the ‘bass’ end to gamma rays at the ‘treble’ end. Although we can’t see outside the tiny band of visible light, from red to violet, we have instruments that can detect these invisible rays.
Scientists called radio astronomers take ‘photographs’ of stars using radio waves rather than light waves or X-rays. The instrument they use is called a radio telescope. Other scientists take photographs of the sky at the other end of the spectrum, in the X-ray band. We learn different things about the stars and about the universe by using different parts of the spectrum. The fact that our eyes can see through only a tiny slit in the middle of the vast spectrum, that we can see only a slender band in the huge range of rays that scientific instruments can see, is a lovely illustration of the power of science to excite our imagination: a lovely example of the magic of the real.
In the next chapter we shall learn something even more wonderful about the rainbow. Splitting the light from a distant star into a spectrum can tell us not only what the star is made of but also how old it is. And it is evidence of this kind – rainbow evidence – that enables us to work out how old the universe is: when did it all begin? That may sound unlikely, but all will be revealed in the next chapter.
8
WHEN AND HOW
DID EVERYTHING
BEGIN?
LET’S START WITH an African myth from a Bantu tribe, the Boshongo of the Congo. In the beginning there was no land, just watery darkness, and also – importantly – the god Bumba. Bumba got a stomach-ache and vomited up the sun. Light from the sun dispelled the darkness, and heat from t
he sun dried up some of the water, leaving land. Bumba’s stomach-ache still hadn’t gone, though, so he then sicked up the moon, the stars, animals and people.
Many Chinese origin myths involve a character called Pan Gu, sometimes depicted as a giant hairy man with a dog’s head. Here’s one of the Pan Gu myths. In the beginning there was no clear distinction between Heaven and Earth: it was all one gooey mess surrounding a big black egg. Curled up inside the egg was Pan Gu. Pan Gu slept inside the egg for 18,000 years. When he finally awoke he wanted to escape, so he picked up his axe and hewed his way out. Some of the contents of the egg were heavy and sank to become the Earth. Some of them were light and floated up to become the sky. The Earth and the sky then swelled at a rate of (the equivalent of) 3 metres a day for another 18,000 years.
Some versions of the story have Pan Gu pushing the sky and the Earth apart, after which he was so exhausted that he died. Various bits of him then became the universe that we know. His breath became the wind, his voice became thunder; his two eyes became the moon and the sun, his muscles farmland and his veins roads. His sweat became rain, and his hairs became stars. Humans are descended from the fleas and lice that once lived on his body.
By the way, the story of Pan Gu pushing the sky and the Earth apart is rather like the (probably unrelated) Greek myth of Atlas, who also held up the sky (although, weirdly, pictures and statues usually show him carrying the whole Earth on his shoulders).
Now here is one of many origin myths from India. Before the beginning of time there was a great dark ocean of nothingness, with a giant snake coiled up on the surface. Sleeping in the coils of the snake was Lord Vishnu. Eventually Lord Vishnu was awakened by a deep humming sound from the bottom of the ocean of nothingness, and a lotus plant grew out of his navel. In the middle of the lotus flower sat Brahma, Vishnu’s servant. Vishnu commanded Brahma to create the world. So Brahma did just that. No problem! And all living creatures too, while he was about it. Easy!
What I find a little disappointing about all these origin myths is that they begin by assuming the existence of some kind of living creature before the universe itself came into being – Bumba or Brahma or Pan Gu, or Unkulukulu (the Zulu creator) or Abassie (Nigeria) or ‘Old Man in the Sky’ (Salish, a tribe of native Americans from Canada). Wouldn’t you think that a universe of some kind would have to come first, to provide a place for the creative spirit to go to work? None of the myths gives any explanation for how the creator of the universe himself (and it usually is a he) came into existence.
So they don’t get us very far. Let’s turn instead to what we know of the true story of how the universe began.
How did everything begin, really?
Do you remember from Chapter 1 that scientists work by setting up ‘models’ of how the real world might be? They then test each model by using it to make predictions of things that we ought to see – or measurements that we ought to be able to make – if the model were correct. In the middle of the twentieth century there were two competing models of how the universe came into being, called the ‘steady state’ model and the ‘big bang’ model. The steady state model was very elegant, but eventually turned out to be wrong – that is, predictions based on it were shown to be false. According to the steady state model, there never was a beginning: the universe had always existed in pretty much its present form. The big bang model, on the other hand, suggested that the universe began at a definite moment in time, in a strange kind of explosion. The predictions made on the basis of the big bang model keep turning out to be right, and so it has now been generally accepted by most scientists.
According to the modern version of the big bang model, the entire observable universe exploded into existence between 13 and 14 billion years ago. Why do we say ‘observable’? The ‘observable universe’ means everything for which we have any evidence at all. It is possible that there are other universes that are inaccessible to all our senses and instruments. Some scientists speculate, perhaps fancifully, that there may be a ‘multiverse’: a bubbling ‘foam’ of universes, of which our universe is only one ‘bubble’. Or it may be that the observable universe – the universe in which we live, and the only universe for which we have direct evidence – is the only universe there is. Either way, in this chapter we are limiting ourselves to the observable universe. The observable universe seems to have begun in the big bang, and this remarkable event happened just under 14 billion years ago.
Some scientists will tell you that time itself began in the big bang, and we should no more ask what happened before the big bang than we should ask what is north of the North Pole. You don’t understand that? Nor do I. But I do understand, sort of, the evidence that the big bang happened, and when. That is what this chapter is about.
First, I need to explain what a galaxy is. We’ve already seen, in our analogy with footballs in Chapter 6, that the stars are spaced out at incredibly huge distances from one another compared with the planets orbiting our sun. But, vastly spaced out as they are, the stars are still actually clustered together into groups; and these groups are called galaxies. A galaxy is seen through astronomers’ powerful telescopes as a swirling pattern that is actually made up of billions of stars, and also clouds of dust and gas.
Our sun is just one of the stars that make up the particular galaxy called the Milky Way. It is called that because on dark nights we get an end-on view of part of it. We see it as a mysterious streak or path of milky white across the sky, which you might mistake for a long, wispy cloud until you realize what it really is – and when you do, the thought should strike you dumb with awe. Since we are in the Milky Way galaxy, we can never see it in its full glory. The universe – our observable universe – is a very big place.
The next important point is this. It is possible to measure how far away from us each galaxy is. How? How, for that matter, do we know how far away anything in the universe is? For nearby stars the best method uses something called ‘parallax’. Hold your finger up in front of your face and look at it with your left eye closed. Now open your left eye and close your right. Keep switching eyes, and you’ll notice that the apparent position of your finger hops from side to side. That is because of the difference between the viewpoints of your two eyes. Move your finger nearer, and the hops will become greater. Move your finger further away and the hops become smaller. All you need to know is how far apart your eyes are, and you can calculate the distance from eyes to finger by the size of the hops. That is the parallax method of estimating distances.
Now, instead of looking at your finger, look at a star out in the night sky, switching from eye to eye. The star won’t hop at all. It is much too far away. In order to make a star ‘hop’ from side to side, your eyes would need to be millions of miles apart! How can we achieve the same effect as switching eyes millions of miles apart? We can make use of the fact that the Earth’s orbit around the sun has a diameter of 186 million miles. We measure the position of a nearby star, against a background of other stars. Then, six months later, when the Earth is 186 million miles away at the opposite side of its orbit, we measure the apparent position of the star again. If the star is quite close, its apparent position will have ‘hopped’. From the length of the hop, it is easy to calculate how far away the star is.
Unfortunately, though, the parallax method works only for nearby stars. For distant stars, and certainly for other galaxies, our two alternating ‘eyes’ would need to be much further apart than 186 million miles. We have to find another method. You might think you could do it by measuring how brightly the galaxy seems to shine: surely a more distant galaxy should be dimmer than a closer one? The trouble is that the two galaxies might really be of different brightnesses. It’s like estimating how far away a lit candle is. If some candles are brighter than others, how would you know whether you were looking at a bright candle far away, or a dim candle nearby?
Fortunately, astronomers have evidence that certain special kinds of stars are what they call ‘standard candles?
??. They understand enough of what is going on in these stars to know how bright they are – not as we see them, but their actual brightness, the intensity of the light (or it might be X-rays or some other kind of radiation that we can measure) before it starts its long journey to our telescopes. They also know how to identify these special ‘candles’; and so, as long as they can find at least one of them in a galaxy, astronomers can use it, with the assistance of well-established mathematical calculations, to estimate how far away the galaxy is.
So we have the parallax method for measuring very short distances; and there is a ‘ladder’, so to speak, of various kinds of standard candles that we can use for measuring a range of increasingly great distances, stretching out even to very distant galaxies.
Rainbows and red shift
OK, so now we know what a galaxy is, and how to find out its distance from us. For the next step in the argument, we need to make use of the light spectrum, which we met in Chapter 7 on the rainbow. I was once asked to contribute a chapter to a book in which scientists were invited to nominate the most important invention ever. It was fun, but I had left it rather late before joining the party and all the obvious inventions had already been taken: the wheel, the printing press, the telephone, the computer and so on. So I chose an instrument that I was pretty sure nobody else would choose, and is certainly very important even though not many people have ever used one (and I must confess that I’ve never used one myself). I chose the spectroscope.
A spectroscope is a rainbow machine. If it is attached to a telescope, it takes the light from one particular star or galaxy and spreads it out as a spectrum, just as Newton did with his prism. But it is more sophisticated than Newton’s prism, because it allows you to make exact measurements along the spread-out spectrum of starlight. Measurements of what? What is there to measure in a rainbow? Well, this is where it starts to get really interesting. The light from different stars produces ‘rainbows’ that are different in very particular ways, and this can tell us a lot about the stars.