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.
Laplace enounced his hypothesis in 1796, and republished it with
supplementary details in 1808. Herschel had meanwhile ascertained the
retrograde movement of the Uranian satellite-system, a circumstance
highly damaging to the validity of the adopted line of reasoning; yet
its author was content to leave it in jeopardy. He must, to be sure,
have regretted that Nature had seen fit to mar the admirable symmetry
indicative of her presumed plan of action, running counter thereby to
the plainest teachings of the doctrine of probabilities. But he kept
his own counsel on the subject, preferring that it should be discussed,
as it has been in full detail, by posterity; and posterity has, at
any rate, learned that the seeming caprices of Nature are often more
instructive than her most harmonious regularity, and has derived a
warning from her frequent breaches of continuity against the undue
extension of apparently well-grounded inferences.
Nevertheless, the constructive scheme handed on by the eighteenth to
the nineteenth century has not, up to the present, been consigned
to the limbo of vanities. It accorded too profoundly with undoubted
realities to be thus summarily disposed of. No one then living had
studied the mechanism of the solar system so attentively, or was so
intimately acquainted with its workings, as Pierre Simon Laplace.
None knew better how admirable, yet how far from inevitable, were the
adjustments by which its stability was secured. Long meditation upon
their poise and plan persuaded him that the subsisting congruities
of arrangement must have had their source in a community of origin.
He thus acquired the settled conviction that the sun engendered his
cortège, or was together with it engendered from one parent-mass.
And this virtually new truth (for Kant's speculation had attracted a
negligible amount of notice) was set forth by him with a directness
and lucidity which won for it an immediate place among the permanent
acquisitions of the human intellect. Few, perhaps, any longer believe
that planetary formation took the precise course laid down for it in
the _Système du Monde_, but fewer still doubt that the entire ambit of
the solar system was once occupied by an inchoate sun, and that its
component bodies came into being incidentally to that sun's progressive
contraction.
In favour of this view Laplace could allege no clinching argument; it
recommended itself to him solely through its inherent probability.
Unexpected confirmation has, none the less, been afforded to it by the
modern theorem of the conservation of energy, applied by Helmholtz with
widely illuminative effect to solve the problem of the maintenance of
solar heat. Laplace assumed an enormously high initial temperature. It
was the only way open to him, and he took it. But a transcendentally
hot nebula is not easily conceivable; an exalted thermal state seems,
and probably is, incompatible with a high degree of attenuation.
The key to the enigma was given by the demonstration that a diffuse
mass, although actually cold, might contain vast stores of potential
heat. There was then no need to postulate a primitive 'fire-mist'; the
surrendered energy of position amply sufficed to meet the requirements
of the case. The temperature of the nebula necessarily rose as it
contracted through gravitational stress; shrinkage and heat-evolution
proceeded together; and they in all likelihood proceed together still.
Our existence depends in part, or wholly, upon the collapse of the sun.
If its particles ceased to descend, their incandescence would become
less intense, and terrestrial vitality would be seriously compromised.
Their number, however, being finite, the store of energy they can
supply in falling even from an infinite distance is also finite. The
process of solar sustentation is then terminable; it had a beginning,
and it will assuredly come to an end. Now the _terminus ad quem_
is of a calculable remoteness: it can be located (unless shifted
by radio-active processes) within certain limits of time. But the
_terminus a quo_ depends upon too many conditions to be satisfactorily
defined. It is only certain that the sun is to-day slightly more
condensed than it was a year ago. It might a few millenniums back have
been measurably larger, had modern micrometrical methods been available
in the Stone Age; while, looking into the geological past, we discern a
continually more diffuse globe, filling the orbit of Mercury when the
earth was perhaps still red-hot, then successively ampler spheres, out
to, and beyond, that of Neptune. And just such a vastly diffused sun
realizes the nebula of Laplace. The state of things he imagined can be
reached accordingly, either by tracing forward the development of a
tenuous rotating mass, or by pursuing backward the surely indicated,
unceasing, and inevitable distension of the sun. Hence, no sooner
was it acknowledged that energy may be transformed, but cannot be
destroyed, than the nebular cosmogony assumed a new and authoritative
aspect.
But here a _caveat_ has been entered by the latest inquirers--a
_caveat_ not to be ignored, though based upon modes of action still
exceedingly obscure. Radio-activity is a fledgling science; its
capabilities, though immense, are vaguely outlined. Until they more
fully approve themselves, it would be unwise to admit conclusions which
they may eventually enforce. Subversive ideas are in the air; the
theory of atomic dissociation goes to the very root of things, and it
insistently claims assent. Its verification, by disclosing the presence
in the universe of a measureless store of unsuspected energy, would
overthrow all the calculations of cosmic time heretofore attempted, and
might protract indefinitely the radiative span of the sun.
Mr. W. E. Wilson pointed out in 1903[6] that its entire thermal output
could be supplied by the spontaneous liberation of energy from 3·6
grammes of radium in each cubic metre of its volume; and although we
have no evidence of the actual existence of radium in the sun, the
possibility that chromospheric helium represents the decay of solar
radio-active elements[7] must be taken into consideration. The ground
here is undermined with pitfalls. We can only see that although
Helmholtz's gravitational rationale of the sun's long life-history
remains true, the results derived from it may be profoundly modified
by co-ordinate processes, variously efficacious according to
circumstances, perhaps knowable, but as yet unknown.
The scope of the nebular hypothesis had widened prodigiously by the
time Helmholtz took it in hand. Five years before its promulgation at
Paris, Herschel gave at Slough the first hint of a corresponding scheme
of sidereal evolution. The discovery of a nebulous star in Taurus
(N.G.C. 1514) set him pondering; and he found himself, as the upshot of
his meditations, reduced to the dilemma either of concluding nucleus
and _chevelure_ to be alike stellar, though composed of stars differing
enormously in real magnitude, or of admitting the possession by the
star of a voluminous appendage constituted of a peculiar and unknown
'shining fluid.' He chose the latter alternative, adding the pregnant
remark: 'The shining fluid might exist independently of stars,' and
'seems more fit to produce a star by its condensation than to depend on
the star for its existence.'[8]
Thus tentatively, and under the compulsion of phenomena rather than
by the deliberate choice of its inventor, the universal theory of
the genesis of stars from nebulæ took its rise. Herschel shaped it
definitively in 1811 and 1814 into a formal plan for the interpretation
of celestial appearances, but in a large and general way. He made no
attempt to realize the particularities of a _modus operandi_ vaguely
conceived of as involving growth by absorption or assimilation. He and
Laplace thought out their separate schemes quite irrespectively one
of the other. There is no evidence of their having exchanged views
personally or by correspondence, nor does their mutual influence appear
to have been appreciable.[9] Yet Laplace needed as the raw material for
his solar system precisely the 'shining fluid' elaborated, one might
say, by Herschel, partly through the revelations of his telescopes,
partly as the outcome of his reasonings concerning the _chevelure_ of
the star in Taurus. Halley, it is true, had, by a sagacious intuition,
surmised the composition of nebulæ out of a 'lucid medium.' But the
ineffectual phrase remained stranded in the pages of the _Philosophical
Transactions_, and has only of late been set floating on the stream of
scientific literature.
Down to the end of the eighteenth century world-building had been a
purely speculative undertaking. It lacked actuality; it was concerned
with operations thought of as belonging exclusively to a past order
of things, now over and done with, and lying wholly outside the
range of experience. Through Herschel's synthesis, however, those
dimly apprehended operations were brought into view as variously
progressing even now in different parts of the cosmos, as incipient
in some regions, far advanced in others, the rubbish of the workshop
here half masking the rising edifice, while elsewhere signs of
decay and exhaustion give legible presage of an appointed end. And
this stupendous vision of a forming universe has not vanished on
critical scrutiny. It is no dream-tissue; it cannot dissolve into
airy nothingness; it is based upon a firm substratum of reality.
The immeasurable purposes of creative wisdom are still only in
part fulfilled. It has become the strange privilege of humanity to
contemplate from its little shoal of time the oceanic flow of their
development. Thus, in the swing of the ages, Laplace's thought was
caught up and vitalized. He himself was scarcely sensible of their
movement. He recognised very imperfectly, if at all, his obligations to
Herschel's nebulous star. His means were inadequate; his field of view
narrow; his knowledge, though co-extensive with that of his time, fell
short of what his boundless task demanded. In some respects his mode
of procedure was faulty; his forecasts have been belied; the behaviour
imputed by him to a nebula such as he devised is questionable, if
not impossible. But with the instinct of consummate intelligence he
hit off the 'psychological moment,' and, divining the genetic import
of harmonies of construction obvious to perception, but arduous of
interpretation, he laid down with masterly simplicity the ground-plan
of a structure likely to maintain its substantial integrity despite
innumerable additions and rectifications.
FOOTNOTES:
[Footnote 3: Quoted by Dr. Hastie in the preface to his translation of
Kant's _Cosmogony_, Glasgow, 1900.]
[Footnote 4: _Sur l'Origine du Monde_, 3e éd., p. 136.]
[Footnote 5: This also was pointed out by M. Faye, _loc. cit._, p. 150.]
[Footnote 6: _Nature_, July 9, 1903.]
[Footnote 7: Rutherford, _Radio-activity_, p. 342.]
[Footnote 8: _Philosophical Transactions_, vol. lxxxi., p. 85.]
[Footnote 9: Herschel met Laplace during a visit to Paris in July,
1801, but what passed between them is unrecorded. In the sixth edition,
however, of the _Exposition du Système du Monde_, Laplace referred to
Herschel's observations of nebulæ as confirmatory of his own genetic
scheme.]