Does this mean that starlight has a whole variety of strange new colours, colours that we never see on Earth? No, definitely not. You have already seen, on Earth, all the colours that your eyes are capable of seeing. Do you find that disappointing? I did, when I first understood it. When I was a child, I used to love Hugh Lofting’s Doctor Dolittle books. In one of the books the doctor flies to the moon, and is enchanted to behold a completely new range of colours, never before seen by human eyes. I loved this thought. For me it stood for the exciting idea that our own familiar Earth may not be typical of everything in the universe. Unfortunately, though the idea is worthwhile, the story was not true – could not be true. That follows from Newton’s discovery that the colours we see are all contained in white light and are all revealed when white light is spread out by a prism. There are no colours outside the range we are used to. Artists may come up with any number of different tints and shades, but all these are combinations of those basic component colours of white light. The colours we see inside our heads are really just labels made up by the brain to identify light of different wavelengths. We’ve already encountered the complete range of wavelengths here on Earth. Neither the moon nor the stars have any surprises to offer in the colour department. Alas.
So what did I mean when I said that different stars produce different rainbows, with differences we can measure using a spectroscope? Well, it turns out that when starlight is splayed out by a spectroscope, strange patterns of thin black lines appear in very particular places along the spectrum. Or sometimes the lines are not black but coloured, and the background is black. The pattern of lines looks like a barcode, the sort of barcode you see on things you buy in shops to identify them at the cash till. Different stars have the same rainbow but different patterns of lines across it – and this pattern really is a kind of barcode, because it tells us a lot about the star and what it is made of.
It isn’t only starlight that shows the barcode lines. Lights on Earth do too, so we’ve been able to investigate, in the laboratory, what makes them. And what makes the barcodes, it turns out, is different elements. Sodium, for example, has prominent lines in the yellow part of the spectrum. Sodium light (produced by an electric arc in sodium vapour) glows yellow. The reason for this is understood by physical scientists, but not by me because I’m a biological scientist who doesn’t understand quantum theory.
When I went to school in the city of Salisbury in southern England, I remember being utterly fascinated by the weird sight of my bright red school cap in the yellow light of the street lamps. It didn’t look red any more, but a yellowish brown. So did the bright red double-decker buses. The reason was this. Like many other English towns in those days, Salisbury used sodium vapour lamps for its street lights. These give off light only in the narrow regions of the spectrum covered by sodium’s characteristic lines, and by far the brightest of sodium’s lines are in the yellow. To all intents and purposes, sodium lights glow with a pure yellow light, very different from the white of sunlight or the vaguely yellowish light of an ordinary electric bulb. Since there was virtually no red at all in the light supplied by the sodium lamps, no red light could be reflected from my cap. If you are wondering what makes a cap, or a bus, red in the first place, the answer is that the molecules of dye, or paint, absorb most of the light of all colours except red. So in white light, which contains all wavelengths, mostly red light is reflected. Under sodium vapour street lamps, there is no red light to be reflected – hence the yellowy brown colour.
Sodium is just one example. You’ll remember from Chapter 4 that every element has its own unique ‘atomic number’, which is the number of protons in its nucleus (and also the number of electrons orbiting it). Well, for reasons connected with the orbits of its electrons, every element also has its own unique effect upon light. Unique like a barcode … in fact, a barcode is pretty much what the pattern of lines in the spectrum of starlight is. You can tell which of the 92 naturally occurring elements are present in a star by spreading the star’s light out in a spectroscope and looking at the barcode lines in the spectrum.
Since every element has a different barcode pattern, we can look at the light from any star and see which elements are present in that star. Admittedly, it is quite tricky because the barcodes of several different elements are likely to be muddled up together. But there are ways of sorting them out. What a wonderful tool the spectroscope is!
It gets even better. The sodium spectrum we would measure in light from a Salisbury street lamp is the same as that from a star that is not very far away. Most of the stars we see – for example, the stars in the well-known constellations of the zodiac – are in our own galaxy. But if you look at the sodium spectrum from a star in a different galaxy, you get a fascinatingly different picture. Sodium light from the distant galaxy has the same pattern of bars, spaced the same distance from each other. But the whole pattern is shifted towards the red end of the spectrum. How do we know it is still sodium, then? The answer is because the pattern of spacing between the bars is the same. That might not seem totally convincing if it only happened with sodium. But the same thing happens with all the elements. In every case we see the same spacing pattern, characteristic of the element concerned, but shifted bodily along the spectrum towards the red end. What’s more, for any given galaxy, all the barcodes are shifted the same distance along the spectrum.
If you look at the sodium barcode in light from a galaxy that is somewhat close to ours – closer than the very distant galaxies I talked about in the previous paragraph but further away than the stars in our own Milky Way galaxy – you see an intermediate shift. You see the same spacing pattern, which is the signature of sodium, but not shifted so far. The first line is shifted along the spectrum away from deep blue, but not as far as green: only as far as light blue. And the yellow line responsible for the yellow colour of the Salisbury street lamps is shifted in the same direction, towards the red end of the spectrum, but not all the way into the red as it is in light from the distant galaxy: only a little way into the orange.
Sodium is just one example. Any other element shows the same shift along the spectrum in the red direction. The more distant the galaxy, the greater the shift towards the red. This is called the ‘Hubble shift’, because it was discovered by the great American astronomer Edwin Hubble, who also gave his name, after his death, to the Hubble telescope. It is also called a ‘red shift’, because the shift is along the spectrum in the direction of red.
Backwards to the big bang
What does the red shift mean? Fortunately, scientists understand it well. It is an example of what is called a ‘Doppler shift’. Doppler shifts can happen wherever we have waves – and light, as we saw in the previous chapter, consists of waves. It’s often called the ‘Doppler effect’ and it is more familiar to us from sound waves. When you are standing at a roadside watching the cars whizz by at high speed, the sound of every car’s engine seems to drop in pitch as it passes you. You know the car’s engine note really stays the same, so why does the pitch seem to drop? The answer is the Doppler shift, and the explanation for it is as follows.
Sound travels through the air as waves of changing air pressure. When you listen to the note of a car engine – or let’s say a trumpet, because it is more pleasant than an engine – sound waves travel through the air in all directions from the source of the sound. Your ear happens to lie in one of those directions, it picks up the changes in air pressure produced by the trumpet, and your brain hears them as sound. Don’t imagine molecules of air flowing from the trumpet all the way to your ear. It isn’t like that at all: that would be a wind, and winds travel in one direction only, whereas sound waves travel outwards in all directions, like the waves on the surface of a pond when you drop a pebble in.
The easiest kind of wave to understand is the so-called Mexican Wave, in which people in a large sports stadium stand up and then sit down again in order, each person doing so immediately after the person on one side of them (say th
eir left side). A wave of standing and then sitting moves swiftly around the stadium. Nobody actually moves from their place, yet the wave travels. Indeed, the wave travels far faster than anybody could run.
What travels in the pond is a wave of changing height in the surface of the water. The thing that makes it a wave is that the water molecules themselves are not rushing outwards from the pebble. The water molecules are just going up and down, like the people in the stadium. Nothing really travels outwards from the pebble. It only looks like that because the high points and low points of the water move outwards.
Sound waves are a bit different. What travels in the case of sound is a wave of changing air pressure. The air molecules move a little bit, to and fro, away from the trumpet, or whatever is the source of the sound, and back again. As they do so, they knock against neighbouring air molecules and set them moving backwards and forwards too. Those in turn knock against their neighbours and the result is that a wave of molecule-knocking – which amounts to a wave of changing pressure – travels outwards from the trumpet in all directions. And it is the wave that travels from the trumpet to your ear, not the air molecules themselves. The wave travels at a fixed speed, regardless of whether the source of the sound is a trumpet or a speaking voice or a car: about 768 miles per hour in air (four times faster under water, and even faster in some solids). If you play a higher note on your trumpet, the speed at which the waves travel remains the same, but the distance between the wave crests (the wavelength) becomes shorter. Play a low note, and the wave crests space out more but the wave still travels at the same speed. So high notes have a shorter wavelength than low ones.
That is what sound waves are. Now for the Doppler shift. Imagine that a trumpeter standing on a snow-covered hillside plays a long, sustained note. You get on a toboggan and speed past the trumpeter (I chose a toboggan rather than a car because it is quiet, so you can hear the trumpet). What will you hear? The successive wave crests leave the trumpet at a definite distance from each other, defined by the note the trumpeter chose to play. But when you are whizzing towards the trumpeter, your ear will gobble up the successive wave crests at a higher rate than if you were standing still on the hilltop. So the trumpet’s note will sound higher than it really is. Then, after you have whizzed past the trumpeter, your ear will hit the successive wave crests at a lower rate (they’ll seem more spaced out, because each wave crest is travelling in the same direction as your toboggan), so the apparent pitch of the note will be lower than it really is. The same thing works if your ear is still and the source of the sound moves. It is said (I don’t know whether it is true, but it is a nice story) that Christian Doppler, the Austrian scientist who discovered the effect, hired a brass band to play on an open railway truck, in order to demonstrate it. The tune the band was playing suddenly dropped into a lower key as the train puffed past the amazed audience.
Light waves are different again – not really like a Mexican Wave and not really like sound waves. But they do have their own version of the Doppler effect. Remember that the red end of the spectrum has a longer wavelength than the blue end, with green in the middle. Suppose the bandsmen on Christian Doppler’s railway truck are all wearing yellow uniforms. As the train speeds towards you, your eyes ‘gobble up’ the wave crests at a faster rate than they would if the train was still. So there is a slight shift in the colour of the uniform towards the green part of the spectrum. Now, when the train goes past you and is speeding away from you, the opposite happens, and the band uniforms appear slightly redder.
There’s only one thing wrong with this illustration. In order for you to notice the blue shift or the red shift, the train would have to be travelling at millions of miles per hour. Trains don’t travel anywhere near fast enough for the Doppler effect on colour to be noticed. But galaxies do. The shift of the spectrum towards the red end shows that very distant galaxies are travelling away from us at a rate of hundreds of millions of miles per hour. And the key point is that the more distant they are (as measured by the ‘standard candles’ mentioned before), the faster they are travelling away from us (the greater the red shift).
All the galaxies in the universe are rushing away from each other, which means that they are rushing away from us too. It doesn’t matter which direction you point your telescope in, the more distant galaxies are moving away from us (and from one another) at ever-increasing speed. The entire universe – space itself – is expanding at a colossal rate.
In that case, you might ask, why is it only at the level of galaxies that space is seen to expand? Why don’t the stars within a galaxy rush away from each other? Why aren’t you and I rushing away from each other? The answer is that clusters of things that are close to each other, like everything in a galaxy, feel the strongest pull from the gravity of their neighbours. This holds them together, while distant objects – other galaxies – recede with the expansion of the universe.
And now here is something amazing. Astronomers have looked at the expansion and worked backwards through time. It is as though they constructed a movie of the expanding universe, with the galaxies rushing apart, and then ran the film in reverse. Instead of hurtling away from each other, in the backwards film the galaxies converge. And from that film the astronomers can calculate back to the moment when the expansion of the universe must have begun. They can even calculate when that moment was. That’s how they know it was somewhere between 13 and 14 billion years ago. That was the moment when the universe itself began – the moment called the ‘big bang’.
Today’s ‘models’ of the universe assume that it wasn’t only the universe that began with the big bang: time itself and space itself began with the big bang too. Don’t ask me to explain that, because, not being a cosmologist, I don’t understand it myself. But perhaps you can now see why I nominated the spectroscope as one of the most important inventions ever. Rainbows are not just beautiful to look at. In a way, they tell us when everything began, including time and space. I think that makes the rainbow even more beautiful.
9
ARE WE
ALONE?
SO FAR AS I know there are few, if any, ancient myths about alien life elsewhere in the universe, perhaps because the very idea of there being a universe vastly bigger than our own world hasn’t been around all that long. It took until the 1500s for scientists to see clearly that the Earth orbits the sun, and that there are other planets that do so too. But the distance and number of the stars, let alone other galaxies, were unknown and undreamed of until relatively modern times. And it isn’t that long since people first realized that the direction we call straight up in one part of the world (for example Borneo) would be straight down in another part of the world (in this case Brazil). Before then, people thought that ‘up’ was the same direction everywhere, towards the place where the gods lived, ‘above’ the sky.
There have long been numerous legends and beliefs about strange alien creatures near at hand: demons, spirits, djinns, ghosts … the list goes on. But in this chapter when I ask ‘Are we alone?’ I am going to mean ‘Are there alien life forms on other worlds elsewhere in the universe?’ As I said, myths about aliens in this sense are rare among primitive tribes. They are all too common, however, among modern city dwellers. These modern myths are interesting because, unlike ancient myths, we can actually watch as they start. We see myths being dreamed up before our very eyes. So the myths in this chapter will be modern.
In California in March 1997 a religious cult called Heaven’s Gate came to a sad end when all 39 of its members took poison. They killed themselves because they believed that a UFO from outer space would take their souls to another world. At the time a bright comet called Hale–Bopp was prominent in the sky and the cult believed – because their spiritual leader told them so – that an alien spacecraft was accompanying the comet on its journey. They bought a telescope to observe it, but then sent it back to the shop because it ‘didn’t work’. How did they know it didn’t work? Because they couldn’t see the s
pacecraft through it!
Did the cult leader, a man called Marshall Applewhite, believe the nonsense he taught his followers? Probably he did, because he was one of those who took the poison, so it looks as though he was sincere! Many cult leaders are in the business only so they can take possession of their female followers, but Marshall Applewhite was one of several cult members who had earlier had themselves castrated, so perhaps sex was not uppermost in his mind.
One thing most such people seem to have in common is a love of science fiction. The members of the Heaven’s Gate cult were obsessed with Star Trek. Of course, there is no shortage of science fiction stories about aliens from other planets, but most of us know that’s just what they are: fiction, imagined, invented stories, not accounts of things that actually happened. But there are quite a lot of people who firmly, sincerely and unshakeably believe that they have personally been captured (‘abducted’) by aliens from outer space. So eager are they to believe this that they will do so on the flimsiest of ‘evidence’.
One man, for instance, believed he had been abducted, for no better reason than that he often got nosebleeds. His theory was that the aliens had put a radio transmitter in his nose to spy on him. He also thought he might be part alien himself, on the grounds that his colouring was a little darker than his parents’. A surprisingly large number of Americans, many of them otherwise normal, sincerely believe that they personally have been taken aboard flying saucers and been the victims of horrific experiments conducted by little grey men with large heads and huge, wraparound eyes. There is a whole mythology of ‘alien abductions’, which is as rich, as colourful and as detailed as the mythology of ancient Greece and the gods of Mount Olympus. But these alien abduction myths are recent, and you can actually go and talk to people who believe they have been abducted: apparently normal, sane, level-headed people, who will tell you they saw the aliens face to face; actually tell you what the aliens look like, and what they say while performing their nasty experiments and sticking needles into people (the aliens speak English, of course!).