THE NEBULAR HYPOTHESIS
Immanuel Kant was, in 1751, still in the plastic stage. His period of 'pure reason' was remote, and might have appeared improbable. Such as they were, his distinctions had been won in the field of concrete science, and the world of phenomena invited his speculations more seductively than the subtleties of logic. A seed was accordingly thrown into fertile soil by his reading of Thomas Wright's New Theory of the Universe, as summarized in a Hamburg journal. It set him thinking, and his thoughts proved to be of the dynamic order. Wright regarded the heavens under a merely statical aspect. He laid down the first definite plan of their construction, showing that the stars were not scattered at random, but aggregated by method; and this was much for one necessitous human being to have accomplished unaided.
But the young professor of Königsberg could not rest satisfied with the idle contemplation of any subsisting arrangement. His mind was incapable of acquiescing in things simply as they presented themselves; it craved to know further how they came to stand to each other in just such mutual relations. He was, moreover, permeated with Epicurean doctrines. Not in any reprehensible sense. He could not be reproached either as a hedonist or as an atheist. His pleasures were intellectual, his morals austere, his convictions orthodox. Behind the veil of material existence he divined its supreme immaterial Originator, and his perception of the activity in Nature of an ordering First Cause remained equally vivid, whether its disclosures were taken to be by immediate creation or through tedious processes of modification and growth. His large and luminous view embraced besides the ethical significance which such processes adumbrate. The following sentence shows an appreciation of the place of man in Nature truer and more profound than was attained perhaps by any other of his philosophical contemporaries: 'The cosmic evolution of Nature,' he wrote in memorable words, 'is continued in the historic development of humanity, and completed in the moral perfection of the individual.'[3]
Nevertheless, he owned to a community of ideas with Democritus as to the origin of the universe. Lucretius had cast over him the spell of his lofty diction, and captured his scientific adhesion by the stately imagery of his verse. With reservations, however. Docile discipleship was not in his line. He availed, then, of the Democritean atoms, but by no means admitted their concourse to be fortuitous. Chaos itself, as he conceived it, half concealed, half revealed the rough draft of a 'perfect plan.' His postulates were few. He demanded only a limitless waste of primordial matter, animated by no forces save those of gravitation and molecular repulsion, and undertook to produce from it a workable solar system. The attempt was no more than partially successful. Retrogressive investigations lead at the best to precarious results, and this one, in particular, was vitiated by a fundamental error of principle. Its author clearly perceived that planetary circulation must be the outcome of a vortical swirl in the nebulous matrix; but he failed to see that no interaction of its constituent particles could have set this swirl going.
Systems cannot of themselves add to their 'moment of momentum.' No changes of internal configuration avail to increase or diminish the sum of the products obtained by multiplying the mass of each of the connected bodies into its areal velocity projected on a common plane. The sum is of the algebraic kind. Equal and opposite motions cancel each other, the total representing only the aggregate excess of speed in either direction. A system with all its parts in rapid motion might then conceivably be devoid of moment of momentum. And if this were its state to begin with, it should be its state to the end of time, unless external force were applied to alter it. But the possibility may be dismissed as ideal. The establishment of so nice a balance as it would require is not practically feasible. In the actual world one side of the velocity account would be sure to exceed the other, albeit very slightly, and the smallest predominance would suffice to set on foot an eventual rotation of the system.
Had Kant been better acquainted with mechanical principles, he might then have safely trusted to the minute beginnings supplied by aboriginal inequalities of movement and dissymmetry of arrangement for the development in his colossal dust-cloud of the wheeling movement necessary for his purpose; and he would thus have escaped stumbling at the threshold of his daring inquiry. Rightly averse to employing arbitrary expedients, he piqued himself on the simplicity of his postulates, and was thus misled into substituting an imaginary for a real cause. The hypothesis adopted by him was that the particles forming the initial inchoate mass fell together by gravity, but were deviated from rectilinear courses through the effects of unequal resistance. And he derived from the combination of these multitudinous encounters a common axial rotation for the entire agglomeration. The futility of this mode of procedure was adverted to by M. Faye in 1885.[4] The deviations in question would, in fact, exactly balance one another, there being no reason why movement in one sense should prevail over movement in the opposite; consequently a general rotatory movement could not even begin to affect the seething mass, which would condense in sterile rigidity. Kant should then, as Laplace did when his turn came, have assumed the gyration indispensable to his purpose. He asked too little from Nature on one side, and too much on the other, with the result of arresting the machinery he designed to set going.
Kant made the germ of the future sun to consist in an aggregation of atoms at the core of the nebula, which, growing by successive innumerable accessions, provided the motive power for the machinery of planetary construction. For it was, as we have seen, the jostling of the particles drawn towards the gradually preponderating centre of attraction which set on foot, it was supposed, the whirl eventually transformed into the tangential velocities of the sun's attendant bodies. They were formed, like the sun, by the perpetuation and increase of subordinate nuclei sure to arise in the elemental tumult. They were formed, not under the guidance of a definite law, but just where chance—or what seemed like chance—favoured an accretion.
The progressive increase of planetary distances noted by Titius and Bode could never have arisen in the Kantian system. Nor could the Kantian planets have had a direct rotation.[5] Under the given conditions retrograde systems should have originated. This would have necessarily ensued from the incoherence of their materials. Particles revolving independently one of the other have smaller velocities the more remote they are from the focus of movement. Should they agglomerate into a globe, the inner flights must, as being the swiftest, determine the direction of its rotation, which will consequently reverse the direction of its orbital revolution. Hence, it depends upon the nature of their generating stuff no less than upon the advance of central condensation whether planets, in their domestic arrangements, contravene or obey the larger law of circulation prevailing in the system to which they belong, and Kant's nebula was undoubtedly such as to involve its contravention.
Yet his scheme, with all its deficiencies, bore the authentic stamp of genius—of genius imperfectly equipped with knowledge, but original, penetrative, divinatory. The very entitling of the work, A Natural History of the Heavens was an audacity implying a radical change of conception. It was in this remarkable treatise that 'island universes' made their definitive appearance. Wright, it is true, had, five years previously (in 1750), thrown out the idea that 'cloudy spots' might represent 'external creations,' but as a mere vagary of the scientific imagination. Kant unhesitatingly laid hold of it, classed nebulæ as so many separate galaxies, and regarded them as combining with our own into a revolving system on a surpassing scale of grandeur. Kant was also the first to take into account the effects on their development of the plasticity of the heavenly bodies. He published in 1754, in a Königsberg paper, by way of preliminary to his forthcoming Natural History, an outline of the workings of tidal friction in the earth-moon system. He saw clearly that it had acted in the past to reduce our satellite's rotation to its present minimum rate, and that it even now, by very slow degrees, tended to retard the spinning of the earth. This brilliant forecast remained unnoticed for well-nigh a century.
The assertion, however, that Kant's cosmogony was an anticipatory 'Meteoritic Hypothesis' lacks foundation. It is only true in the sense that his building materials were pulverulent, not 'fluid.' Laplace's primitive nebula was a coherent mass. It rotated as a whole; it divided only under considerable strain; its separated parts had individual unity—they held together with, so to speak, a purpose of concentration. Kant's elemental matter, on the contrary, was a loose aggregate of independent particles, each pursuing its way, disturbed, indeed, by its neighbours, but essentially isolated from them. They were, in short, genuine Lucretian atoms, intended to stand for the irreducible minima of Nature. The chaos that they formed was in nowise a 'meteoritic plenum,' unless the phrase be emptied of all distinctive meaning. Meteorites, so far from being primordial units, have the show and semblance of advanced cosmical products. They raise special questions in chemistry, mineralogy, geology, and physics, claiming to be dealt with by experts in each branch. Before serving for explanatory purposes, in fact, they themselves need to be explained.