God's glory in the heavens/The Nebular Hypothesis
Nebulous Rings.
XIV.
THE NEBULAR HYPOTHESIS.
We have seen that, apart altogether from any theory of the formation of the solar system, there is abundant evidence of unity of style. The fact of types in nature, does not at all depend on the soundness of the theories devised to account for their existence; and the argument for the necessity of a divine origin for the typal idea, is not, in the least, affected, though many links of natural causation be discovered between the mental conception and the material impress. We may accurately describe the mechanical; process by which the figure on a medal is struck from a die; but trace the process as minutely as we may, we never can eliminate the mental conception of which the figure on the medal is a material embodiment.
No astronomical speculation, in modern times, has given rise to greater controversy than that known by the name of the "nebular hypothesis." Considered in its purely scientific aspect, it possesses great interest, but its peculiar claim lies in its religious bearings. To understand these, it will be necessary shortly to advert to its physical character.
A slight glance at the motions and relations of the bodies of the planetary system, at once reveals a striking amount of uniformity, not all accounted for by the principle of gravitation, or by any known cause. For example, there is no reason, that we know of, why all the planets should revolve round the sun, and rotate on their axes, in the same direction. Laplace devised his nebular hypothesis for the purpose of grouping all such uniformities under one cause or law. The following are the phenomena which he attempts thus to account for: first, the motion of the planets in the same direction, and very nearly in the same plane; secondly, the motion of the satellites in the same direction, and very nearly in the same plane with the primaries; thirdly, the motion of rotation of all the above bodies, and likewise of the sun, in the same direction as their orbitual motion, and in planes but little inclined to one another; fourthly, the small eccentricity of the orbits of the planets and satellites. Now the theory of gravitation throws no light whatever on this extraordinary amount of uniformity. Consistently with the law of gravitation, there might be a wide diversity, instead of such a marked uniformity. There is no reason, as far as the mechanical action of gravity is concerned, why the planets should not move in opposite directions, and why their orbits should not have every possible degree of inclination to one another. It is utterly impossible, Laplace argued, to ascribe this amount of uniformity to chance, and therefore the human mind is irresistibly impelled to seek some key to it. He was further stimulated to seek a common explanation, from the circumstance that a like uniformity is not to be found in the motions of the comets. These bodies are regarded as not belonging to our system. It is indeed ascertained, that a few revolve regularly in elliptic orbits round the sun; but these few, out of the thousands that undoubtedly exist, are regarded as children of adoption, and not of birth.[1] Here then we have the case of bodies not belonging to our system, exhibiting none of that uniformity which characterises the various bodies constituting the system.
Besides the phenomena above enumerated, there are other traces of uniformity not accounted for by the law of gravitation. It is found that there is a remarkable regularity in the relative distances of the planets. This disposition is known as Bode's law of distances. Mercury being regarded as the point of departure, the distance of any planet from this point is double the distance of the next inferior planet from the same point. For example, the Earth is twice farther from Mercury than Venus is, Venus being the next inferior planet to the Earth.[2] The law of gravitation requires no such regular progression. It would hold equally well though no such regularity could be discovered. There is also something like a trace of law in the disposition of the planets in regard to their magnitude and density—the densest being, generally speaking, nearest the sun, and the largest more remote.
The nebular hypothesis professes to explain, more or less explicitly, the above cases of uniformity, unaccounted for by the theory of gravitation. Laplace supposes that the matter of which the sun and planets are formed, consisted originally of a vast nebula of extreme tenuity. He starts with the hypothesis that this vapour-like mass assumed, in some way, a rotatory motion. The problem which he undertook to solve may be thus stated—Given a nebulous mass in rotation, to shew how the various bodies of our system might be evolved from it, and their regularity of motion and disposition accounted for. The solution consists in supposing the rotating nebula to cool and condense, so that the central portion increases its rate of rotation, while an equatorial zone or ring of vapour is left behind, which rotates as a body separate from the central mass. A conception of the process may be formed by supposing the solid globe of the earth to contract its dimensions very much. A corresponding increase in the rate of its rotation would consequently follow, and the atmosphere would be left behind, whenever the centrifugal force exactly balanced the force of gravity. The atmosphere thus abandoned, would form a gaseous body revolving round the earth.
The successive rings abandoned by the rotatory nebula, would continue to rotate in the same direction as before. But it would be highly improbable that the integrity of each should be preserved. From inequalities in the internal forces, the ring would most probably be broken up, and form distinct globular masses, all revolving in the former plane of the ring, and in the same direction. These globes would most likely attract each other, and form one nebulous mass, the germ of some future planet. Now this mass would exhibit the same phenomena as the original one. It would condense and abandon successive zones, which would ultimately break up and form the satellites. It might be that the ring preserved its equilibrium; and the rings of Saturn form an illustration of this. It might happen, on the other hand, that the globular masses, formed from the disruption of the ring, preserved their identity; and we have a case of this kind in the asteroids between Mars and Jupiter. This hypothesis ingeniously accounts for the points of harmony above enumerated. The planets and satellites revolve nearly in circles and in one plane, because they have been thrown off from the equator of the rotating mass. The direction of rotation is the same as that of revolution, because the outer portion of the ring, having a greater absolute velocity than the inner, would necessarily originate rotation, and rotation in the same direction as that of the ring. By assuming a suitable law of condensation in the nebulous matter, the relative distances, densities, and magnitudes may be accounted for. The assumptions of Laplace regarding the rotating mass, and the law of condensation, are wholly gratuitous; but, admitting these, the results which he deduced from them flow, as a necessary consequence, according to the unerring laws of dynamics.
It has been attempted to support the nebular hypothesis by what is known as "Kirkwood's law " of the rotation of the primary planets. We have seen, that the direction of the rotation of the planets cannot be accounted for merely by the law of gravitation; but the period of rotation, or the length of the planet's day, is, also, altogether unaccounted for. It has been, therefore, a matter of interest to trace such an analogy in these rotations as might lead to the determining cause. Generally speaking, the period of rotation is greater in the planets least remote from the sun, though there are particular cases which contravene this rule. No definite law, then, can be derived from the mere element of distance. The nebular hypothesis led Kirkwood to the surmise, that the rotation might be dependent on the masses of the planets, and, with this clue to guide him, he was led to the following law: That the square of the number of rotations or days in the year of the planet, is proportional to the cube of the diameter of the sphere of attraction. The diameter of the sphere of attraction is determined in this way: The planet in question is supposed to be in conjunction with the two nearest planets, one on each side of it,—the three planets being thus in line. It is plain that there are two neutral points on each side of the middle planet, where its attraction is equally balanced by the attraction of either extreme. For example there is a point between the Earth and Venus, where, if a body were placed, it would be stationary, being equally attracted by both. A similar point exists between the Earth and Mars. The line joining these two points is called the diameter of the sphere of attraction. Now the law asserts, that the number of days in the year of each planet bears a definite proportion to this line; and as this line depends on the masses of the planets, a definite relation is asserted between the day of each planet and its mass. As this holds through all the solar system, it is maintained that the law clearly points to some common origin for the rotation of the planets, and that the hypothesis of an original rotating nebulous mass furnishes such a common origin.
It is very much to be doubted, whether astronomers will be disposed to accept of this as an established law. The reason is obvious. The data on which the law is founded are very uncertain in many cases. The masses and periods of rotation of various planets are merely conjectural, and such numbers are assumed as will suit the theory; and the law, even in the most favourable cases, does not pretend to anything like absolute accuracy.[3]The nebular hypothesis, then, can only derive very doubtful support from Kirkwood's law.
In directing the eye to the heavens, it is at once obvious, that the background from which the stars appear to shine is not uniform. There are brighter patches, which are distinguishable from the general dark background. The Milky Way appears as a zone of faint light passing round the heavens nearly in a great circle. But besides this great zone, there are isolated spots, distinguishable some of them by the naked eye, which appear like very small luminous clouds. Now these isolated cloudy objects are denominated nebulæ, and the question arises, What is their constitution? Are they composed, as they appear to be, of nebulous matter like comets? The telescope, when turned to the Milky Way, which presents a cloud-like appearance, at once resolves it into distinct stars. The unaided eye is not sensitive enough to separate the individual stars; so they appear to blend in one uniform surface of light. But the telescope enables the eye to effect the separation, and to reveal its true character. Are we to conclude, that the nebulse are of the same character, and that they are congeries of stars, so closely set, that they present a cloudy appearance without distinct isolation? Thus, among the innumerable nebulas which the telescope reveals, there are some which are at once resolved into stars by moderate powers. Nebulae, according to their resolvability, may be divided into three grades. The first consists of those in which the stars are distinctly separated. The second consists of those presenting a granular appearance, which merely indicates the resolvability under a sufficient power. But besides these, there is a third class in which no indication of resolvability could for a long time be found. Some of them could be descried with the naked eye, and, yet, the highest telescopic power that could be applied did not resolve them. It was, therefore, concluded that they differed essentially in their constitution from the resolvable nebula. It was held that they consisted of nebulous matter, in the process of condensation. The advocate of the nebular hypothesis regarded them as quite analogous to the nebula, from which the sun and planets were supposed to be evolved. Nebula3 were pointed out in all the various stages of condensation, from the diffuse mist to the perfect star, and they were maintained to be demonstrative of actual growth, just as we conclude that there has been growth when in a forest we see all gradations, from the tender sapling to the full-grown tree.
The large telescopes of Lord Rosse and other observers have, however, dissipated such speculations. The nebulas which presented the most obstinate character, and long resisted all attempts at resolution, have at last yielded. The great test-objects, such as the nebula in Orion and the one in Andromeda, which are faintly discernible by the naked eye, and which resisted all former attempts, have now been resolved. It was on such remarkable cases, that the speculation chiefly rested, and now that the foundation is removed, its plausibility is gone. It is true that many nebula are still unresolved, but this we must expect to be the case, however great the telescopic power may be. Every increase of power, while it resolves nebulæ hitherto unresolved, at the same time descries new ones defying resolution. It may be argued that, possibly, some of these may consist of genuine nebulous matter; but the question is not one of possibility but probability, and the revelations of the telescope have reduced such probability to its lowest point. The different degrees of apparent condensation, are explained by the various ways in which the stars are arranged in these clusters. Sometimes they appear to be pretty equally distributed, but at others there is a rapid concentration, so that at an unresolvable distance, the appearance is that of a single star surrounded by a nebulosity. The actual changes alleged to have been observed in these nebulte, are ascribed to the difference of appearance presented by telescopes of different optical power; features being brought into view by very powerful telescopes, which are altogether invisible in telescopes of inferior power. But though change were established, it would only refer to a difference in the aggregation of distinct stars, not in the chemical condensation of nebulous matter. The existence of anything like nebulous matter in space, is now only countenanced by the constitution of comets, the new ring of Saturn, the zodiacal light, and the zones of meteoric matter. The existence of nebulæ in the process of condensation, can no longer be used in support of the nebular hypothesis. The result then of the whole is, that as a purely mechanical speculation, there can be no scientific objection to the nebular hypothesis; for the data are assumed at will, and in such a manner, that the required results flow as a necessary sequence. When, however, it is attempted to elevate the hypothesis to the rank of a vera causa in nature, by the analogy of nebulous condensation in the remote regions of the universe, the proof entirely fails. The conclusion to which Sir John Herschel has come, after the light thrown upon the subject by recent revelations, is, that the nebular hypothesis is "a physical conception of processes which may yet, for aught we know, have formed part of that mysterious chain of causes and effects antecedent to the existence of separate, self-luminous solid bodies."' He views it not as an established theory, but as an ingenious hypothesis, still seeking that confirmation which it has hitherto wanted.
We are now in a position to understand the religious bearings of the question, and perhaps no astronomical point, since the time of Galileo, has given rise to keener theological discussion. By the atheistical inquirer, the hypothesis was hailed as the greatest triumph. He held that God might be dispensed with altogether, when the universe could be evolved so readily from this nebulous mist. It was argued, that the proofs of Divine wisdom, discerned in the constitution of the solar system, were at once dispelled, when the adaptation in question could be traced to a primordial unintelligent vapour. The nebular hypothesis was, in short, regarded as a satisfactory substitute for a God. It is to be regretted, that this cavil of the atheist should have been sometimes met by the defenders of Christianity in a manner so little satisfactory. Instead of inquiring whether the hypothesis, though granted, warranted such an inference, it was attempted to rob the atheist of the argument by proving the hypothesis to be altogether unfounded. This physical hypothesis was denounced as atheistical, and the question of a God was staked upon the refutation of it. No line of argument could be more unfortunate, and more uncalled for. The principle implied in such a position is, that the traces of a God disappear, as we reduce the instances of adaptation around us to general laws. Now the atheist, if this principle were granted, could afford to relinquish the nebular hypothesis. The granting of this principle would be tantamount to the admission, that the conception of a God must disappear, as a spectre, before the advancing light of science—the function of science being the reduction of special cases of adaptation to general laws. The atheist who appeals to the nebular hypothesis, can be met without making such a fatal admission. Instead of attacking the scientific theory, the proper attitude is to deny the theological inference. Instead of denouncing the theory as atheistical, the only tenable position is to shew that, though granted, it would not warrant the atheistical deduction.
1. CRAB NEBULA. 2. STRATIFIED NEBULA.
It seems almost incredible, that the mere tracing of special adaptations to general laws, should ever be regarded by the human mind as in the least degree weakening the evidence for a wise Creator and Governor. Granting that the solar system was developed from a nebulous mist, according to the rigid laws of mechanics, the question at once arises: Who endowed the atoms of this mist with such properties and capabilities, as to form worlds, with their wondrous adaptations? The primordial atoms, with their original susceptibilities, just as urgently demand a wise Intelligence, as the worlds evolved from them. In judging of human skill, our estimate is only enhanced, by finding some contrivance of exceeding ingenuity accomplished by the simplest means. The highest achievements of art are those in which the simplicity of the means strongly contrasts with the effects produced.
Our conclusion, then, in regard to the nebular hypothesis is, that it must be dealt with purely as a question of science, and that it would be exceedingly unwise to regard it as hostile to religion. Natural theology can only gain by the discovery of another wisely-adapted wheel in the celestial mechanism. The hypothesis does not clash with revealed religion; for the interpretation of the first chapter of Genesis, which admits of the long periods of the geologists, also allows a like extension to the speculations of the astronomer. It is now almost universally admitted by divines, that the Scriptures indicate no limit, beyond which the history of the universe can be traced. The period of the Mosaic creation is historically defined in Scripture, but of the absolute creation of worlds, it is only asserted that it was in time, and that the material universe is not self-existent.
- ↑ Laplace indeed points out a uniformity in the case of the comets, viz., the great eccentricity of their orbits; but this is employed as a confirmation of the nebular hypothesis. He holds that the motions of the less eccentric were destroyed by the resistance of the nebulous matter in the process of condensation, and that only the more eccentric were spared such a fate.
- ↑ The truth of Bode's law was assumed in the calculations which led to the discovery of Neptune, but the distance of this planet is found to be a fact irreconcilable with this law.
- ↑ The discordance for the various planets is indicated by the following numbers:— 928, 961, 1275, 3138, 833, 985, 3614. If the law held strictly, there should be no difference in these numbers; and therefore the least compared with the greatest indicates the maximum amount of error, which is obviously far too great.