Author: Sir James Jeans, M. A., D. Sc., Sc. D., LL. D., F. R. S.
[p. 325] We have seen how the solid substance of the material universe is continually dissolving away into intangible radiation. The sun weighed 360,000 million tons more yesterday than to-day, the difference being the weight of 24-hours’ emission of radiation which is now travelling through space, and, so far as direct observation goes, is destined to journey on through space until the end of time. The same transformation of material weight into radiation is in progress in all the stars, and to a lesser degree on earth, where complex atoms such as uranium are continually changing into the simpler atoms of lead and helium, and setting radiation free in the process. But against the sun’s daily loss of weight of 360,000 million tons, the earth is only losing weight from this cause at the rate of about ninety pounds a day.
Cyclic processes. It is natural to ask whether a study of the universe as a whole reveals these processes as part only of a closed cycle, so that the wastage which we see in progress in the sun and stars and on the earth is made good elsewhere. When we stand on the banks of a river and watch its current ever carrying water out to sea, we know that this water is in due course transformed into clouds and rain which replenish the river. Is the physical universe a similar cyclic system, or ought it rather to be compared to a stream which, having no source of replenishment, must cease flowing after it has spent itself? [p. 326]
To this question, the wide scientific principle known as the second law of thermodynamics provides an answer in very general terms. If we ask what is the underlying cause of all the varied animation we see around us in the world, the answer is in every case, energy — the chemical energy of the fuel which drives our ships, trains and cars, or of the food which keeps our bodies alive and is used in muscular effort, the mechanical energy of the earth’s motion which is responsible for the alternations of day and night, of summer and winter, of high tide and low tide, the heat energy of the sun which makes our crops grow and provides us with wind and rain.
The first law of thermodynamics, which embodies the principle of “conservation of energy,” teaches that energy is indestructible; it may change about from one form to another, but its total amount remains unaltered through all these changes, so that the total energy of the universe remains always the same. As the energy which is the cause of all the life of the universe is indestructible, it might be thought that this life could go on for ever undiminished in amount.
Availability Of Energy. The second law of thermodynamics rules out any such possibility. Energy is indestructible as regards its amount, but it continually changes in form, and generally speaking there are upward and downward directions of change. It is the usual story — the downward journey is easy, while the upward is either hard or impossible. As a consequence, more energy passes in one direction than in the other. For instance, both light and heat are [p. 327] forms of energy, and a million ergs of light- energy can be transformed into a million ergs of heat with the utmost ease ; let the light fall on any cool, black surface, and the thing is done. But the reverse transformation is impossible; a million ergs which have once assumed the form of heat, can never again assume the form of a million ergs of light. This is a special example of the general principle that radiative energy tends always to change into a form of longer, not shorter, wave-length. In general, for instance, fluorescence increases the wave-length of the light; it changes blue light into green, yellow or red, but not red light into yellow, green or blue. Exceptions to the general principle are known, but they are of special type, admitting of special explanations, and do not affect the general principle.
It may be objected that the everyday act of lighting a fire disproves all this. Has not the sun’s heat been stored up in the coal we burn, and cannot we produce light by burning coal? The answer is that the sun’s radiation is a mixture of both light and heat, and indeed of radiation of all wave-lengths. What is stored up in the coal is primarily the sun’s light and other radiation of still shorter wave-length. When we burn coal we get some light, but not as much as the sun originally put into the coal; we also get some heat, and this is more than the amount of heat which was originally put in. On balance, the net result of the whole transaction is that a certain amount of light has been transformed into a certain amount of heat.
All this shews that we must learn to think of energy, not only in terms of quantity, but also in terms of quality. Its total quantity remains always the same; this is the first law of thermodynamics. But its [p. 328] quality changes, and tends to change always in the same direction. Turnstiles are set up between the different qualities of energy ; the passage is easy in one direction, impossible in the other. A human crowd may contrive to find a way round without jumping over turnstiles, but in nature there is no way round ; this is the second law of thermodynamics. Energy flows always in the same direction, as surely as water flows downhill.
Part of the downward path consists, as we have seen, of the transition from radiation of short wave-length into radiation of longer wave-length. In terms of quanta (p. 126) the transition is from a few quanta of high energy to a large number of quanta of low energy, the total amount of energy of course remaining unaltered. The downfall of the energy accordingly consists in the breaking of its quanta into smaller units. And when once the fall and breakage have taken place, it is as impossible to reconstitute the original large quanta as it was to put Humpty-Dumpty back on his wall.
Although this is the main part of the downward path, it is not the whole of it. Thermodynamics teaches that all the different forms of energy have different degrees of “availability,” and that the downward path is always from higher to lower availability.
And now we may return to the question with which we started the present chapter: “What is it that keeps the varied life of the universe going? ” Our original answer “energy” is seen to be incomplete. Energy is no doubt essential, but the really complete answer is that it is the transformation of energy from a more available to a less available form; it is the running downhill of energy. To argue that the total energy of the universe cannot diminish, and therefore the [p. 329] universe must go on for ever, is like arguing that as a clock- weight cannot diminish, the clock-hand must go round and round for ever.
Energy cannot run downhill for ever, and, like the clock-weight, it must touch bottom at last. And so the universe cannot go on for ever; sooner or later the time must come when its last erg of energy has reached the lowest rung on the ladder of descending availability, and at this moment the active life of the universe must cease. The energy is still there, but it has lost all capacity for change; it is as little able to work the universe as the water in a flat pond is able to turn a water-wheel. We are left with a dead, although possibly a warm, universe — a “heat-death.”
Such is the teaching of modern thermodynamics. There is no reason for doubting or challenging it, and indeed it is so fully confirmed by the whole of our terrestrial experience, that it is difficult to see at what point it could be open to attack. It disposes at once of any possibility of a cyclic universe in which the events we see are as the pouring of river water into the sea, while events we do not see restore this water back to the river. The water of the river can go round and round in this way, just because it is not the whole of the universe; something extraneous to the river-cycle keeps it continually in motion — namely, the heat of the sun. But the universe as a whole cannot so go round and round. Short of postulating continuous action from outside the universe, whatever this may mean, the energy of the universe must continually lose availability; a universe in which the energy had no [p. 330] further availability to lose would be dead already. Change can occur only in the one direction, which leads to the heat-death. With universes as with mortals, the only possible life is progress to the grave.
Even the flow of the river to the sea, which we selected as an obvious instance of true cyclic motion, is seen to illustrate this, as soon as all the relevant factors are taken into account. As the river pours seaward over its falls and cascades, the tumbling of its waters generates heat, which ultimately passes off into space in the form of heat radiation. But the energy which keeps the river pouring along comes ultimately from the sun in the form mainly of light; shut off the sun’s radiation and the river will soon stop flowing. The river flows only by continually transforming light-energy into heat-energy, and as soon as the cooling sun ceases to supply energy of sufficiently high availability the flow must cease.
The same general principles may be applied to the astronomical universe. There is no question as to the way in which energy runs down here. It is first liberated in the hot interior of a star in the form of quanta of extremely short wave-length and excessively high energy. As this radiant energy struggles out to the star’s surface, it continually adjusts itself, through repeated absorption and re-emission, to the temperature of that part of the star through which it is passing. As longer wave-lengths are associated with lower temperatures (p. 140), the wave-length of the radiation is continually lengthened; a few energetic quanta are being transformed into numerous feeble quanta. Once these are free in space, they travel onward unchanged until they meet dust particles, stray atoms, free electrons, or some other form of interstellar matter. [p. 331] Except in the highly improbable event of this matter being at a higher temperature than the surfaces of the stars, these encounters still further increase the wave-length of the radiation, and the final result of innumerable encounters is radiation of very great wave-length. The quanta have increased enormously in numbers, but have paid for their increase by a corresponding decrease in individual strength. In all probability, the original very energetic quanta had their source in the annihilation of protons and electrons, so that the main process of the universe consists in the energy of exceedingly high availability which is bottled up in electrons and protons being transformed into heat-energy at the lowest level of availability.
Many, giving rein to their fancy, have speculated that this low-level heat-energy may in due course re-form itself into new electrons and protons. As the existing universe dissolves away into radiation, their imagination sees new heavens and a new earth coming into being out of the ashes of the old. But science can give no support to such fancies. Perhaps it is as well; it is hard to see what advantage could accrue from an eternal reiteration of the same theme, or even from endless variations of it.
The final state of the universe will, then, be attained when every atom which is capable of annihilation has been annihilated, and its energy transformed into heat-energy wandering for ever round space, and when all the weight of any kind whatever which is capable of being transformed into radiation has been so transformed.
We have mentioned Hubble’s estimate that matter is distributed in space at an average rate of 1.5 x 10-31 grammes per cubic centimetre. The annihilation of a [p. 332] gramme of matter liberates 9 x 1020 ergs of energy, so that the annihilation of 1.5 x 10-31 grammes of matter liberates 1.35 x 10-10 ergs of energy. It follows that the total annihilation of all the substance of the existing universe would only fill space with energy at the rate of 1.35 x 10-10 ergs per cubic centimetre. This amount of energy is only enough to raise the temperature of space from absolute zero to a temperature far below that of liquid air; it would only raise the temperature of the earth’s surface by a 6000th part of a degree Centigrade. The reason why the effect of annihilating a whole universe is so extraordinarily slight is of course that space is so extraordinarily empty of matter ; trying to warm space by annihilating all the matter in it is like trying to warm a room by burning a speck of dust here and a speck of dust there. As compared with any amount of radiation that is ever likely to be poured into it, the capacity of space is that of a bottomless pit. Indeed, so far as scientific observation goes, it is entirely possible that the radiation of thousands of dead universes may even now be wandering round space without our suspecting it.
Such is the final end of things to which, so far as present-day science can see, the material universe must inevitably come in some far-off age, unless the course of nature is changed in the meantime. Let us now try to peer back towards the beginnings of things.
As we go forwards in time, material weight continually changes into radiation. Conversely, as we go backwards in time, the total material weight of the universe must continually increase. We have seen how the present [p. 333] weights of the stars are incompatible with their having existed for more than some 5 or 10 million million years, and that they would need approximately the whole of this enormous period to acquire certain signs of age which their present arrangement and motions reveal.
We have seen that the break-up of the huge extragalactic nebulae must result in the birth of stars, and have found that the most consistent account of the origin of the galactic system of stars is provided by the supposition that the whole system originated out of the break-up of a single huge nebula some 5 to 10 million million years ago.
Let us pause for a moment to compare this with an alternative hypothesis, which some astronomers have favoured, that stars are being created all the time. On this hypothesis we picture the stars as passing in an endless steady stream from creation to extinction, just as men pass in an endless steady stream from their cradles to their graves, a new generation always coming into being to step into the place vacated by the old. On this view Plaskett’s star, with about a hundred times the weight of the sun, must be a recent creation, while Kruger 60, with only a fraction of the sun’s weight, would be very, very old — perhaps 100 million million years older than Plaskett’s star.
At present direct observation cannot definitely decide between the two conflicting hypotheses, but it rather frowns upon the “steady stream” view of the stars. In a steady population the number of people in any assigned condition is exactly proportional to the time taken to pass through that condition. Suppose for instance that human beings possess infant teeth for a quarter as long as they possess adult teeth. If [p. 334] examination of the teeth of a population shewed that four times as many had adult teeth as infant teeth, this would create a prima facie expectation that we were dealing with a steady population. If, on the contrary, 100 times as many people were found with adult teeth as with infant teeth, we should know we were not dealing with a steady population. If other evidence pointed to the population all being of approximately the same age, we should be inclined to accept this and regard the 1 per cent, of cases of infant teeth as cases of arrested development.
We do not judge the ages of stars by their teeth but by their weights and luminosities. And the luminosities of the stars are not found to conform to the statistical laws which would prevail in a steady population of stars. There appear to be so many middle-aged stars and so few infants and veterans as to make the hypothesis of a steady continuous creation hardly tenable. Indeed there is rather distinct evidence of a special creation of stars at about the time our sun was born. This leads back again quite naturally to the view that the galactic system was born out of a spiral nebula whose main activity as a parent of stars occurred some 5 to 10 million million years ago.
Pre-stellar existence. On the whole it seems likely that we must assign ages of 5 to 10 million million years to most or all of the stars in the galactic system. This is as far as we can probe back into time with any reasonable plausibility. The atoms which now form the sun and stars must no doubt have had a previous existence as atoms of a nebula, but we cannot say for how long. The temperatures at the centres of the spiral nebulae may be, and in all probability are, so high that atoms are stripped bare of electrons and so shielded [p. 335] from annihilation. We may in fact regard the gaseous centres of nebulae as a sort of “white dwarfs” built on a colossal scale. This fits on to the fact that the nebulae generate very little energy for their weights and so shine very feebly.
We have seen that the weights of two extra-galactic nebulae can be estimated to a reasonable degree of accuracy. The great Andromeda nebula M 31 has the weight of 3500 million suns, its total luminosity being that of 660 million suns. The nebula N.G.C. 4594 has the weight of 2000 million suns, and the luminosity of 260 million suns. A simple calculation shews that the atoms in the Andromeda nebula have an average expectation of life of 80 million million years, while the corresponding figure in N.G.C. 4594 is 115 million million years. From these two instances, we may guess that the average life, before annihilation, of the atoms in such nebulae must be of the order of 100 million million years. It cannot be claimed that this calculation is either very convincing or very exact, but it supplies the only evidence at present available as to the probable length of life of matter in the nebular state. We can say that the stars have existed as such for from 5 to 10 million million years, and that their atoms may have previously existed in nebulae for at least a comparable, and possibly for a much longer, time.
Apart from detailed figures, however, it is clear that we cannot go backward in time for ever. Each step back in time involves an increase in the total weight of the matter of the universe, and, just as with individual stars, we cannot go so far back that this total weight becomes infinite. Indeed a limit may quite possibly be set by considerations which we have [p. 336] already mentioned. The complete annihilation of all the matter now in the universe would raise the temperature of the earth’s surface by the six-thousandth part of a degree ; the annihilation of a million times as much matter would raise it by 160 degrees. We cannot admit that as much radiation as this can be wandering about space. The earth’s temperature is determined by the amount of radiation it receives from the sun; it adjusts its temperature so that it radiates away just as much energy as it receives. A small correction is required on account of the earth’s own radio-activity, but this need not bother us. What would bother us, and would indeed upset the balance entirely, would be the radiation of a million dead universes if this were for ever streaming on to us out of space; in this event the earth’s surface would have to rise to a temperature well above that of boiling water before it could restore the balance between the radiation it received and that it emitted. In a word, the radiation of a million dead universes would boil our seas, rivers and ourselves.
The creation of matter. All this makes it clear that the present matter of the universe cannot have existed for ever: indeed we can probably assign an upper limit to its age of, say, some such round number as 200 million million years. And, wherever we fix it, our next step back in time leads us to contemplate a definite event, or series of events, or continuous process, of creation of matter at some time not infinitely remote. In some way matter which had not previously existed, came, or was brought, into being.
If we want a naturalistic interpretation of this creation of matter, we may imagine radiant energy of any wave-length less than 1.3 x 10-13 cms. being poured into empty space; this is energy of higher [p. 337] “availability ” than any known in the present universe, and the running down of such energy might well create a universe similar to our own. The table on p. 144 shews that radiation of the wave-length just mentioned might conceivably crystallise into electrons and protons, and finally form atoms. If we want a concrete picture of such a creation, we may think of the finger of God agitating the ether.
We may avoid this sort of crude imagery by insisting on space, time, and matter being treated together and inseparably as a single system, so that it becomes meaningless to speak of space and time as existing at all before matter existed. Such a view is consonant not only with ancient metaphysical theories, but also with the modern theory of relativity (p. 74). The universe n ow becomes a finite picture whose dimensions are a certain amount of space and a certain amount of time; the protons and electrons are the streaks of paint which define the picture against its space-time background. Travelling as far back in time as we can, brings us not to the creation of the picture, but to its edge; the creation of the picture lies as much outside the picture as the artist is outside his canvas. On this view, discussing the creation of the universe in terms of time and space is like trying to discover the artist and the action of painting, by going to the edge of the picture. This brings us very near to those philosophical systems which regard the universe as a thought in the mind of its Creator, thereby reducing all discussion of material creation to futility.
Both these points of view are impregnable, but so also is that of the plain man who, recognising that it is impossible for the human mind to comprehend the full plan of the universe, decides that his own efforts shall [p. 338] stop this side of the creation of matter. This last point of view is perhaps the most justifiable of all from the purely philosophic standpoint. It is now a full quarter of a century since physical science, largely under the leadership of Poincare, left off trying to explain phenomena and resigned itself merely to describing them in the simplest way possible. To take the simplest illustration, the Victorian scientist thought it necessary to “explain” light as a wave-motion in the mechanical ether which he was for ever trying to construct out of jellies and gyroscopes; the scientist of to-day, fortunately for his sanity, has given up the attempt and is well satisfied if he can obtain a mathematical formula which will predict what light will do under specified conditions. It does not matter much whether the formula admits of a mechanical explanation or not, or whether such an explanation corresponds to any thinkable ultimate reality. The formulae of modern science are judged mainly, if not entirely, by their capacity for describing the phenomena of nature with simplicity, accuracy, and completeness. For instance, the ether has dropped out of science, not because scientists as a whole have formed a reasoned judgment that no such thing exists, but because they find they can describe all the phenomena of nature quite perfectly without it. It merely cumbers the picture, so they leave it out. If at some future time they find they need it, they will put it back again.
This does not imply any lowering of the standards or ideals of science ; it implies merely a growing conviction that the ultimate realities of the universe are at present quite beyond the reach of science, and may be — and probably are — for ever beyond the comprehension of the human mind. It is a priori probable that only the [p. 339] artist can understand the full significance of the picture he has painted, and that this will remain for ever impossible for a few specks of paint on the canvas. It is for this kind of reason that, when, as in Chapter II, we try to discuss the ultimate structure of the atom, we are driven to speak in terms of similes, metaphors, and parables. There is no need even to worry overmuch about apparent contradictions. The higher unity of ultimate reality must no doubt reconcile them all, although it remains to be seen whether this higher unity is within our comprehension or not. In the meantime a contradiction worries us about as much as an unexplained fact, but hardly more; it may or may not disappear in the progress of science.
If some such train of thought may be applied to our efforts to understand the most minute processes of the universe (and it is the common everyday train of thought of those who are working in this field), then it must surely be still more applicable to our efforts to understand the universe as a whole. Phenomena come to us disguised in their frameworks of time and space ; they are messages in cypher of which we shall not understand the ultimate significance until we have discovered how to decode them out of their space-time wrappings. Whatever may be thought about our final ability to decode the difficult messages we have recently received about the ultimate structure of the minutest parts of matter, it seems natural that we should feel some apprehension with regard to those about the structure of the universe as a whole, and particularly those about its beginnings and endings. Often enough the message itself may help us to discover the code in which it reaches us — with sufficient skill we can often do this — but we are now speaking of problems as to [p. 340] when, by whom, and for what purpose, the code was devised. There is no reason why a code message should throw any light on this.
The astronomer must leave the problem at this stage. The message of astronomy is of obvious concern to philosophy, to religion and to humanity in general, but it is not the business of the astronomer to decode it. The observing astronomer watches and records the dots and dashes of the needle which delivers the message, the theoretical astronomer translates these into words — and according as they are found to form known consistent words or not, it is known whether he has done his job well or ill — but it is for others to try to understand and explain the ultimate decoded meaning of the words he writes down.
Abandoning our efforts to understand the universe as a whole, let us glance for a moment at the relation of life to the universe we know.
The old view that every point of light in the sky represented a possible home for life is quite foreign to modern astronomy. The stars themselves have surfacetemperatures of anything from 1650 degrees to 30,000 degrees or more, and are of course at far higher temperatures inside. By far the greater part of the matter of the universe is at a temperature of millions of degrees, so that its molecules are broken up into atoms, and the atoms are broken up, partially at least, into their constituent parts. Now the very concept of life implies duration in time; there can be no life where atoms change their make-up millions of times a second and no pair of atoms can ever stay joined together. It also implies a certain mobility in space, and these two [p. 341] implications restrict life to the small range of physical conditions in which the liquid state is possible. Our survey of the universe has shewn how small this range is in comparison with that exhibited by the universe as a whole. It is not to be found in the stars, nor in the nebulae out of which the stars are born. We know of no type of astronomical body in which the conditions can be favourable to life except planets like our own revolving round a sun.
Now, to the best of our present knowledge, planets are very rare. We have seen how a single star cannot of itself produce planets. A family of planets must have two parents; it only comes into being as the result of the close approach of two stars, and stars are so sparsely scattered in space that it is an inconceivably rare event for one to pass near to a neighbour. On the Tidal Theory, explained on p. 236, planets cannot be born except when two stars pass within about three diameters of one another. As we know how the stars are scattered in space, we can estimate fairly closely how often two stars will approach within this distance of one another. The calculation shews that even after a star has lived its life of millions of millions of years, the chance is still about a hundred thousand to one against its being a sun surrounded by planets.
Even so, if life is to obtain a footing, the planets must not be too hot or too cold. In the solar system, for instance, we cannot imagine life existing on Mercury or on Neptune ; liquids boil on the former and freeze hard on the latter. These planets are unsuitable for life be cause they are too near to, or too far from, the sun. We can imagine other planets which are unsuitable because their substance itself generates energy at such a rate as to make them unsuitable for habitation. The inert [p. 342] atoms which form our earth seem to be the end products of a long series of atomic changes, a sort of final ash resulting from the combustion of the universe. We have seen how such atoms probably float to the top in every star, as being the lightest in weight, but it is by no means a foregone conclusion that all planets will consist of nothing but inert atoms, and so will cool down until life can obtain a footing on them. This has happened with our earth, but we do not know how many planets and planetary systems may be unsuited for life because it has not happened with them.
All this suggests that only an infinitesimally small corner of the universe can be in the least suited to form an abode of life. Primaeval matter must go on transforming itself into radiation for millions of millions of years to produce a minute quantity of the inert ash on which life can exist. Then by an almost incredible accident this ash, and nothing else, must be torn out of the sun which has produced it, and condense into a planet. Even then, this residue of ash must not be too hot or too cold, or life will be impossible.
Finally, after all these conditions are satisfied, will life come or will it not? We must probably discard the at one time widely accepted view that once life had come into the universe in any way whatsoever, it would rapidly spread from planet to planet and from one planetary system to another until the whole universe teemed with life; space now seems too cold, and planetary systems too far apart. Our terrestrial life must in all probability have originated on the earth itself. What we would like to know is whether it originated as the result of still another amazing accident or succession of coincidences, or whether it is the normal event for inanimate matter to produce life in due course, when the physical [p. 343] environment is suitable. We look to the biologist for the answer, which so far he has not been able to produce.
The astronomer might be able to give a partial answer if he could find evidence of life on some other planet, for we should then at least know that life had occurred more than once in the history of the universe, but so far no convincing evidence has been forthcoming. Some astronomers interpret certain markings on Mars as canals, which they believe to be the handiwork of intelligent beings, but this interpretation is not generally accepted. Again, seasonal changes necessarily occur on Mars as on the earth, and certain phenomena accompany these which many astronomers are inclined to ascribe to the growth and decline of vegetation, although they may represent nothing more than rains watering the desert. There is no definite evidence of life, and certainly no evidence of conscious life, on Mars — or indeed anywhere else in the universe.
It seems at first somewhat surprising that oxygen figures so largely in the earth’s atmosphere, in view of its readiness to enter into chemical combination with other substances. We know, however, that vegetation is continually discharging oxygen into the atmosphere, and it has often been suggested that the oxygen of the earth’s atmosphere may be mainly or entirely of vegetable origin. If so, the presence or absence of oxygen in the atmospheres of other planets should shew whether vegetation similar to that we have on earth exists on these planets or not.
Oxygen certainly exists in the Martian atmosphere, but its amount is small. Adams and St John estimate that there cannot be more than 15 per cent, as much, per square mile, as on earth. On the other hand it is either completely absent, or of negligible amount, in [p. 343] the atmosphere of Venus. If any is present at all, St John estimates that the amount above the clouds which cover the surface of Venus is less than 0.1 per cent, of the terrestrial amount. The evidence, for what it is worth, goes to suggest that Venus, the only planet in the solar system outside Mars and the earth on which life could possibly exist, possesses no vegetation and no oxygen for higher forms of life to breathe.
Apart from the certain knowledge that life exists on earth, we have no definite knowledge whatever except that, at the best, life must be limited to a tiny fraction of the universe. Millions of millions of stars exist which support no life, which have never done so and never will do so. Of the rare planetary systems in the sky, many must be entirely lifeless, and in others life, if it exists at all, is probably limited to a few planets. The three centuries which have elapsed since Giordano Bruno suffered martyrdom for believing in the plurality of worlds have changed our conception of the universe almost beyond description, but they have not brought us appreciably nearer to understanding the relation of life to the universe. We can still only guess as to the meaning of this life which, to all appearances, is so rare. Is it the final climax towards which the whole creation moves, for which the millions of millions of years of transformation of matter in uninhabited stars and nebulae, and of the waste of radiation in desert space, have been only an incredibly extravagant preparation? Or is it a mere accidental and possibly quite unimportant by-product of natural processes, which have some other and more stupendous end in view? Or, to glance at a still more modest line of thought, must we regard it as something of the nature of a disease, which affects matter in its old age when it has lost the high [p. 345] temperature and capacity for generating high-frequency radiation with which younger and more vigorous matter would at once destroy life? Or, throwing humility aside, shall we venture to imagine that it is the only reality, which creates, instead of being created by, the colossal masses of the stars and nebulae and the almost inconceivably long vistas of astronomical time?
Again it is not for the astronomer to select between these alternative guesses ; his task is done when he has delivered the message of astronomy. Perhaps it is over-rash for him even to formulate the questions this message suggests.
Let us leave these rather abstract regions of thought and come down to earth. We feel the solid earth under our feet, and the rays of the sun overhead. Somehow, but we know not how or why, life also is here; we ourselves are part of it. And it is natural to enquire what astronomy has to say as to its future prospects.
The central facts which dominate the whole situation are that we are dependent on the light and heat of the sun, and that these cannot remain for ever as they now are. So far as we can at present see, solar conditions can hardly have changed much since the earth was born; the earth’s 2000 million years form so small a fraction of the sun’s whole life that we can almost suppose the sun to have stood still throughout it. This of itself suggests that, in so far as astronomical factors are concerned, life may look to a tenancy of the earth of far longer duration than the total past age of the earth.
The earth, which started life as a hot mass of gas, has gradually cooled, until it has now about touched bottom, and has almost no heat beyond that which it [p. 346] receives from the sun. This just about balances the amount it radiates away into space, so that it would stay at its present temperature for ever if external conditions did not change, and any changes in its condition will be forced on it by changes occurring outside.
These external changes may be of many kinds. The sun’s loss of weight causes the earth to recede from it at the rate of about a yard a century, so that after a million million years, the earth will be 10 per cent, further away from the source of its light and life than now. Consequently even if the sun then radiated as much light and heat as now, the earth would receive 20 per cent, less of this radiation, and its mean temperature would be some 15 degrees Centigrade or so lower than at present. But after a million million years the sun will not radiate as much light and heat as now; it will have lost some 6 per cent, of its present weight through radiation, and, judging from other stars, this loss will probably reduce its energy-generating capacity by about 20 per cent. This will reduce the earth’s temperature by about another 15 degrees, so that after a million million years the inevitable course of events will have reduced the earth’s temperature by about 30 degrees Centigrade.
It would be rash to attempt to predict how such a fall of temperature may affect terrestrial life, and human life in particular. Given sufficient time, life has such an enormous capacity for adapting itself to its environment that it seems possible that, even with a temperature 30 degrees Centigrade lower than now, life may still exist on earth a million million years hence. If so, I am glad that my life has not fallen in this far distant future. Mountains and seas, which provide some of [p. 347] the keenest pleasures of our present life, will exist only as traditions handed down from a remote and almost incredible past. The denudation of a million million years will have reduced the mountains almost to plains, while seas and rivers will be frozen packs of solid ice. We may well imagine that man will have infinitely more knowledge than now, but he will no longer know the thrill of pleasure of the pioneer who opens up new realms of knowledge. Disease, and perhaps death, will have been conquered, and life will doubtless be safer and incomparably better-ordered than now. It will seem incredible that a time could have existed when men risked, and lost, their lives in traversing unexplored country, in climbing hitherto unclimbed peaks, in fighting wild beasts for the fun of it. Life will be more of a routine and less of an adventure than now ; it will also be more purposeless when the human race knows that within a measurable space of time it must face extinction and the eternal destruction of all its hopes, endeavours, and achievements.
Without laying too much stress on these visionary concepts of life a million million years hence, we may nevertheless think of this as the period in round numbers after which the inevitable wastage of the sun’s weight is likely to drive life off the earth. Venus, with a mean temperature some 60 degrees higher than the earth, is probably rather too hot for life at present. But after a million million years, the temperature of Venus will have fallen by 40 degrees, and what the earth is now, Venus may perhaps be somewhere between one and two million million years hence. Whether life will then inhabit Venus we cannot know, and it would be futile to guess, but there is at least a chance that as the earth fails, Venus may step into its place. Possibly [p. 348] Venus may be followed by Mercury in due course, but the present evidence is that Mercury is devoid of atmosphere, in which case it is hard to imagine it as a home for life at all resembling that which now inhabits the earth.
So far we have considered only the normal course of events; a variety of accidents may bring the human race to an end long before a million million years have elapsed. To mention only possible astronomical occurrences, the sun may run into another star, any asteroid may hit any other asteroid and, as a result, be so deflected from its path as to strike the earth, any of the stars in space may wander into the solar system and, in so doing, upset all the planetary orbits to such an extent that the earth becomes impossible as an abode of life. It is difficult to estimate the likelihood of any of these events happening but they all seem very improbable, and the first and last highly so. Let us disregard them all.
A danger remains which cannot be so lightly dismissed. Let us first state it in technical language. The sun is a main-sequence star, and is moreover very near to the left-hand edge of the main-sequence in the Russell diagram (p. 278). Beyond this edge is a region of the diagram which is completely untenanted by stars. We have supposed this region to be untenanted by stars because the stellar configurations it represents would be unstable. Stars pass through it rapidly until they find a stable configuration, and so end up in a region which can be permanently tenanted by stars. Now the next stable configurations beyond this region are those of the white dwarfs, and as these are less massive as a class than the main-sequence stars, the general trend of stellar evolution appears to be from main-sequence star [p. 349] to white dwarf. On this view the white dwarfs must have previously been mainsequence stars which wandered across the left-hand edge of the band of stable configurations and then fell through the unstable region until they resumed stability as white dwarfs.
The danger lies in the fact that the sun is already perilously near to the left-hand edge of the mainsequence. According to Redman’s determinations, which are probably by far the most reliable at present available, the main-sequence belt of stable configurations for stars of the same spectral type as the sun (G 0) extends roughly between stellar absolute magnitudes, 4.88 and 3.54, the former marking the dangerous left- hand edge. The sun’s present absolute magnitude is estimated as 4.85. Thus if the sun were to become 0.03 magnitudes fainter, this representing a reduction of only 3 per cent, in its luminosity, it would arrive exactly at the edge of the main-sequence, and would proceed to contract precipitately to the white dwarf state. In so doing, its light and heat would diminish to such an extent that life would be banished from the earth. The known white dwarf star which it would most closely resemble is the companion of Sirius, and this emits only a four-hundredth part as much light and heat as the sun.
To put the same thing in non-technical language, the sun is in, or is not far from, a precarious state in which stars are liable to begin to shrink and in so doing to reduce their radiation to a tiny fraction of that at present emitted by the sun. The shrinkage of the sun to this state would transform our oceans into ice and our atmosphere into liquid air; it seems impossible that terrestrial life could survive. The vast museum of the sky must almost certainly contain examples of [p. 350] shrunken suns of this type with planets like our earth revolving round them. Whether these planets carry on them the frozen remains of a life which was once as active as our present life on earth we can hardly even surmise.
This may be thought to open up a startling prospect for the earth, but we can take courage for several reasons. In the first place a 3 per cent, decrease in the sun’s luminosity can hardly occur in less than about 150,000 million years. This in itself is not too bad, but the prospect becomes enormously more hopeful when we reflect that the evolution of the stars, including the sun, takes place in a direction almost parallel to the edge of the main-sequence. The sun is not heading for the precipice, so much as skirting along its edge. Whether it is approaching the edge, and is ultimately destined to fall over, we do not know, but it is in any case unlikely to reach the edge within the next million million years.
Finally, the sun’s distance from the edge of the main-sequence cannot be estimated with anything like the degree of accuracy assumed in the foregoing calculations. The figure of 0.03 appeared as the difference of two much larger numbers, and although both of these can be estimated with fair accuracy, neither can be estimated with sufficient accuracy to justify us in treating their small difference of 0.03 as exact. The most we can say is that the sun is quite fairly near to the dangerous edge, but that any appreciable motion towards this edge is a matter of millions of millions of years.
Another danger, of a more speculative kind, must also be mentioned. We have seen (p. 61) how every now and then a new star appears in the sky, shines [p. 351] with terrific brilliancy for a short time, and then either fades away entirely or continues to shine as an ordinary star. These apparitions are known as “novae” — new stars. In many cases the nova has been proved to be an ordinary star which was visible as a very faint star long before it appeared as a nova, flashed into brilliance for a brief span of life, and then lapsed back into commonplaceness, and it seems reasonable to suppose that all novae are of this kind, although the star may often escape detection until it assumes its brilliant nova state. These apparitions are by no means rare; something like six appear every year in the galactic system alone. Now if we suppose the galactic system to consist of 300,000 million stars, this means that, on the average, each star becomes a nova once in every 50,000 million years. What we would like to know is whether our sun is in danger of becoming a nova; for, if all kinds of stars run equal chances, it is likely to become a nova some twenty times in the next million million years.
So far there is no agreement among astronomers either as to the physical causes which turn an ordinary star into a nova, or as to the physical conditions which prevail in novae. Various suggestions are in the field, but none of them wins general acceptance. It seems fairly certain that if our sun were suddenly to become a nova, its emission of light and heat would so increase as to scorch all life off the earth, but we are completely in the dark as to whether our sun runs any risk of entering the nova stage. If it does, this is probably the greatest of all the risks to which life on earth is exposed.
Apart from accidents, we have seen that if the solar system is left to the natural course of evolution, the [p. 352] earth is likely to remain a possible abode of life for something of the order of a million million years to come.
This is some five hundred times the past age of the earth, and over three million times the period through which humanity has so far existed on earth. Let us try to see these times in their proper proportion by the help of yet another simple model. Take a postage-stamp, and stick it on to a penny. Now climb Cleopatra’s needle and lay the penny flat, postage-stamp upper- most, on top of the obelisk. The height of the whole structure may be taken to represent the time that has elapsed since the earth was born. On this scale, the thickness of the penny and postage-stamp together represents the time that man has lived on earth. The thickness of the postage-stamp represents the time he has been civilised, the thickness of the penny representing the time he lived in an uncivilised state. Now stick another postage-stamp on top of the first to represent the next 5000 years of civilisation, and keep sticking on postage-stamps until you have a pile as high as Mont Blanc. Even now the pile forms an inadequate representation of the length of the future which, so far as astronomy can see, probably stretches before civilised humanity, unless an accident cuts it short. The first postage-stamp was the past of civilisation ; the column higher than Mont Blanc is its future. Or, to look at it in another way, the first postage-stamp represents what man has already achieved; the pile which outtops Mont Blanc represents what he may achieve, if his future achievement is proportional to his time on earth.
Yet we have seen that we cannot count on such a length of future with any certainty. Accidents may [p. 353] happen to the race as to the individual. Celestial collisions may occur; shrinking into a white dwarf, the sun may freeze terrestrial life out of existence ; bursting out as a nova it may scorch our race to death. Accident may replace our Mont Blanc of postage-stamps by a truncated column of only a fraction of the height of Mont Blanc. Even so, there is a prospect of tens of thousands of millions of years before our race. And the human mind, as apart from the mind of the mathematician, can hardly distinguish clearly between such a period as this and the million million years to which we may look forward if accidents do not overtake us. For all practical purpose the only statement that conveys any real meaning is that our race may look forward to occupying the earth for a time incomparably longer than any we can imagine.
Looked at in terms of space, the message of astronomy is at best one of melancholy grandeur and oppressive vastness. Looked at in terms of time, it becomes one of almost endless possibility and hope. As denizens of the universe we may be living near its end rather than its beginning; for it seems likely that most of the universe had melted into radiation before we appeared on the scene. But as inhabitants of the earth, we are living at the very beginning of time. We have come into being in the fresh glory of the dawn, and a day of almost unthinkable length stretches before us with unimaginable opportunities for accomplishment. Our descendants of far-off ages, looking down this long vista of time from the other end, will see our present age as the misty morning of the world’s history; our contemporaries of to-day will appear as dim heroic figures who fought their way through jungles of ignorance, error and superstition to discover truth, to [p. 354] learn how to harness the forces of nature, and to make a world worthy for mankind to live in. We are still too much engulfed in the greyness of the morning mists to be able to imagine, however vaguely, how this world of ours will appear to those who will come after us and see it in the full light of day. But by what light we have, we seem to discern that the main message of astronomy is one of hope to the race and of responsibility to the individual — of responsibility because we are drawing plans and laying foundations for a longer future than we can well imagine.