Myths and Marvels of Astronomy

It is singular how variously this question of life in other worlds has been viewed at various stages of astronomical progress. From the time of Pythagoras, who first, so far as is known, propounded the general theory of the plurality of worlds, down to our own time, when Brewster and Chalmers on the one hand, and Whewell on the other, have advocated rival theories probably to be both set aside for a theory at once intermediate to and more widely ranging in time and space than either, the aspect of the subject has constantly varied, as new lights have been thrown upon it from different directions. It may be interesting briefly to consider what has been thought in the past on this strangely attractive question, and then to indicate the view towards which modern discoveries seem manifestly to point—a view not likely to undergo other change than that resulting from clearer vision and closer approach. In other words, I shall endeavour to show that the theory to which we are now led by all the known facts is correct in general, though, as fresh knowledge is obtained, it may undergo modification in details. We now see the subject from the right point of view, though as science progresses we may come to see it more clearly and definedly.

When men believed the earth to be a flat surface above which the heavens were arched as a tent or canopy, they were not likely to entertain the belief in other worlds than ours. During the earlier ages of mankind ideas such as these prevailed. The earth had been fashioned into its present form and condition, the heavens had been spread over it, the sun, and moon, and stars had been set in the heavens for its use and adornment, and there was no thought of any other world.

But while this was the general belief, there was already a school of philosophy where another doctrine had been taught. Pythagoras had adopted the belief of Apollonius Pergæus that the sun is the centre of the planetary paths, the earth one among the planets—a belief inseparable from the doctrine of the plurality of worlds. Much argument has been advanced to show that this belief never was adopted before the time of Copernicus, and unquestionably it must be admitted that the theory was not presented in the clear and simple form to which we have become accustomed. But it is not necessary to weigh the conflicting arguments for and against the opinion that Pythagoras and others regarded the earth as not the fixed centre of the universe. The certain fact that the doctrine of the plurality of worlds was entertained (I do not say adopted) by them, proves sufficiently that they cannot have believed the earth to be fixed and central. The idea of other worlds like our earth is manifestly inconsistent with the belief that the earth is the central body around which the whole universe revolves.

That this is so is well illustrated by the fate of the unfortunate Giordano Bruno. He was one of the first disciples of Copernicus, and, having accepted the doctrine that the earth travels round the sun as one among his family of planets, was led very naturally to the belief that the other planets are inhabited. He went farther, and maintained that as the earth is not the only inhabited world in the solar system, so the sun is not the only centre of a system of inhabited worlds, but each star a sun like him, about which many planets revolve. This was one of the many heresies for which Bruno was burned at the stake. It is easy, also, to recognise in the doctrine of many worlds as the natural sequel of the Copernican theory, rather than in the features of this theory itself, the cause of the hostility with which theologians regarded it, until, finding it proved, they discovered that it is directly taught in the books which they interpret for us so variously. The Copernican theory was not rejected—nay, it was even countenanced—until this particular consequence of the theory was recognised. But within a few years from the persecution of Bruno, Galileo was imprisoned, and the last years of his life made miserable, because it had become clear that in setting the earth adrift from its position as centre of the universe, he and his brother Copernicans were sanctioning the belief in other worlds than ours. Again and again, in the attacks made by clericals and theologians upon the Copernican theory, this lamentable consequence was insisted upon. Unconscious that they were advancing the most damaging argument which could be conceived for the cause they had at heart, they maintained, honestly but unfortunately, that with the new theory came the manifest inference that our earth is not the only and by no means the most important world in the universe—a doctrine manifestly inconsistent (so they said) with the teachings of the Scriptures.

It was naturally only by a slow progression that men were able to advance into the domain spread before them by the Copernican theory, and to recognise the real minuteness of the earth both in space and time. They more quickly recognised the earth's insignificance in space, because the new theory absolutely forced this fact upon them. If the earth, whose globe they knew to be minute compared with her distance from the sun, is really circling around the sun in a mighty orbit many millions of miles in diameter, it follows of necessity that the fixed stars must lie so far away that even the span of the earth's orbit is reduced to nothing by comparison with the vast depths beyond which lie even the nearest of those suns. This was Tycho Brahe's famous and perfectly sound argument against the Copernican theory. 'The stars remain fixed in apparent position all the time, yet the Copernicans tell us that the earth from which we view the stars is circling once a year in an orbit many millions of miles in diameter; how is it that from so widely ranging a point of view we do not see widely different celestial scenery? Who can believe that the stars are so remote that by comparison the span of the earth's path is a mere point?' Tycho's argument was of course valid.31 Of two things one. Either the earth does not travel round the sun, or the stars are much farther away than men had conceived possible in Tycho's time. His mistake lay in rejecting the correct conclusion because simply it made the visible universe seem many millions of times vaster than he had supposed. Yet the universe, even as thus enlarged, was but a point to the universe visible in our day, which in turn will dwindle to a point compared with the universe as men will see it a few centuries hence; while that or the utmost range of space over which men can ever extend their survey is doubtless as nothing to the real universe of occupied space.

Such has been the progression of our ideas as to the position of the earth in space. Forced by the discoveries of Copernicus to regard our earth as a mere point compared with the distances of the nearest fixed stars, men gradually learned to recognise those distances which at first had seemed infinite as in their turn evanescent even by comparison with that mere point of space over which man is able by instrumental means to extend his survey.

Though there has been a similar progression in men's ideas as to the earth's position in time, that progression has not been carried to a corresponding extent. Men have not been so bold in widening their conceptions of time as in widening their conceptions of space. It is here and thus that, in my judgment, the subject of life in other worlds has been hitherto incorrectly dealt with. Men have given up as utterly idle the idea that the existence of worlds is to be limited to the special domain of space to which our earth belongs; but they are content to retain the conception that the domain of time to which our earth's history belongs, 'this bank and shoal of time' on which the life of the earth is cast, is the period to which the existence of other worlds than ours should be referred.

This, which is to be noticed in nearly all our ordinary treatises on astronomy, appears as a characteristic peculiarity of works advocating the theory of the plurality of worlds. Brewster and Dick and Chalmers, all in fact who have taken that doctrine under their special protection, reason respecting other worlds as though, if they failed to prove that other orbs are inhabited now, or are at least now supporting life in some way or other, they failed of their purpose altogether. The idea does not seem to have occurred to them that there is room and verge enough in eternity of time not only for activity but for rest. They must have all the orbs of space busy at once in the one work which they seem able to conceive as the possible purpose of those bodies—the support of life. The argument from analogy, which they had found effective in establishing the general theory of the plurality of worlds, is forgotten when its application to details would suggest that not all orbs are at all times either the abode of life or in some way subserving the purposes of life.

We find, in all the forms of life with which we are acquainted, three characteristic periods—first the time of preparation for the purposes of life; next, the time of fitness for those purposes; and thirdly, the time of decadence tending gradually to death. We see among all objects which exist in numbers, examples of all these stages existing at the same time. In every race of living creatures there are the young as yet unfit for work, the workers, and those past work; in every forest there are saplings, seed-bearing trees, and trees long past the seed-bearing period. We know that planets, or rather, speaking more generally, the orbs which people space, pass through various stages of development, during some only of which they can reasonably be regarded as the abode of life or supporting life; yet the eager champion of the theory of many worlds will have them all in these life-bearing or life-supporting stages, none in any of the stages of preparation, none in any of the stages of decrepitude or death.

This has probably had its origin in no small degree from the disfavour with which in former years the theory of the growth and development of planets and systems of planets was regarded. Until the evidence became too strong to be resisted, the doctrine that our earth was once a baby world, with many millions of years to pass through before it could be the abode of life, was one which only the professed atheist (so said too many divines) could for a moment entertain; while the doctrine that not the earth alone, but the whole of the solar system, had developed from a condition utterly unlike that through which it is now passing, could have had its origin only in the suggestions of the Evil One. Both doctrines were pronounced to be so manifestly opposed to the teachings of Moses, and not only so, but so manifestly inconsistent with the belief in a Supreme Being, that—that further argument was unnecessary, and denunciation only was required. So confident were divines on these points, that it would not have been very wonderful if some few students of science had mistaken assertion for proof, and so concluded that the doctrines towards which science was unmistakably leading them really were inconsistent with what they had been taught to regard as the Word of God. Whether multiplied experiences taught men of science to wait before thus deciding, or however matters fell out, it certainly befell before very long that the terrible doctrine of cosmical development was supported by such powerful evidence, astronomical and terrestrial, as to appear wholly irresistible. Then, not only was the doctrine accepted by divines, but shown to be manifestly implied in the sacred narrative of the formation of the earth and heavens, sun, and moon, and stars; while upon those unfortunate students of science who had not changed front in good time, and were found still arguing on the mistaken assumption that the development of our system was not accordant with that ancient narrative, freshly forged bolts were flung from the Olympus of orthodoxy.

So far as the other argument—from the inconsistency of the development theory with belief in a Supreme Being—was concerned, the student of science was independent of the interpretations which divines claim the sole right of assigning to the ancient books. Science has done so much more than divinity (which in fact has done nothing) to widen our conceptions of space and time, that she may justly claim full right to deal with any difficulties arising from such enlargement of our ideas. With the theological difficulty science would not care to deal at all, were she not urged to do so by the denunciations of divines; and when, so urged, she touches that difficulty, she is quickly told that the difficulty is insuperable, and not long after that it has no existence, and (on both accounts) that it should have been left alone. But with the difficulty arising from the widening of our ideas respecting space and time, science may claim good, almost sole, right to deal. The path to a solution of the problem is not difficult to find. At a first view, it does seem to those whose vision had been limited to a contracted field, that the wide domain of time and space in which processes of development are found to take place is the universe itself, that to deny the formation of our earth by a special creative act is to deny the existence of a Creator, that to regard the beginning of our earth as a process of development is to assert that development has been in operation from the beginning of all things. But when we recognise clearly that vastness and minuteness, prolonged and brief duration, are merely relative, we perceive that in considering our earth's history we have to deal only with small parts of space and brief periods of time, by comparison with all space and all time. Our earth is very large compared with a tree or an animal, but very small compared with the solar system, a mere point compared with the system of stars to which the sun belongs, and absolutely as nothing compared with the universe of space; and in like manner, while the periods of her growth and development occupy periods very long-lasting compared with those required for the growth and development of a tree or an animal, they are doubtless but brief compared with the eras of the development of our solar system, a mere instant compared with the eras of the development of star-systems, and absolutely evanescent compared with eternity. We have no more reason for rejecting the belief in a Creator because our earth or the solar system is found to have developed to its present condition from an embryonic primordial state, than we have had ever since men first found that animals and trees are developed from the germ. The region of development is larger, the period of development lasts longer, but neither the one nor the other is infinite; and being finite, both one and the other are simply nothing by comparison with infinity. It is a startling thought, doubtless, that periods of time compared with which the life of a man, the existence of a nation, nay, the duration of the human race itself, sink into insignificance, should themselves in turn be dwarfed into nothingness by comparison with periods of a still higher order. But the thought is not more startling than that other thought which we have been compelled to admit—the thought that the earth on which we live, and the solar system to which it belongs, though each so vast that all known material objects are as nothing by comparison, are in turn as nothing compared with the depths of space separating us from even the nearest among the fixed stars. One thought, as I have said, we have been compelled to admit, the other has not as yet been absolutely forced upon us. Though men have long since given up the idea that the earth and heavens have endured but a few thousand years, it is still possible to believe that the birth of our solar system, whether by creative act or by the beginning of processes of development, belongs to the beginning of all time. But this view cannot be regarded as even probable. Although it has never been proved that any definite relation must subsist between time (occupied by events) and space (occupied by matter), the mind naturally accepts the belief that such a relation exists. As we find the universe enlarging under the survey of science, our conceptions of the duration of the universe enlarge also. When the earth was supposed to be the most important object in creation, men might reasonably assign to time itself (regarded as the interval between the beginning of the earth and the consummation of all things when the earth should perish) a moderate duration; but it is equally reasonable that, as the insignificance of the earth's domain in space is recognised, men should recognise also the presumable insignificance of the earth's existence in time.

In this respect, although we have nothing like the direct evidence afforded by the measurement of space, we yet have evidence which can scarcely be called in question. We find in the structure of our earth the signs of its former condition. We see clearly that it was once intensely hot! and we know from experimental researches on the cooling of various earths that many millions of years must have been required by the earth in cooling down from its former igneous condition. We may doubt whether Bischoff's researches can be relied upon in details, and so be unwilling to assign with him a period of 350 millions of years to a single stage of the process of cooling. But that the entire process lasted tens of millions and probably hundreds of millions of years cannot be doubted. Recognising such enormous periods as these in the development of one of the smallest fruits of the great solar tree of life, we cannot but admit at least the reasonableness of believing that the larger fruits (Jupiter, for instance, with 340 times as much matter, and Saturn with 100 times) must require periods still vaster, probably many times larger. Indeed, science shows not only that this view is reasonable, but that no other view is possible. For the mighty root of the tree of life, the great orb of the sun, containing 340 thousand times as much matter as the earth, yet mightier periods would be needed. The growth and development of these, the parts of the great system, must of necessity require much shorter time-intervals than the growth and development of the system regarded as a whole. The enormous period when the germs only of the sun and planets existed as yet, when the chaotic substance of the system had not yet blossomed into worlds, the mighty period which is to follow the death of the last surviving member of the system, when the whole scheme will remain as the dead trunk of a tree remains after the last leaf has fallen, after the last movement of sap within the trunk—these periods must be infinite compared with those which measure the duration of even the mightiest separate members of the system.

But all this has been left unnoticed by those who have argued in support of the Brewsterian doctrine of a plurality of worlds. They argue as if it had never been shown that every member of the solar system, as of all other such systems in space, has to pass through an enormously long period of preparation before becoming fit to be the abode of life, and that after being fit for life (for a period very long to our conceptions, but by comparison with the other exceedingly short) it must for countless ages remain as an extinct world. Or else they reason as though it had been proved that the relatively short life-bearing periods in the existence of the several planets must of necessity synchronise, instead of all the probabilities lying overwhelmingly the other way.

While this has been (in my judgment) a defect in what may be called the Brewsterian theory of other worlds, a defect not altogether dissimilar has characterised the opposite or Whewellite theory. Very useful service was rendered to astronomy by Whewell's treatise upon, or rather against, the plurality of worlds, calling attention as it did to the utter feebleness of the arguments on which men had been content to accept the belief that other planets and other systems are inhabited. But some among the most powerfully urged arguments against that belief tacitly relied on the assumption of a similarity of general condition among the members of the solar system. For instance, the small mean density of Jupiter and Saturn had, on the Brewsterian theory, been explained as probably due to vast hollow spaces in those planets' interiors—an explanation which (if it could be admitted) would leave us free to believe that Jupiter and Saturn may be made of the same materials as our own earth. With this was pleasantly intermixed the conception that the inhabitant of these planets may have his 'home in subterranean cities warmed by central fires, or in crystal caves cooled by ocean tides, or may float with the Nereids upon the deep, or mount upon wings as eagles, or rise upon the pinions of the dove, that he may flee away and be at rest,' with much more in the same fanciful vein. We now know that there can be no cavities more than a few miles below the crust of a planet, simply because, under the enormous pressures which would exist, the most solid matter would be perfectly plastic. But while Whewell's general objection to the theory that Jupiter or Saturn is in the same condition as our earth thus acquires new force, the particular explanation which he gave of the planet's small density is open to precisely the same general objection. For he assumes that, because the planet's mean density is little greater than that of water, the planet is probably a world of water and ice with a cindery nucleus, or in fact just such a world as would be formed if a sufficient quantity of water in the same condition as the water of our seas were placed at Jupiter's greater distance from the sun, around a nucleus of earthy or cindery matter large enough to make the density of the entire planet thus formed equal to that of Jupiter, or about one-third greater than the density of water. In this argument there are in reality two assumptions, of precisely the same nature as those which Whewell set himself to combat. It is first assumed that some material existing on a large scale in our earth, and nearly of the same density as Jupiter, must constitute the chief bulk of that planet, and secondly that the temperature of Jupiter's globe must be that which a globe of such material would have if placed where Jupiter is. The possibility that Jupiter may be in an entirely different stage of planetary life—or, in other words, that the youth, middle life, and old age of that planet may belong to quite different eras from the corresponding periods of our earth's life—is entirely overlooked. Rather, indeed, it may be said that the extreme probability of this, on any hypothesis respecting the origin of the solar system, and its absolute certainty on the hypothesis of the development of that system, are entirely overlooked.

A fair illustration of the erroneous nature of the arguments which have been used, not only in advocating rival theories respecting the plurality of worlds, but also in dealing with subordinate points, may be presented as follows:

Imagine a wide extent of country covered with scattered trees of various size, and with plants and shrubs, flowers and herbs, down to the minutest known. Let us suppose a race of tiny creatures to subsist on one of the fruits of a tree of moderate size, their existence as a race depending entirely on the existence of the fruit on which they subsist, while the existence of the individuals of their race lasts but for a few minutes. Furthermore, let there be no regular fruit season either on their tree or in their region of vegetable life, but fruits forming, growing, and decaying all the time.

Let us next conceive these creatures to be possessed of a power of reasoning respecting themselves, their fruit world, the tree on which it hangs, and to some degree even respecting such other trees, plants, flowers, and so forth, as the limited range of their vision might be supposed to include. It would be a natural thought with them, when first they began to exercise this power of reasoning, that their fruit home was the most important object in existence, and themselves the chief and noblest of living beings. It would also be very natural that they should suppose the formation of their world to correspond with the beginning of time, and the formation of their race to have followed the formation of their world by but a few seconds. They would conclude that a Supreme Being had fashioned their world and themselves by special creative acts, and that what they saw outside their fruit world had been also specially created, doubtless to subserve their wants.