The Dying Sun by Sir James Jeans
A few stars are known which are hardly bigger than the earth, but the majority are so large that hundreds of thousands of earths could be packed inside each and leave room to spare; here and there we come upon a giant star large enough to contain millions of millions of earths. And the total number of stars in the universe is probably something like the total number of grains of sand on all the seashores of the world. Such is the littleness of our home in space when measured up against the total substance of the universe.
This vast multitude of stars are wandering about in space. A few form groups which journey in company, but the majority are solitary travellers. And they travel through a universe so spacious that it is an event of almost unimaginable rarity for a star to come anywhere near to another star. For the most part each voyages in splendid isolation, like a ship on an empty ocean. In a scale model in which the stars are ships, the average ship will be well over a million miles from its nearest neighbour, whence it is easy to understand why a ship seldom finds another within hailing distance.
We believe, nevertheless, that some two thousand million years ago this rare event took place, and that a second star, wandering blindly through space, happened to come within hailing distance of the sun. Just as the sun and moon raise tides on the earth, so this second star must have raised tides on the surface of the sun. But they would be very different from the puny tides which the small mass of the moon raises in our oceans; a huge tidal wave must have travelled over the surface of the sun, ultimately forming a mountain of prodigious height, which would rise ever higher and higher as the cause of the disturbance came nearer and nearer. And, before the second star began to recede, its tidal pull had become so powerful that this mountain was torn to pieces and threw off small fragments of itself, much as the crest of a wave throws off spray. These small fragments have been circulating around their parent sun ever since. They are the planets, great and small, of which our earth is one.
The sun and the other stars we see in the sky are all intensely hot—far too hot for life to be able to obtain or retain a footing on them. So also no doubt were the ejected fragments of the sun when they were first thrown off. Gradually they cooled, until now they have but little intrinsic heat left, their warmth being derived almost entirely from the radiation which the sun pours down upon them. In course of time, we know not how, when, or why, one of these cooling fragments gave birth to life. It started in simple organisms whose vital capacities consisted of little beyond reproduction and death. But from these humble beginnings emerged a stream of life which, advancing through ever greater and greater complexity, has culminated in beings whose lives are largely centred in their emotions and ambitions, their aesthetic appreciations, and the religions in which their highest hopes and noblest aspirations lie enshrined.
Although we cannot speak with any certainty, it seems most likely that humanity came into existence in some such way as this. Standing on our microscopic fragment of a grain of sand, we attempt to discover the nature and purpose of the universe which surrounds our home in space and time. Our first impression is something akin to terror. We find the universe terrifying because of its vast meaningless distances, terrifying because of its inconceivably long vistas of time which dwarf human history to the twinkling of an eye, terrifying because of our extreme loneliness, and because of the material insignificance of our home in space —a millionth part of a grain of sand out of all the seasand in the world. But above all else, we find the universe terrifying because it appears to be indifferent to life like our own; emotion, ambition and achievement, art and religion all seem equally foreign to its plan. Perhaps indeed we ought to say it appears to be actively hostile to life like our own. For the most part, empty space is so cold that all life in it would be frozen; most of the matter in space is so hot as to make life on it impossible; space is traversed, and astronomical bodies continually bombarded, by radiation of a variety of kinds, much of which is probably inimical to, or even destructive of, life.
Into such a universe we have stumbled, if not exactly by mistake, at least as the result of what may properly be described as an accident. The use of such a word need not imply any surprise that our earth exists, for accidents will happen, and if the universe goes on for long enough, every conceivable accident is likely to happen in time. It was, I think, Huxley who said that six monkeys, set to strum unintelligently on typewriters for millions of millions of years, would be bound in time to write all the books in the British Museum. If we examined the last page which a particular monkey had typed, and found that it had chanced, in its blind strumming, to type a Shakespeare sonnet, we should rightly regard the occurrence as a remarkable accident, but if we looked through all the millions of pages the monkeys had turned off in untold millions of years, we might be sure of finding a Shakespeare sonnet somewhere amongst them, the product of the blind play of chance. In the same way, millions of millions of stars wandering blindly through space for millions of millions of years are bound to meet with every kind of accident; a limited number are bound to meet with that special kind of accident which calls planetary systems into being. Yet calculation shews that the number of these can at most be very small in comparison with the total number of stars in the sky; planetary systems must be exceedingly rare objects in space.
This rarity of planetary systems is important, because so far as we can see, life of the kind we know on earth could only originate on planets like the earth. It needs suitable physical conditions for its appearance, the most important of which is a temperature at which substances can exist in the liquid state.
The stars themselves are disqualified by being far too hot. We may think of them as a vast collection of fires scattered throughout space, providing warmth in a climate which is at most some four degrees above absolute zero—about 484 degrees of frost on our Fahrenheit scale—and is even lower in the vast stretches of space which lie out beyond the Milky Way. Away from the fires there is this unimaginable cold of hundreds of degrees of frost; close up to them there is a temperature of thousands of degrees, at which all solids melt, all liquids boil.
Life can only exist inside a narrow temperate zone which surrounds each of these fires at a very definite distance. Outside these zones life would be frozen; inside, it would be shrivelled up. At a rough computation, these zones within which life is possible, all added together, constitute less than a thousand million millionth part of the whole of space. And even inside them, life must be of very rare occurrence, for it is so unusual an accident for suns to throw off planets as our own sun has done, that probably only about one star in 100,000 has a planet revolving round it in the small zone in which life is possible.
Just for this reason it seems incredible that the universe can have been designed primarily to produce life like our own; had it been so, surely we might have expected to find a better proportion between the magnitude of the mechanism and the amount of the product. At first glance at least, life seems to be an utterly unimportant by-product; we living things are somehow off the main line.
We do not know whether suitable physical conditions are sufficient in themselves to produce life. One school of thought holds that as the earth gradually cooled, it was natural, and indeed almost inevitable, that life should come. Another holds that after one accident had brought the earth into being, a second was necessary to produce life. The material constituents of a living body are perfectly ordinary chemical atoms—carbon, such as we find in soot or lampblack; hydrogen and oxygen, such as we find in water; nitrogen, such as forms the greater part of the atmosphere; and so on. Every kind of atom necessary for life must have existed on the new-born earth. At intervals, a group of atoms might happen to arrange themselves in the way in which they are arranged in the living cell. Indeed, given sufficient time, they would be certain to do so, just as certain as the six monkeys would be certain, given sufficient time, to type off a Shakespeare sonnet. But would they then be a living cell? In other words, is a living cell merely a group of ordinary atoms arranged in some non-ordinary way, or is it something more? Is it merely atoms, or is it atoms plus life? Or, to put it in another way, could a sufficiently skillful chemist create life out of the necessary atoms, as a boy can create a machine out of " Meccano," and then make it go? We do not know the answer. When it comes it will give us some indication whether other worlds in space are inhabited like ours, and so must have the greatest influence on our interpretation of the meaning of life—it may well produce a greater revolution of thought than Galileo's astronomy or Darwin's biology.
We do, however, know that while living matter consists of quite ordinary atoms, it consists in the main of atoms which have a special capacity for coagulating into extraordinary large bunches or "molecules."
Most atoms do not possess this property. The atoms of hydrogen and oxygen, for instance, may combine to form molecules of hydrogen (H2 or H3), of oxygen or ozone (O2 or O3), of water (H2O), or of hydrogen peroxide (H2O2), but none of these compounds contains more than four atoms. The addition of nitrogen does not greatly change the situation; the compounds of hydrogen, oxygen and nitrogen all contain comparatively few atoms. But the further addition of carbon completely transforms the picture; the atoms of hydrogen, oxygen, nitrogen and carbon combine to form molecules containing hundreds, thousands, and even tens of thousands, of atoms. It is of such molecules that living bodies are mainly formed. Until a century ago it was commonly supposed that some "vital force" was necessary to produce these and the other substances which entered into the composition of the living body. Then Wohler produced urea (CC^NH^), which is a typical animal product, in his laboratory, by the ordinary processes of chemical synthesis, and other constituents of the living body followed in due course. To-day one phenomenon after another which was at one time attributed to "vital force" is being traced to the action of the ordinary processes of physics and chemistry. Although the problem is still far from solution, it is becoming increasingly likely that what specially distinguishes the matter of living bodies is the presence not of a " vital force," but of the quite commonplace element carbon, always in conjunction with other atoms with which it forms exceptionally large molecules.
If this is so, life exists in the universe only because the carbon atom possesses certain exceptional properties. Perhaps carbon is rather noteworthy chemically as forming a sort of transition between the metals and non-metals, but so far nothing in the physical constitution of the carbon atom is known to account for its very special capacity for binding other atoms together. The carbon atom consists of six electrons revolving around the appropriate central nucleus, like six planets revolving around a central sun; it appears to differ from its two nearest neighbours in the table of chemical elements, the atoms of boron and nitrogen, only in having one electron more than the former and one electron fewer than the latter. Yet this slight difference must account in the last resort for all the difference between life and absence of life. No doubt the reason why the six electron atom possesses these remarkable properties resides somewhere in the ultimate laws of nature, but mathematical physics has not yet fathomed it.
Other similar cases are known to chemistry. Magnetic phenomena appear in a tremendous degree in iron, and in a lesser degree in its neighbours, nickel and cobalt. The atoms of these elements have 26, 27 and 28 electrons respectively. The magnetic properties of all other atoms are almost negligible in comparison. Somehow, then, although again mathematical physics has not yet unravelled how, magnetism depends on the peculiar properties of the 26, 27 and 28 electron atoms, especially the first. Radio-activity provides a third instance, being confined, with insignificant exceptions, to atoms having from 83 to 92 electrons; again we do not know why.
Thus chemistry can only tell us to place life in the same category as magnetism and radio-activity. The universe is built so as to operate according to certain laws. As a consequence of these laws, atoms having certain definite numbers of electrons, namely 6, 26 to 28, and 83 to 92, have certain special properties, which shew themselves in the phenomena of life, magnetism and radio-activity respectively. An omnipotent creator, subject to no limitations whatever, would not have been restricted to the laws which prevail in the present universe; he might have elected to build the universe to conform to any one of innumerable other sets of laws. If some other set of laws had been chosen, other special atoms might have had other special properties associated with them. We cannot say what, but it seems a priori unlikely that either radio-activity or magnetism or life would have figured amongst them. Chemistry suggests that, like magnetism and radio-activity, life may merely be an accidental consequence of the special set of laws by which the present universe is governed.
Again the word "accidental" may be challenged. For what if the creator of the universe selected one special set of laws just because they led to the appearance of life? What if this were his way of creating life? So long as we think of the creator as a magnified man-like being, activated by feelings and interests like our own, the challenge cannot be met, except perhaps by the remark that, when such a creator has once been postulated, no argument can add much to what has already been assumed. If, however, we dismiss every trace of anthropomorphism from our minds, there remains no reason for supposing that the present laws were specially selected in order to produce life. They are just as likely, for instance, to have been selected in order to produce magnetism or radio-activity—indeed more likely, since to all appearances physics plays an incomparably greater part in the universe than biology. Viewed from a strictly material standpoint, the utter insignificance of life would seem to go far towards dispelling any idea that it forms a special interest of the Great Architect of the universe.
A trivial analogy may exhibit the situation in a clearer light. An unimaginative sailor, accustomed to tying knots, might think it would be impossible to cross the ocean if tying knots were impossible. Now the capacity for tying knots is limited to space of three dimensions; no knot can be tied in a space of 1, 2, 4, 5 or any other number of dimensions. From this fact our unimaginative sailor might reason that a beneficent creator must have had sailors under his special patronage, and have chosen that space should have three dimensions in order that tying knots and crossing the ocean should be possibilities in the universe he had created—in brief, space was of three dimensions so that there could be sailors. This and the argument outlined above seem to be much on a level, because life as a whole and the tying of knots are pretty much on a level in that neither of them forms more than an utterly insignificant fraction of the total activity of the material universe.
So much for the surprising manner in which, so far as science can at present inform us, we came into being. And our bewilderment is only increased when we attempt to pass from our origins to an understanding of the purpose of our existence, or to foresee the destiny which fate has in store for our race.
Life of the kind we know can only exist under suitable conditions of light and heat; we only exist ourselves because the earth receives exactly the right amount of radiation from the sun; upset the balance in either direction, of excess or defect, and life must disappear from the earth. And the essence of the situation is that the balance is very easily upset.
Primitive man, living in the temperate zone of the earth, must have watched the ice-age descending on his home with something like terror; each year the glaciers came farther down into the valleys; each winter the sun seemed less able to provide the warmth needed for life. To him, as to us, the universe must have seemed hostile to life.
We of these later days, living in the narrow temperate zone surrounding our sun and peering into the far future, see an ice-age of a different kind threatening us. Just as Tantalus, standing in a lake so deep that he only just escaped drowning, was yet destined to die of thirst, so it is the tragedy of our race that it is probably destined to die of cold, while the greater part of the substance of the universe still remains too hot for life to obtain a footing. The sun, having no extraneous supply of heat, must necessarily emit ever less and less of its life-giving radiation, and, as it does so, the temperate zone of space, within which alone life can exist, must close in around it. To remain a possible abode of life, our earth would need to move in ever nearer and nearer to the dying sun. Yet, science tells us that, so far from its moving inwards, inexorable dynamical laws are even now driving it ever farther away from the sun into the outer cold and darkness. And, so far as we can see, they must continue to do so until life is frozen off the earth, unless indeed some celestial collision or cataclysm intervenes to destroy life even earlier by a more speedy death. This prospective fate is not peculiar to our earth; other suns must die like our own, and any life there may be on other planets must meet the same inglorious end.
Physics tells the same story as astronomy. For, independently of all astronomical considerations, the general physical principle known as the second law of thermodynamics predicts that there can be but one end to the universe—a "heat-death" in which the total energy of the universe is uniformly distributed, and all the substance of the universe is at the same temperature. This temperature will be so low as to make life impossible. It matters little by what particular road this final state is reached; all roads lead to Rome, and the end of the journey cannot be other than universal death.
Is this, then, all that life amounts to—to stumble, almost by mistake, into a universe which was clearly not designed for life, and which, to all appearances, is either totally indifferent or definitely hostile to it, to stay clinging on to a fragment of a grain of sand until we are frozen off, to strut our tiny hour on our tiny stage with the knowledge that our aspirations are all doomed to final frustration, and that our achievements must perish with our race, leaving the universe as though we had never been?
Astronomy suggests the question, but it is, I think, mainly to physics that we must turn for an answer. For astronomy can tell us of the present arrangement of the universe, of the vastness and vacuity of space, and of our own insignificance therein; it can even tell us something as to the nature of the changes produced by the passage of time. But we must probe deep into the fundamental nature of things before we can expect to find the answer to our question. And this is not the province of astronomy; rather we shall find that our quest takes us right into the heart of modern physical science.
The Dying Sun by Sir James Jeans
(a) What is the role of gasses in the creation of life?
(b) How did the Earth and other planets come into being?
(c) What are planets?
(d) Compare the size of the Earth and other planets.
(e) How did life begin on the Earth?