The Faber Book of Science

by John Carey

  Starting with the Sun’s visible rays, let us explore outwards in each direction and see (though that word is hardly applicable) what we can discover. On the long-wave side we come first to the infra-red rays, which can be perceived by our skin but not by our eyes. Infra-red rays are heat radiation; go out of doors on a summer’s day, and you can tell where the Sun is even though your eyes may be tightly closed.

  Thanks to special photographic films, we have all had glimpses of the world of infra-red. It is an easily recognizable world, though tone values are strangely distorted. Sky and water are black, leaves and grass dazzling white as if covered with snow. It is a world of clear, far horizons, for infra-red rays slice through the normal haze of distance – hence their great value in aerial photography.

  The further we go down into the infra-red, the stranger are the sights we encounter and the harder it becomes to relate them to the world of our normal senses. It is only very recently (partly under the spur of guided missile development) that we have invented sensing devices that can operate in the far infra-red. They see the world of heat; they can ‘look’ at a man wearing a brilliantly coloured shirt and smoking a cigarette – and see only the glowing tip. They can also look down on a landscape hidden in the darkness of night and see all the sources of heat from factories, cars, taxiing aircraft. Hours after a jet has taken off, they can still read its signature on the warm runway.

  Some animals have developed an infra-red sense, to enable them to hunt at night. There is a snake which has two small pits near its nostrils, each holding a directional infra-red detector. These allow it to ‘home’ upon small, warm animals like mice, and to strike at them even in complete darkness. Only in the last decade have our guided missiles learned the same trick.

  Below the infra-red, for several octaves, is a no man’s land of radiation about which very little is known. It is hard to generate or to detect waves in this region, and until recently few scientists paid it much attention. But as we press on to more familiar territory, first we encounter the inch-long waves of radar, then the yard-long one of the shortwave bands, then the hundred-yard waves of the broadcast band.

  The existence of all these radiations was quite unknown a century ago; today, of course, they are among the most important tools of our civilization. It is a bare twenty years since we discovered that the Sun also produces them, on a scale we cannot hope to match with our puny transmitters.

  The Sun’s radio output differs profoundly from its visible light, and the difference is not merely one of greater length. Visible sunlight is practically constant in intensity; if there are any fluctuations, they are too slight to be detected. Not only has the Sun shone with unvarying brightness throughout the whole span of human history, but we would probably notice no difference if we could see it through the eyes of one of the great reptiles.

  But if you saw only the ‘radio’ Sun, you would never guess that it was the same object. Most of the time it is very dim – much dimmer, in fact, than many other celestial bodies. To the eye able to see only by radio waves, there would be little difference between day and night; the rising of the Sun would be a minor and inconspicuous event.

  From time to time, however, the radio Sun explodes into nova brightness. It may, within seconds, flare up to a hundred, a thousand or even a million times its normal brilliance. These colossal outbursts of radio energy do not come from the Sun as a whole, but from small localized areas of the solar disc, often associated with sunspots.

  This is one excellent reason why no animals have ever developed radio senses. Most of the time, such a sense would be useless, because the radio landscape would be completely dark – there would be no source of illumination.

  In any event, ‘radio eyes’ would pose some major biological problems, because radio waves are millions of times larger than normal eyes, if they were to have the same definition. Even a radio eye which showed the world as fuzzily as a badly out-of-focus TV picture would have to be hundreds of yards in diameter; the gigantic antennas of our radar systems and radio telescopes dramatize the problem involved. If creatures with radio senses do exist anywhere in the Universe, they must be far larger than whales and can, therefore, only be inhabitants of gravity-free space.

  Meanwhile, back on Earth, let us consider the other end of the spectrum – the rays shorter than visible light. As the blue deepens into indigo and then violet, the human eye soon fails to respond. But there is still ‘light’ present in solar radiation: the ultraviolet. As in the case of the infra-red, our skins can react to it, often painfully; for ultraviolet rays are the cause of sunburn.

  And here is a very strange and little-known fact. Though I have just stated that our eyes do not respond to ultraviolet, the actual situation is a good deal more complicated. (In nature, it usually is.) The sensitive screen at the back of the eye – the retina, which is the precise equivalent of the film in a camera – does react strongly to ultraviolet. If it were the only factor involved, we could see by the invisible ultraviolet rays.

  Then why don’t we? For a purely technical reason. Though the eye is an evolutionary marvel, it is a very poor piece of optics. To enable it to work properly over the whole range of colours, a good camera has to have four, six or even more lenses, made of different types of glass and assembled with great care into a single unit. The eye has only one lens, and it already has trouble coping with the two-to-one range of wavelengths in the visible spectrum. You can prove this by looking at a bright red object on a bright blue background. They won’t both be in perfect focus; when you look at one, the other will appear slightly fuzzy.

  Objects would be even fuzzier if we could see by ultraviolet as well as by visible light, so the eye deals with this insoluble problem by eliminating it. There is a filter in the front of the eye which blocks the ultraviolet, preventing it from reaching the retina. The haze filter which photographers often employ when using colour film does exactly the same job, and for a somewhat similar reason.

  The eye’s filter is the lens itself – and here at last is the punch line of this rather long-winded narrative. If you are ever unlucky enough to lose your natural lenses (say through a cataract operation) and have them replaced by artificial lenses of clear glass, you will be able to see quite well in the ultraviolet. Indeed, with a source of ultraviolet illumination, like the so-called ‘black light’ lamps, you will be able to see perfectly in what is, to the normal person, complete darkness! I hereby donate this valuable information to the CIA, James Bond, or anyone else who is interested.

  Normal sunlight, as you can discover during a day at the beach, contains plenty of ultraviolet. It all lies, however, in a narrow band – the single octave just above the visible spectrum in frequency. As we move beyond this to still higher frequencies, the scene suddenly dims and darkens. A being able to see only in the far ultraviolet would be in a very unfortunate position. To him, it would always be night, whether or not the Sun was above the horizon.

  What has happened? Doesn’t the Sun radiate in the far ultraviolet? Certainly it does, but this radiation is all blocked by the atmosphere, miles above our head. In the far ultraviolet, a few inches of ordinary air are as opaque as a sheet of metal.

  Only with the development of rocket-borne instruments has it become possible to study this unknown region of the solar spectrum – a region, incidentally, which contains vital information about the Sun and the processes which power it by the atmosphere, miles above our head. In the far ultraviolet, if you started off from ground level on a bright, sunny day, this is what you would see.

  At first, you would be in utter darkness, even though you were looking straight at the Sun. Then, about twenty miles up, you would notice a slow brightening, as you climbed through the opaque fog of the atmosphere. Beyond this, between twenty and thirty miles high, the ultraviolet Sun would break through in its awful glory.

  I use that word ‘awful’ with deliberate intent. These rays can kill, and swiftly. They do not bother astronauts, because they can b
e easily filtered out by special glass. But if they reached the surface of the Earth – if they were not blocked by the upper atmosphere – most existing forms of life would be wiped out.

  If you regard the existence of this invisible ultraviolet umbrella as in any way providential, you are confusing cause and effect. The screen was not put in the atmosphere to protect terrestrial life: it was put there by life itself, hundreds of millions of years before man appeared on Earth.

  The Sun’s raw ultraviolet rays, in all probability, did reach the surface of the primeval Earth; the earliest forms of life were adapted to it, perhaps even thrived upon it. In those days, there was no oxygen in the atmosphere; it is a by-product of plant life, and over geological aeons its amount slowly increased, until at last those oxygen-burning creatures called animals had a chance to thrive.

  That filter in the sky is made of oxygen – or, rather, the grouping of three oxygen atoms known as ozone. Not until Earth’s protective ozone layer was formed, and the short ultraviolet rays were blocked twenty miles up, did the present types of terrestrial life evolve. If there had been no ozone layer, they would doubtless have evolved into different forms. Perhaps we might still be here, but our skins would be very, very black.*

  Life on Mars must face this problem, for that planet has no oxygen in its atmosphere and, therefore, no ozone layer. The far ultraviolet rays reach the Martian surface unhindered, and must profoundly affect all living matter there. It has been suggested that these rays are responsible for the colour changes which astronomers have observed on the planet. Whether or not this is true, we can predict that one of the occupational hazards of Martian explorers will be severe sunburn.

  Just as ultraviolet lies beyond the violet, so still shorter rays lie beyond it. These are X-rays, which are roughly a thousand times shorter than visible light. Like the ultraviolet, these even more dangerous rays are blocked by the atmosphere; few of them come to within a hundred miles of Earth, and they have been detected by rocket instruments only during the last few years. The solar X-rays are quite feeble – only a millionth of the intensity of visible light – but their importance is much greater than this figure would indicate. We know now that blasts of X-rays from the Sun, impinging upon the upper atmosphere, can produce violent changes in radio communications, even to the extent of complete blackouts.

  Men have lost their lives because the Sun has disrupted radio; nations are equally vulnerable, in this age of the ICBM.

  You will recall that though the Sun shines with remarkable steadiness in the visible spectrum, it flares and sparkles furiously on the long (radio) waves. Exactly the same thing happens with its X-ray emission, even though these waves are a billion times shorter. Moreover, both the Sun’s radio waves and its X-rays appear to come from the same localized areas of the solar surface – disturbed regions in the neighbourhood of sunspots, where clouds of incandescent gas larger than the Earth erupt into space at hundreds of miles a second.

  For reasons not yet understood (there is not much about the Sun that we do thoroughly understand) solar activity rises and falls in an eleven-year cycle. The Sun was most active around 1957, which is why that date was chosen for the International Geophysical Year. In the 1960s it headed for a minimum but unfortunatley threatened to come back to the boil at around the time the first major space expeditions were being planned. The astronauts might have run into some heavy weather, since the Sun by then was shooting out not only vast quantities of ultraviolet, X-rays and radio waves, but other radiations which cannot be so easily blocked. (As it turned out, however, the risks were far less than had at one time been feared.)

  We see, then, how complicated and how variable sunlight is, if we use that word in the widest sense to describe all the waves emitted by the Sun. Nevertheless, when we accept the evidence of our unaided eyes and describe the Sun as a yellow star, we have summed up the most important single fact about it – at this moment in time. It appears probable, however, that sunlight will be the colour we know for only a negligibly small part of the Sun’s history.

  For stars, like individuals, age and change. As we look out into space, we see around us stars at all stages of evolution. There are faint blood-red dwarfs so cool that their surface temperature is a mere 4,000 degrees Fahrenheit; there are searing ghosts blazing at 100,000 degrees, and almost too hot to be seen, for the greater part of their radiation is in the invisible ultraviolet. Obviously, the ‘daylight’ produced by any star depends upon its temperature; today (and for ages past, as for ages to come) our Sun is at about 10,000 degrees Fahrenheit, and this means that most of its light is concentrated in the yellow band of the spectrum, falling slowly in intensity towards both the longer and the shorter waves.

  That yellow ‘bump’ will shift as the Sun evolves, and the light of day will change accordingly. It is natural to assume that as the Sun grows older and uses up its hydrogen fuel – which it is now doing at the spanking rate of half a billion tons a second – it will become steadily colder and redder.

  But the evolution of a star is a highly complex matter, involving chains of interlocking nuclear reactions. According to one theory, the Sun is still growing hotter and will continue to do so for several billion years. Probably life will be able to adapt itself to these changes, unless they occur catastrophically, as would be the case if the Sun exploded into a nova. In any event, whatever the vicissitudes of the next five or ten billion years, at long last the Sun will settle down to the white dwarf stage.

  It will be a tiny thing, not much bigger than the Earth, and therefore too small to show a disc to the naked eye. At first, it will be hotter than it is today, but because of its minute size it will radiate very little heat to its surviving planets. The daylight of that distant age will be as cold as moonlight, but much bluer, and the temperature of Earth will have fallen to 300 degrees below zero. If you think of mercury lamps on a freezing winter night, you have a faint mental picture of high noon in the year ad 7,000 million.

  Yet that does not mean that life – even life as we know it today – will be impossible in the Solar System; it will simply have to move in towards the shrunken Sun. The construction of artificial planets would be child’s play to the intelligences we can expect at this date; indeed, it will be child’s play to us in a few hundred years’ time.

  Around the year 10,000 million the dwarf Sun will have cooled back to its present temperature, and hence to the yellow colour that we know today. From a body that was sufficiently close to it – say only a million miles away – it would look exactly like our present Sun, and would give just as much heat. There would be no way of telling, by eye alone, that it was actually a hundred times smaller, and a hundred times closer.

  So matters may continue for another five billion years; but at last the inevitable will happen. Very slowly, the Sun will begin to cool, dropping from yellow down to red. Perhaps by the year 15,000 million it will become a red dwarf, with a surface temperature of a mere 4,000 degrees. It will be nearing the end of the evolutionary track, but reports of its death will be greatly exaggerated. For now comes one of the most remarkable, and certainly least appreciated, results of modern astrophysical theories.

  When the Sun shrinks to a dull red dwarf, it will not be dying. It will just be starting to live – and everything that has gone before will be merely a fleeting prelude to its real history.

  For a red dwarf, because it is so small and so cool, loses energy at such an incredibly slow rate that it can stay in business for thousands of times longer than a normal-sized white or yellow star. We must no longer talk in billions but of trillions of years if we are to measure its life span. Such figures are, of course, inconceivable. (For that matter, who can think of a thousand years?) But we can nevertheless put them into their right perspective if we relate the life of a star to the life of a man.

  On this scale, the Sun is but a week old. Its flaming youth will continue for another month; then it will settle down to a sedate adult existence which may last at least eigh
ty years.

  Life has existed on this planet for two or three days of the week that has passed; the whole of human history lies within the last second, and there are eighty years to come.

  In the wonderful closing pages of The Time Machine, the young H. G. Wells described the world of the far future, with a blood-red Sun hanging over a freezing sea. It is a sombre picture that chills the blood, but our reaction to it is wholly irrelevant and misleading. For we are creatures of the dawn, with eyes and senses adapted to the hot light of today’s primeval Sun. Though we should miss beyond measure the blues and greens and violets which are the fading afterglow of Creation, they are all doomed to pass with the brief billion-year infancy of the stars.

  But the eyes that will look upon that all-but-eternal crimson twilight will respond to the colours that we cannot see, because evolution will have moved their sensitivity away from the yellow, somewhere out beyond the visible red. The world of rainbow-hued heat they see will be as rich and colourful as ours – and as beautiful; for a melody is not lost if it is merely transposed an octave down into the bass.

  So now we know that Shelley, who was right in so many things, was wrong when he wrote:

  Life, like a dome of many-coloured glass,

  Stains the white radiance of eternity.

  For the radiance of eternity is not white: it is infra-red.

  Source: Arthur C. Clarke, By Space Possessed, London, Gollancz, 1993.

  *I never imagined that, thirty years later, the ozone layer would be headline news!

  Can We Know the Universe? Reflections on a Grain of Salt

  Carl Sagan, Professor of Astronomy and Space Sciences at Cornell University, played a leading role in the Mariner, Viking and Voyager space programmes. Deeply interested in the possibility of life on other planets, he compiled the Voyager interstellar record – a message from earthdwellers to other civilizations in space. One of the most distinguished popular science writers, he won the Pulitzer Prize for The Dragons of Eden: Speculations on the Evolution of Human Intelligence. The following extract is from Broca’s Brain (1979).