FOOTNOTES:
[62] Lorentz, Proceedings Amsterdam Academy, 1900, p. 565.
[63] Sir W. Thomson, Philosophical Magazine, vol. xlv., fourth series, 1873. Many hints have been taken, in what is above written, from this valuable paper.
[64] Sir W. Thomson (Lord Kelvin), loc. cit., p. 323.
[65] Electricity and Matter, p. 159.
[66] Loc. cit., p. 331.
[67] Traité de Mécanique Céleste, tome. iv., p. 495.
[68] Electricity and Matter, p. 160.
[69] Proceedings Amsterdam Academy of Sciences, 1900, p. 559.
[70] Monthly Notices, vol. lxii., p. 619; vol. lxiii., p. 258.
[71] Monthly Notices, vol. lxiii., p. 424.
[72] J. J. Thomson, Electricity and Matter, p. 88.
[73] Ibid., p. 47.
[CHAPTER XI]
THE INEVITABLE ETHER
Ether is the fundamental postulate of physics. Its existence, nowise apparent, is in all manner of ways implied. The properties that must be assigned to it are certainly arduous of conception. We need the aid of forced analogies to enable us to realize, even imperfectly and indistinctly, the mode in which it discharges functions obviously somehow discharged. But in the last resort everything is obscure; if our thought-borings go deep enough, they always reach the incomprehensible.
The original ether was the 'quintessence' of the ancients—a kind of matter vaguely imagined as pure and incorruptible enough to serve for the raw material of the heavenly bodies, the four common elements being reserved exclusively for sublunary use. The distinction, however, eventually broke down. All the spheres, from the primum mobile to the very surface of our low earth, are pervaded by a subtle mode of action, demanding for its transmission machinery of a finer kind than could be constructed out of gross everyday matter. The phenomena of light, when they came to be attentively studied, imperatively required a medium, universally diffused, evasive to sense, accessible only by processes of reasoning. Hooke and Newton accordingly brought the ether through the Horn Gate of dreamland into a region of reality, where it became a subject of legitimate speculation to men of science. The task, nevertheless, of definitely specifying its qualities was not taken seriously in hand until the beginning of the nineteenth century, when the establishment of the undulatory theory of light supplied tangible holding-ground for ideas regarding the vehicle of propagation, and rendered the ether a fixture of thought.
A great deal is demanded from it. We cannot afford to set up an establishment of ethers; one factotum must suffice. Incongruous offices are devolved upon it. It has to be Atlas and Mercury in one. It is the universal supporter and the universal messenger. Whatever kind of influence or form of energy can pass from world to world is conveyed by its means. If 'action at a distance' be inadmissible (as Newton himself held it to be), the pull of gravity must be exerted through a medium; and common-sense insists upon its identification with the transmitting medium of light, as well as upon the identification of that with the transmitting medium of electricity. A genuine conformity to these demands of reason is vouched for, not only by Hertz's discovery that an electrical explosion starts an undulatory disturbance indistinguishable, except in scale, from luminous waves; but also by Dr. Lorentz's indicated conclusion that strains of the same ethereal essence bear the all-pervading mandates of gravity. The unity of the medium may, then, be regarded as finally ascertained; the complex interactions of sundry different 'fluids' need no longer be taken into account. To provide one with the capabilities implied by the services we perceive it to render is, indeed, a sufficiently formidable task.
In popular apprehension the ether of space figures as a finer kind of air. No idea could be more misleading. The elasticity by which air transmits the longitudinal waves of sound is totally different from the elasticity by which ether propagates the transversal waves of light. Air yields to pressure; disturbance hence produces in it undulatory condensations due to oscillations of the gaseous molecules along the line in which the audible commotion travels. Ether, on the contrary, appears to be entirely incompressible; it conveys no vibrations directed lengthwise. Now this is extremely perplexing. We have no experience of a kind of matter absolutely rigid to pressure, while yielding, albeit with intense reluctance, to distortional stresses.
A jelly-like solid makes the nearest, though a very distant, approach to fulfilling the indispensable conditions; and a solid ether was accordingly in vogue until long past the middle of the nineteenth century. For a solid it had very peculiar qualities; that, for instance, of offering no resistance to motion. It was, in truth, obviously a mere temporary expedient—a scientific fiction which might pass muster until replaced by something corresponding less distantly with the fundamental fact. At last, on the advent of the electro-magnetic theory of light and the modified conceptions which it brought in its train, the solid ether withdrew behind the scenes. Its properties, though inconsistent and unconvincing, had not been chosen arbitrarily; they were imposed by the necessities of the situation; and when these varied, speculators naturally had recourse to fresh inventions.
The most plausible is that of a medium neither solid, liquid, nor gaseous in the ordinary sense, but in the ideal state of a 'perfect fluid.' Out of such an ether Lord Kelvin, with exquisite ingenuity, constructed his 'vortex-atoms,' which 'had their day and ceased to be.' Other ideas now prevail. 'The present tendency of physical science,' the late Mr. Preston wrote in 1890,[74] 'is to regard all the phenomena of Nature, and even matter itself, as manifestations of energy stored in the ether.' The more closely we look into the things around us, the more strongly the persuasion is forced upon us that what we call ether, electricity, and matter are really varied forms of one primal substance.
Two comprehensive schemes of molecular physics, resting upon the basis of this unifying thought, have lately been elaborated—one by Dr. Larmor, the other by Professor Osborne Reynolds. They have nothing in common beyond the largeness of their synthesis. In every respect they are radically unlike, save in regarding the intangible ether as the one material reality. Dr. Larmor, however, is not quite confidently explanatory. He presents no cut-and-dried theory of the universe; its haunting mysteries are not ignored in his efforts to rationalize them. He is vividly aware of the difficulties besetting the endowment of the ether with the type of elasticity which it is recognised to possess. He can only surmise that it results from particular modes of motion—from 'kinetic stability' ensuing upon a special dynamical state. The medium may thus be thought of as pervaded by 'a structure of tangled or interlaced vortex filaments, which might resist deformation by forming a stable configuration.'[75] But the details of any such scheme of action are evidently far too intricate to be easily unravelled; what concerns us here is to point out that no simple structureless fluid can avail to maintain cosmical communications.
Reduced to its lowest terms, Dr. Larmor's conception of the ether is that of a 'rotationally elastic medium.'[76] In other words, it resists being turned round an axis. The forces continually acting upon it are nevertheless of a gyratory nature; and hence arise strains, betrayed to our apprehension by electrical manifestations. Each 'electron' is held to be the nucleus of some kind of distortion or displacement,[77] and carries with it, as it moves, a field of force. Out of these 'point charges' material atoms are variously built up. They are 'structures in the ether,' encompassed by 'atmospheres of ethereal strain,' not—as they were formerly taken to be—'small bodies exerting direct action at a distance on other atoms according to extraneous laws of force.'[78] Obviously the new view brings to the front extremely subtle questions regarding the nature of 'dynamical transmission'[79]—what the propagation of energy essentially consists in, and by what mechanism it is effected; and they are, for the present, unanswerable. Electricity is, on the theory we are attempting to sketch, positive or negative according to the direction of the originating strain. A positive electron might be imagined to resemble a spiral nebula of the right-handed sort, a negative one a left-handed spiral, or vice versâ. The analogy is, perhaps, fanciful; yet it helps towards obtaining a mental picture of objects which, insignificant and elusive though they appear, may be the initials and ultimates of this strange world.
The forces, at any rate, by which it is at present kept going are evoked ad libitum by the pioneers of modern research from the ethereal plenum. The actualities of matter are potentialities in the ether. 'All mass,' in Professor J. J. Thomson's opinion, 'is mass of the ether, all momentum, momentum of the ether, and all kinetic energy, kinetic energy of the ether.[80] Only if this be so,' he adds, 'the density of the ether must be immensely greater than that of any known substance.'
The condition is startling, but in dealing with such subjects we must be prepared to meet with anomalies. They come, as the ghosts appeared to Odysseus in Hades, at first one by one, then in an awe-inspiring swarm. Yet, in spite of the perplexities they occasion, we can discern, with growing sureness of insight, the amazing reality of the universal medium. It is, in a manner, the only reality. For what is manifest to sense is subject to change. We can conceive that the visible framework of material existence might crumble and dissolve, like 'the baseless fabric of a vision,' into seeming nothingness. But a substance that is inapprehensible is, to our limited ideas, imperishable. The ether is assuredly the seat of intense activities, which lie at the root, most likely, of all the processes in Nature. An absolutely uniform medium, however, can scarcely be imagined to energize or react. Some kind of heterogeneity it must possess; and the heterogeneity produced, in Dr. Larmor's view, by strains, is associated, in Professor Reynolds's theory, with intrinsic texture.
The 'Sub-Mechanics of the Universe' are here made to depend upon the fitting together of ineffably small, ideally rigid grains. A misfit gives rise to matter, which might hence be defined as 'ether out of joint'; and the misfit can be propagated endlessly from one range of granules to the next. This propagation through the ether of an abnormal arrangement of its constituent particles, without any transference of the particles themselves, explains the phenomena of matter in motion. A concrete existence belongs to the ether alone. It is composed of round aboriginal atoms, the diameters of which measure the seven hundred thousand millionth part of the wave-length of violet light.[81] They are packed closely together, yet not so closely but that free paths are left to them averaging in length the four hundred thousand millionth part of their diameters.
This inconceivably small relative motion suffices, nevertheless, to render the medium elastic; is, indeed, 'the only cause of elasticity in the universe, and hence is the prime cause of the elasticity of matter.' The medium so formed is ten thousand times denser than water; it exerts a mean pressure of 750,000 tons on the square inch; the coefficient of its transverse elasticity is 9 + 1024 (in C.G.S. units); which gives a velocity of transmission identical with that of light for vibrations of the same type, while longitudinal waves are propagated 2·4 times more rapidly. The scheme further includes a plausible rationale of gravity and of electrical effects; so that there is much to warrant the claim of its author to have excogitated 'the one and only conceivable purely mechanical system capable of accounting for all the physical evidence, as we know it, in the universe.'
The machine, to be sure, lacks motive power; but that is a want which no human ingenuity can supply. Its source is obscured in the primal mystery of creation. And as regards the preliminary assumptions required for the constitution of an atomic ether, inclined though we might be to cavil at them, we should, perhaps, act more wisely in following Dr. Larmor's advice by abstaining from attempts to explain 'the simple group of relations which have been found to define the activity of the ether. We should rather rest satisfied,' he tells us, 'with having attained to their exact dynamical correlation, just as geometry explores or correlates, without explaining, the descriptive and metric properties of space.'[82] Yet one cannot help remarking that the properties of space are not ordinarily modified to suit the needs of demonstration, while those of the ethereal medium are varied at the arbitrary discretion of rival cosmogonists. In the future, when they come to be more clearly ascertained, they will, perhaps, form the basis of a genuine new science. Already, the study of ethereal physics excites profound interest and attention. Nor is it possible to ignore the gathering indications that it will impose qualifications upon principles consecrated by authority and hitherto regarded as fundamental.
The grand modern tenet of the conservation of energy, for example, may need a gloss; it may prove to be admissible only with certain restrictions. The second bulwark of the scientific edifice is even more seriously undermined. For the 'strain theory' of atomic constitution necessarily includes the conception of opposite distortions corresponding to positive and negative electricity. And the further inference lies close at hand that these, by combining, may neutralize one another. The coalescence, then, of a positive and negative electron should result in the smoothing out of the complementary strains they stand for; and there would ensue the annihilation of a pair of the supposed ultimates of matter. The event might be called the statical equivalent of the destruction of light through interference. That its possibility should be contemplated even by the most adventurous thinkers is a circumstance fraught with meaning as to the subversive tendencies of recent research.
Already, in May, 1902, Professor J. A. Fleming[83] pointed out that 'if the electron is a strain-centre in the ether, then corresponding to every negative electron there must be a positive one. In other words, electrons must exist in pairs of such kind that their simultaneous presence at one point would result in the annihilation of both of them.' The consequence thus viewed in the abstract finds concrete realization, if Mr. Jeans's suggestion be adopted,[84] in the processes of radio-activity, which possibly consist 'in an increase of material energy at the expense of the destruction of a certain amount of matter. There would, therefore, be conservation neither of mass nor of material energy.'
No longer ago than at the opening of the present century such notions would have been scouted as extravagant and paradoxical; now there is no escape from giving them grave and respectful consideration. Scientific reason has ceased to be outraged by hypotheses regarding the disappearance of mass and the development of energy. Mass and energy may, after all, be interchangeable; they are, at any rate, kept less rigidly apart in our meditations than used formerly to be the case. Nor can we assert with any confidence that partial subsidences into or emergences from the surrounding medium are for either a sheer impossibility; the universal framework, on the contrary, presents itself to us in the guise of an iridescent fountain leaping upward from, and falling back towards, the ethereal reservoir.
To the very brink of that mysterious ocean the science of the twentieth century has brought us; and it is with a thrill of wondering awe that we stand at its verge and survey its illimitable expanse. The glory of the heavens is transitory, but the impalpable, invisible ether inconceivably remains. Such as it is to-day, it already was when the Fiat Lux was spoken; its beginning must have been coeval with that of time. Nothing or everything, according to the manner in which it is accounted of, it is evasive of common notice, while obtrusive to delicate scrutiny. Its negative qualities are numerous and baffling. It has no effect in impeding motion; it does not perceptibly arrest, absorb, or scatter light; it pervades, and may even share in the displacements of gross matter; yet its motion (if it do move), is without effect on the velocity of light.
Looking, however, below the surface of things, we find the semi-fabulous quintessence to be unobtrusively doing all the world's work. It embodies the energies of motion; is, perhaps, in a very real sense, the true primum mobile; the potencies of matter are rooted in it; the substance of matter is latent in it; universal intercourse is maintained by means of the ether; cosmic influences can be exerted only through its aid; unfelt, it is the source of solidity; unseen, it is the vehicle of light; itself non-phenomenal, it is the indispensable originator of phenomena. A contradiction in terms, it points the perennial moral that what eludes the senses is likely to be more permanently and intensely actual than what strikes them.