The Unseen World, and Other Essays

by John Fiske

Our attention should first be directed to the enormous waste of energy which has accompanied this contraction of the solar nebula. The first result of such a contraction is the generation of a great quantity of heat, and when the heat thus generated has been lost by radiation into surrounding space it becomes possible for the contraction to continue. Thus, as concentration goes on, heat is incessantly generated and incessantly dissipated. How long this process is to endure depends chiefly on the size of the contracting mass, as small bodies radiate heat much faster than large ones. The moon seems to be already thoroughly refrigerated, while Jupiter and Saturn are very much hotter than the earth, as is shown by the tremendous atmospheric phenomena which occur on their surfaces. The sun, again, generates heat so rapidly, owing to his great energy of contraction, and loses it so slowly, owing to his great size, that his surface is always kept in a state of incandescence. His surface-temperature is estimated at some three million degrees of Fahrenheit, and a diminution of his diameter far too small to be detected by the finest existing instruments would suffice to maintain the present supply of heat for more than fifty centuries. These facts point to a very long future during which the sun will continue to warm the earth and its companion planets, but at the same time they carry on their face the story of inevitable ultimate doom. If things continue to go on as they have all along gone on, the sun must by and by grow black and cold, and all life whatever throughout the solar system must come to an end. Long before this consummation, however, life will probably have become extinct through the refrigeration of each of the planets into a state like the present state of the moon, in which the atmosphere and oceans have disappeared from the surface. No doubt the sun will continue to give out heat a long time after heat has ceased to be needed for the support of living organisms. For the final refrigeration of the sun will long be postponed by the fate of the planets themselves. The separation of the planets from their parent solar mass seems to be after all but a temporary separation. So nicely balanced are they now in their orbits that they may well seem capable of rolling on in their present courses forever. But this is not the case. Two sets of circumstances are all the while striving, the one to drive the planets farther away from the sun, the other to draw them all into it. On the one hand, every body in our system which contains fluid matter has tides raised upon its surface by the attraction of neighbouring bodies. All the planets raise tides upon the surface of the sun and the periodicity of sun-spots (or solar cyclones) depends upon this fact. These tidal waves act as a drag or brake upon the rotation of the sun, somewhat diminishing its rapidity. But, in conformity with a principle of mechanics well known to astronomers, though not familiar to the general reader, all the motion of rotation thus lost by the sun is added to the planets in the shape of annual motion of revolution, and thus their orbits all tend to enlarge,—they all tend to recede somewhat from the sun. But this state of things, though long-enduring enough, is after all only temporary, and will at any rate come to an end when the sun and planets have become solid. Meanwhile another set of circumstances is all the time tending to bring the planets nearer to the sun, and in the long run must gain the mastery. The space through which the planets move is filled with a kind of matter which serves as a medium for the transmission of heat and light, and this kind of matter, though different in some respects from ordinary ponderable matter, is yet like it in exerting friction. This friction is almost infinitely little, yet it has a wellnigh infinite length of time to work in, and during all this wellnigh infinite length of time it is slowly eating up the momentum of the planets and diminishing their ability to maintain their distances from the sun. Hence in course of time the planets will all fall into the sun, one after another, so that the solar system will end, as it began, by consisting of a single mass of matter.

But this is by no means the end of the story. When two bodies rush together, each parts with some of its energy of motion, and this lost energy of motion reappears as heat. In the concussion of two cosmical bodies, like the sun and the earth, an enormous quantity of motion is thus converted into heat. Now heat, when not allowed to radiate, or when generated faster than it can be radiated, is transformed into motion of expansion. Hence the shock of sun and planet would at once result in the vaporization of both bodies; and there can be no doubt that by the time the sun has absorbed the outermost of his attendant planets, he will have resumed something like his original nebulous condition. He will have been dilated into a huge mass of vapour, and will have become fit for a new process of contraction and for a new production of life-bearing planets.

We are now, however, confronted by an interesting but difficult question. Throughout all this grand past and future career of the solar system which we have just briefly traced, we have been witnessing a most prodigal dissipation of energy in the shape of radiant heat. At the outset we had an enormous quantity of what is called "energy of position," that is, the outer parts of our primitive nebula had a very long distance through which to travel towards one another in the slow process of concentration; and this distance was the measure of the quantity of work possible to our system. As the particles of our nebula drew nearer and nearer together, the energy of position continually lost reappeared continually as heat, of which the greater part was radiated off, but of which a certain amount was retained. All the gigantic amount of work achieved in the geologic development of our earth and its companion planets, and in the development of life wherever life may exist in our system, has been the product of this retained heat. At the present day the same wasteful process is going on. Each moment the sun's particles are losing energy of position as they draw closer and closer together, and the heat into which this lost energy is metamorphosed is poured out most prodigally in every direction. Let us consider for a moment how little of it gets used in our system. The earth's orbit is a nearly circular figure more than five hundred million miles in circumference, while only eight thousand miles of this path are at any one time occupied by the earth's mass. Through these eight thousand miles the sun's radiated energy is doing work, but through the remainder of the five hundred million it is idle and wasted. But the case is far more striking when we reflect that it is not in the plane of the earth's orbit only that the sun's radiance is being poured out. It is not an affair of a circle, but of a sphere. In order to utilize all the solar rays, we should need to have an immense number of earths arranged so as to touch each other, forming a hollow sphere around the sun, with the present radius of the earth's orbit. We may well believe Professor Tyndall, therefore, when he tells us that all the solar radiance we receive is less than a two-billionth part of what is sent flying through the desert regions of space. Some of the immense residue of course hits other planets stationed in the way of it, and is utilized upon their surfaces; but the planets, all put together, stop so little of the total quantity that our startling illustration is not materially altered by taking them into the account. Now this two-billionth part of the solar radiance poured out from moment to moment suffices to blow every wind, to raise every cloud, to drive every engine, to build up the tissue of every plant, to sustain the activity of every animal, including man, upon the surface of our vast and stately globe. Considering the wondrous richness and variety of the terrestrial life wrought out by the few sunbeams which we catch in our career through space, we may well pause overwhelmed and stupefied at the thought of the incalculable possibilities of existence which are thrown away with the potent actinism that darts unceasingly into the unfathomed abysms of immensity. Where it goes to or what becomes of it, no one of us can surmise.

Now when, in the remote future, our sun is reduced to vapour by the impact of the several planets upon his surface, the resulting nebulous mass must be a very insignificant affair compared with the nebulous mass with which we started. In order to make a second nebula equal in size and potential energy to the first one, all the energy of position at first existing should have been retained in some form or other. But nearly all of it has been lost, and only an insignificant fraction remains with which to endow a new system. In order to reproduce, in future ages, anything like that cosmical development which is now going on in the solar system, aid must be sought from without. We must endeavour to frame some valid hypothesis as to the relation of our solar system to other systems.

Thus far our view has been confined to the career of a single star,—our sun,—with the tiny, easily-cooling balls which it has cast off in the course of its development. Thus far, too, our inferences have been very secure, for we have been dealing with a circumscribed group of phenomena, the beginning and end of which have been brought pretty well within the compass of our imagination. It is quite another thing to deal with the actual or probable career of the stars in general, inasmuch as we do not even know how many stars there are, which form parts of a common system, or what are their precise dynamic relations to one another. Nevertheless we have knowledge of a few facts which may support some cautious inferences. All the stars which we can see are undoubtedly bound together by relations of gravitation. No doubt our sun attracts all the other stars within our ken, and is reciprocally attracted by them. The stars, too, lie mostly in or around one great plane, as is the case with the members of the solar system. Moreover, the stars are shown by the spectroscope to consist of chemical elements identical with those which are found in the solar system. Such facts as these make it probable that the career of other stars, when adequately inquired into, would be found to be like that of our own sun. Observation daily enhances this probability, for our study of the sidereal universe is continually showing us stars in all stages of development. We find irregular nebulae, for example; we find spiral and spheroidal nebulae; we find stars which have got beyond the nebulous stage, but are still at a whiter heat than our sun; and we also find many stars which yield the same sort of spectrum as our sun. The inference seems forced upon us that the same process of concentration which has gone on in the case of our solar nebula has been going on in the case of other nebulae. The history of the sun is but a type of the history of stars in general. And when we consider that all other visible stars and nebulae are cooling and contracting bodies, like our sun, to what other conclusion could we very well come? When we look at Sirius, for instance, we do not see him surrounded by planets, for at such a distance no planet could be visible, even Sirius himself, though fourteen times larger than our sun, appearing only as a "twinkling little star." But a comparative survey of the heavens assures us that Sirius can hardly have arrived at his present stage of concentration without detaching, planet-forming rings, for there is no reason for supposing that mechanical laws out there are at all different from what they are in our own system. And the same kind of inference must apply to all the matured stars which we see in the heavens.

When we duly take all these things into the account, the case of our solar system will appear as only one of a thousand cases of evolution and dissolution with which the heavens furnish us. Other stars, like our sun, have undoubtedly started as vaporous masses, and have thrown off planets in contracting. The inference may seem a bold one, but it after all involves no other assumption than that of the continuity of natural phenomena. It is not likely, therefore, that the solar system will forever be left to itself. Stars which strongly gravitate toward each other, while moving through a perennially resisting medium, must in time be drawn together. The collision of our extinct sun with one of the Pleiades, after this manner, would very likely suffice to generate even a grander nebula than the one with which we started. Possibly the entire galactic system may, in an inconceivably remote future, remodel itself in this way; and possibly the nebula from which our own group of planets has been formed may have owed its origin to the disintegration of systems which had accomplished their career in the depths of the bygone eternity.

When the problem is extended to these huge dimensions, the prospect of an ultimate cessation of cosmical work is indefinitely postponed, but at the same time it becomes impossible for us to deal very securely with the questions we have raised. The magnitudes and periods we have introduced are so nearly infinite as to baffle speculation itself: One point, however, we seem dimly to discern. Supposing the stellar universe not to be absolutely infinite in extent, we may hold that the day of doom, so often postponed, must come at last. The concentration of matter and dissipation of energy, so often checked, must in the end prevail, so that, as the final outcome of things, the entire universe will be reduced to a single enormous ball, dead and frozen, solid and black, its potential energy of motion having been all transformed into heat and radiated away. Such a conclusion has been suggested by Sir William Thomson, and it is quite forcibly stated by the authors of "The Unseen Universe." They remind us that "if there be any one form of energy less readily or less completely transformable than the others, and if transformations constantly go on, more and more of the whole energy of the universe will inevitably sink into this lower grade as time advances." Now radiant heat, as we have seen, is such a lower grade of energy. "At each transformation of heat-energy into work, a large portion is degraded, while only a small portion is transformed into work. So that while it is very easy to change all of our mechanical or useful energy into heat, it is only possible to transform a portion of this heat-energy back again into work. After each change, too, the heat becomes more and more dissipated or degraded, and less and less available for any future transformation. In other words," our authors continue, "the tendency of heat is towards equalization; heat is par excellence the communist of our universe, and it will no doubt ultimately bring the system to an end..... It is absolutely certain that life, so far as it is physical, depends essentially upon transformations of energy; it is also absolutely certain that age after age the possibility of such transformations is becoming less and less; and, so far as we yet know, the final state of the present universe must be an aggregation (into one mass) of all the matter it contains, i. e. the potential energy gone, and a practically useless state of kinetic energy, i. e. uniform temperature throughout that mass." Thus our authors conclude that the visible universe began in time and will in time come to an end; and they add that under the physical conditions of such a universe "immortality is impossible."

Concerning the latter inference we shall by and by have something to say. Meanwhile this whole speculation as to the final cessation of cosmical work seems to me—as it does to my friend, Professor Clifford [3]—by no means trustworthy. The conditions of the problem so far transcend our grasp that any such speculation must remain an unverifiable guess. I do not go with Professor Clifford in doubting whether the laws of mechanics are absolutely the same throughout eternity; I cannot quite reconcile such a doubt with faith in the principle of continuity. But it does seem to me needful, before we conclude that radiated energy is absolutely and forever wasted, that we should find out what becomes of it. What we call radiant heat is simply transverse wave-motion, propagated with enormous velocity through an ocean of subtle ethereal matter which bathes the atoms of all visible or palpable bodies and fills the whole of space, extending beyond the remotest star which the telescope can reach. Whether there are any bounds at all to this ethereal ocean, or whether it is as infinite as space itself, we cannot surmise. If it be limited, the possible dispersion of radiant energy is limited by its extent. Heat and light cannot travel through emptiness. If the ether is bounded by surrounding emptiness, then a ray of heat, on arriving at this limiting emptiness, would be reflected back as surely as a ball is sent back when thrown against a solid wall. If this be the case, it will not affect our conclusions concerning such a tiny region of space as is occupied by the solar system, but it will seriously modify Sir William Thomson's suggestion as to the fate of the universe as a whole. The radiance thrown away by the sun is indeed lost so far as the future of our system is concerned, but not a single unit of it is lost from the universe. Sooner or later, reflected back in all directions, it must do work in one quarter or another, so that ultimate stagnation be comes impossible. It is true that no such return of radiant energy has been detected in our corner of the world; but we have not yet so far disentangled all the force-relations of the universe that we are entitled to regard such a return as impossible. This is one way of escape from the consummation of things depicted by our authors. Another way of escape is equally available, if we suppose that while the ether is without bounds the stellar universe also extends to infinity. For in this case the reproduction of nebulous masses fit for generating new systems of worlds must go on through space that is endless, and consequently the process can never come to an end and can never have had a beginning. We have, therefore, three alternatives: either the visible universe is finite, while the ether is infinite; or both are finite; or both are infinite. Only on the first supposition, I think, do we get a universe which began in time and must end in time. Between such stupendous alternatives we have no grounds for choosing. But it would seem that the third, whether strictly true or not, best represents the state of the case relatively to our feeble capacity of comprehension. Whether absolutely infinite or not, the dimensions of the universe must be taken as practically infinite, so far as human thought is concerned. They immeasurably transcend the capabilities of any gauge we can bring to bear on them. Accordingly all that we are really entitled to hold, as the outcome of sound speculation, is the conception of innumerable systems of worlds concentrating out of nebulous masses, and then rushing together and dissolving into similar masses, as bubbles unite and break up—now here, now there—in their play on the surface of a pool, and to this tremendous series of events we can assign neither a beginning nor an end.

We must now make some more explicit mention of the ether which carries through space the rays of heat and light. In closest connection with the visible stellar universe, the vicissitudes of which we have briefly traced, the all-pervading ether constitutes a sort of unseen world remarkable enough from any point of view, but to which the theory of our authors ascribes capacities hitherto unsuspected by science. The very existence of an ocean of ether enveloping the molecules of material bodies has been doubted or denied by many eminent physicists, though of course none have called in question the necessity for some interstellar medium for the transmission of thermal and luminous vibrations. This scepticism has been, I think, partially justified by the many difficulties encompassing the conception, into which, however, we need not here enter. That light and heat cannot be conveyed by any of the ordinary sensible forms of matter is unquestionable. None of the forms of sensible matter can be imagined sufficiently elastic to propagate wave-motion at the rate of one hundred and eighty-eight thousand miles per second. Yet a ray of light is a series of waves, and implies some substance in which the waves occur. The substance required is one which seems to possess strangely contradictory properties. It is commonly regarded as an "ether" or infinitely rare substance; but, as Professor Jevons observes, we might as well regard it as an infinitely solid "adamant." "Sir John Herschel has calculated the amount of force which may be supposed, according to the undulatory theory of light, to be exerted at each point in space, and finds it to be 1,148,000,000,000 times the elastic force of ordinary air at the earth's surface, so that the pressure of the ether upon a square inch of surface must be about 17,000,000,000,000, or seventeen billions of pounds." [4] Yet at the same time the resistance offered by the ether to the planetary motions is too minute to be appreciable. "All our ordinary notions," says Professor Jevons, "must be laid aside in contemplating such an hypothesis; yet [it is] no more than the observed phenomena of light and heat force us to accept. We cannot deny even the strange suggestion of Dr. Young, that there may be independent worlds, some possibly existing in different parts of space, but others perhaps pervading each other, unseen and unknown, in the same space. For if we are bound to admit the conception of this adamantine firmament, it is equally easy to admit a plurality of such."

The ether, therefore, is unlike any of the forms of matter which we can weigh and measure. In some respects it resembles a fluid, in some respects a solid. It is both hard and elastic to an almost inconceivable degree. It fills all material bodies like a sea in which the atoms of the material bodies are as islands, and it occupies the whole of what we call empty space. It is so sensitive that a disturbance in any part of it causes a "tremour which is felt on the surface of countless worlds." Our old experiences of matter give us no account of any substance like this; yet the undulatory theory of light obliges us to admit such a substance, and that theory is as well established as the theory of gravitation. Obviously we have here an enlargement of our experience of matter. The analysis of the phenomena of light and radiant heat has brought us into mental relations with matter in a different state from any in which we previously knew it. For the supposition that the ether may be something essentially different from matter is contradicted by all the terms we have used in describing it. Strange and contradictory as its properties may seem, are they any more strange than the properties of a gas would seem if we were for the first time to discover a gas after heretofore knowing nothing but solids and liquids? I think not; and the conclusion implied by our authors seems to me eminently probable, that in the so-called ether we have simply a state of matter more primitive than what we know as the gaseous state. Indeed, the conceptions of matter now current, and inherited from barbarous ages, are likely enough to be crude in the extreme. It is not strange that the study of such subtle agencies as heat and light should oblige us to modify them; and it will not be strange if the study of electricity should entail still further revision of our ideas.