The Faber Book of Science (54 page)

It seems almost certain that close in time and space to the origin of our solar system, there was a supernova event. A supernova is the explosion of a large star. Astronomers speculate that this fate may overtake a star in the following manner: as a star burns, mostly by fusion of its hydrogen and, later, helium atoms, the ashes of its fire in the form of other heavier elements such as silicon and iron accumulate at the centre. If this core of dead elements, no longer generating heat and pressure, should much exceed the mass of our own sun, the inexorable force of its own weight will be enough to cause its collapse in a matter of seconds to a body no larger than a few thousand cubic miles in volume, although still as heavy as a star. The birth of this extraordinary object, a neutron star, is a catastrophe of cosmic dimensions. Although the details of this and other similar catastrophic processes are still obscure, it is obvious that we have here, in the death throes of a large star, all the ingredients for a vast nuclear explosion. The stupendous amount of light, heat, and hard radiation produced by a supernova event equals at its peak the total output of all the other stars in the galaxy.

Explosions are seldom one hundred per cent efficient. When a star ends as a supernova, the nuclear explosive material, which includes uranium and plutonium together with large amounts of iron and other burnt-out elements, is distributed around and scattered in space just as
is the dust cloud from a hydrogen bomb test. Perhaps the strangest fact of all about our planet is that it consists largely of lumps of fall-out from a star-sized hydrogen bomb. Even today, aeons later, there is still enough of the unstable explosive material remaining in the Earth’s crust to enable the reconstitution on a minute scale of the original event.

Binary, or double, star systems are quite common in our galaxy, and it may be that at one time our sun, that quiet and well-behaved body, had a large companion which rapidly consumed its store of hydrogen and ended as a supernova. Or it may be that the debris of a nearby supernova explosion mingled with the swirl of interstellar dust and gases from which the sun and its planets were condensing. In either case, our solar system must have been formed in close conjunction with a supernova event. There is no other credible explanation of the great quantity of exploding atoms still present on the Earth. The most primitive and old-fashioned Geiger counter will indicate that we stand on fall-out from a vast nuclear explosion. Within our bodies, no less than three million atoms rendered unstable in that event still erupt every minute, releasing a tiny fraction of the energy stored from that fierce fire of long ago.

The Earth’s present stock of uranium contains only 0.72 per cent of the dangerous isotope U235. From this figure it is easy to calculate that about four aeons ago the uranium in the Earth’s crust would have been nearly 15 per cent U235. Believe it or not, nuclear reactors have existed since long before man, and a fossil natural nuclear reactor was recently discovered in Gabon, in Africa. It was in action two aeons ago when U235 was only a few per cent. We can therefore be fairly certain that the geochemical concentration of uranium four aeons ago could have led to spectacular displays of natural nuclear reactions. In the current fashionable denigration of technology, it is easy to forget that nuclear fission is a natural process. If something as intricate as life can assemble by accident, we need not marvel at the fission reactor, a relatively simple contraption, doing likewise.

Thus life probably began under conditions of radioactivity far more intense than those which trouble the minds of certain present-day environmentalists. Moreover, there was neither free oxygen nor ozone in the air, so that the surface of the Earth would have been exposed to the fierce unfiltered ultra-violet radiation of the sun. The hazards of nuclear and of ultra-violet radiation are much in mind these days and
some fear that they may destroy all life on Earth. Yet the very womb of life was flooded by the light of these fierce energies.

No man has ever seen the Sun, or ever will. What we call ‘sunlight’ is only a narrow span of the entire solar spectrum – the immensely broad band of vibrations which the Sun, our nearest star, pours into space. All the colours visible to the eye, from warm red to deepest violet, lie within a single octave of this band – for the waves of violet light have twice the frequency, or ‘pitch’ if we think in musical terms, of red. On either side of this narrow zone are ranged octave after octave of radiations to which we are totally blind.

The musical analogy is a useful one. Think of one octave on the piano – less than the span of the average hand. Imagine that you were deaf to all notes outside this range; how much, then, could you appreciate of a full orchestral score when everything from
contrabassoon
to piccolo is going full blast? Obviously you could get only the faintest idea of the composer’s intentions. In the same way, by eye alone we can obtain only a grossly restricted conception of the true ‘colour’ of the world around us.

However, let us not exaggerate our visual handicap. Though visible light is merely a single octave of the Sun’s radiation, this octave contains most of the power; the higher and lower frequencies are relatively feeble. It is, of course, no coincidence that our eyes are adapted to the most intense band of sunlight; if that band had been somewhere else in the spectrum, as is the case with other stars, evolution would have given us eyes appropriately tuned.

Nevertheless, the Sun’s invisible rays are extremely important, and affect our lives in a manner undreamed of only a few years ago. Some of them, indeed, may control our destinies – and even, as we shall see in a moment, our very existence.

The visible spectrum is, quite arbitrarily, divided up into seven primary colours – the famous sequence, red, orange, yellow, green, blue, indigo, violet, if we start from the longest waves and work down to the shortest. Seven main colours in the one octave; but the complete band of solar radiations covers at least thirty octaves, or a total frequency range of ten thousand million to one. If we could see the whole of it, therefore, we might expect to discern more than two hundred colours as distinct from each other as orange is from yellow, or green is from blue.

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 be 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.