The nature of a power so singularly conditioned is almost inconceivable; yet attempts have not been wanting to fathom the mystery that surrounds it. Professor Osborne Reynolds, in the Rede Lecture for 1902, claimed to have arrived at 'a complete, quantitative, purely mechanical explanation of the cause of gravitation,' based on the 'dilatancy' of a granular medium in close piling. But his working model of the universe will probably be remembered only as a lesson in the 'inversion of ideas,' showing that with skill and ingenuity a fairly concordant outcome of phenomena may be derived from antagonistic hypotheses. In this author's view matter is equivalent to a deficiency of mass, the spaces where his cosmic grains are relatively few, because their arrangement is out of gear, being driven towards one another by the pressure of the surrounding medium, in which they are compactly stowed, and therefore numerous. Thus, the acting forces in nature are made to depend upon the compression by the denser medium of interspatial tracts of rarer consistence, forming what we call matter. The theory is difficult, if not impossible of acceptance, not because it involves the overthrow of conceptions which may be rooted in habitual modes of thought, rather than in absolute truth, but because of its startling postulates and large vacuities. To be valid, it should be complete; and there are obvious chasms in the vast expanse of ground which it covers with surprising, though only partial success.
The multa renascentur of the poet is verified by the revolutions no less of thought than of speech. Flights of minute material particles have served the turn of theorists often, and in more ways than one, and have as frequently been consigned to discredited oblivion; but they are in vogue once more. George Louis Lesage, of Geneva, devoted sixty-three of the seventy-nine years of his life, which came to an end in 1803, to the elaboration of a mechanical rationale of gravity, first given to the world in the Transactions of the Berlin Academy of Sciences for 1782, and with details of amplification in his Traité de Physique Mécanique, edited by Pierre Prévost in 1818.[63] The explanation it offered of molar attractions was by the supposed unceasing impacts of 'ultramundane corpuscles,' speeding in countless numbers and at fabulous velocities, from nowhere everywhere, and thus enforcing the mutual approach of masses of gross matter. This involved the supposition of an infinitesimal screening effect, producing a small inequality in the strength of the bombardment on the sheltered and unsheltered sides of the bodies exposed to it. This inequality, in fact, was taken to be the causa causans of gravity. Yet its production encountered a difficulty. There was required for it a trifling degree of opacity in every kind of matter, while perfect gravitational transparency is asserted by the most delicate observations. Lesage, then, reduced the arrests laid upon his particles to a minimum; one in ten thousand, for instance, might at the utmost be intercepted by the terrestrial globe.[64] Even this insignificant minority, however, would suffice, through the surrender of their momentum to the impeding bodies, to endow them with the noted property of gravitation.
Clerk Maxwell urged the objection that the accompanying loss of kinetic energy by the corpuscles should, if transformed into heat, render all gravitating bodies white-hot. But Professor J. J. Thomson holds that the transferred corpuscular energy might, instead of reappearing in thermal form, be converted into some highly penetrative species of radiation capable of escaping unperceived into surrounding space.[65] 'A simple calculation,' he adds, 'will show that the amount of kinetic energy transformed per second in each gramme of the gravitating body must be enormously greater than that given out in the same time by one gramme of radium.' This consequence of Lesage's theory takes one's breath away; the 'fables of the Talmud' seem, by comparison, easy of belief; nevertheless, Lord Kelvin[66] declared it in 1873 to be more complete in the expository sense, and not more arduous in its assumptions, than the kinetic theory of gases.
Its fundamental postulate, at any rate, has been curiously verified in the course of recent researches into the arcana of physics. Entities in some degree corresponding to the ultramundane corpuscles of the Genevese philosopher do actually exist. Electrons are being continually expelled from bodies in all parts of the universe; they issue forth under all conceivable conditions and in unlimited numbers. Space is perhaps thronged with them; no material object can be exempt from their multitudinous buffetings, which are beginning to be taken account of in many cosmical speculations, and cannot certainly be ignored in efforts to solve the most obvious to superficial apprehension, the most intricate on a profound consideration, of all cosmical problems. But there is one fatal objection to an electronic theory of gravitation. The agency appealed to travels too slowly to be available for the required purpose. The velocity of light, there is reason to believe, sets a limit impossible to be surpassed or even attained by the velocity of electrons; yet it is incomparably smaller than the rate of gravitational transmission.
Tisserand estimated at six million times the quickness of luminous travel the minimum speed at which the sun's attraction must be propagated in view of the imperceptibility in planetary observations of effects corresponding to a time-inequality;[67] and this value may be taken as authentic. So colossal a discrepancy excludes any kind of impact-rationale of the mutual pull of heavy masses; Lesage's corpuscles remain 'ultramundane'; their identification with known atoms or sub-atoms appears to be precluded; no products of ionic disintegration possess the qualities necessarily to be ascribed to them.
We turn, then, inevitably to the menstruum of mysteries, the bank of the insolvent in speculation, to the all-serviceable ether. Ethereal radiations exercise an impulsive power; light-pressure has secured a recognised status among cosmic agencies; and every vibrational system of the luminous type undoubtedly shares the faculty by which light tends to drive minute particles forward along the lines of its propagation. Professor J. J. Thomson, accordingly, considered that but for the drawback of their insufficient velocity, 'very penetrating Röntgen rays' might with advantage be substituted for corpuscular streams as the cause of gravity.[68] They would, in some slight degree, be absorbed by encountered masses, to which they would impart a proportionate amount of momentum. Two bodies mutually shadowing one another would, under such circumstances, be drawn together with a force varying as the inverse square of distance; and if further they absorbed the impinging rays strictly in the measure of their density (as observation shows to be approximately the case), the attraction would increase in the same ratio as the product of their masses. But Röntgen rays travel with the precise velocity of light; they are, in truth, ultra-invisible light; and they must hence be regarded as hopelessly incompetent to explain an influence transmitted at least six million times more rapidly.
This was fully admitted by Dr. H. A. Lorentz,[69] who, five years ago, weighed the vibrational hypothesis of gravity in the balance of rigorous calculation, and found it wanting. His equations yielded the unexpected result that, if its postulates were granted, the noted attractions between massive bodies could subsist only on the terms of an incessant waste of electro-magnetic energy. But this is of course inadmissible. The theory involving such a consequence stands self-condemned, to say nothing of the wholly inadequate rate of propagation afforded by it.
Impulsion hypotheses, whether by corpuscles or rays, being hopelessly discredited, Dr. Lorentz reverted to a half-forgotten speculation by Mosotti, which, though sixty years old, struck him as capable of being adapted to modern requirements. It was of an electrical nature, and, in the novel shape given to it, supposed gravitational action to depend upon strains of the ether due to the disturbing effects of the positive and negative ions constituting ordinary matter. These 'states' of the medium are distinct in kind; they cannot neutralize one another; and the familiar law of attraction represents their resultant effect. To bring it about much has to be taken for granted; yet the hypothesis can lay claim to one singular prerogative. Although the disturbances invoked by it traverse the ether with no more than the standard speed of light, it appears from Dr. Lorentz's investigation that, owing to certain modifications in the properties of the medium produced by moving matter, the planetary perturbations betraying loss of time in gravitational transmission would, on the electrical theory, be so small as to evade detection. As regards this crucial point, the Dutch physicist has hit upon a felicity of explanation entirely original, and, as it were, unsought.
An 'undulatory theory' of gravity, adumbrated, rather than advanced by Mr. Whittaker in 1902,[70] excited hopes that the ideal aim of science—a complete unification of the forces of Nature—might at last be within reach. Based upon a striking mathematical research, it exhibited the attraction between masses as, in a manner, the integration of innumerable wave disturbances, propagated at a rate not strictly definable, but perhaps immensely surpassing that of gravity. No suggestion was made as to the primary mode or cause of agitation, yet it seemed much to learn that the medium we are cognizant of in space might be capable of transmitting the pull of gravity. Unfortunately, however, the physical foundation of this reassuring congruity proves to be weak or unsound. The mathematical mill works magnificently, but the grist put into it is of dubious quality. Stripping Mr. Whittaker's result of its purely analytical form, Dr. Johnstone Stoney showed that an assumption of extreme improbability lay concealed in his equations, which could not, he concluded, be seriously taken to correspond with the reality of things.[71]
There would then seem to be no alternative but to accept ad interim the electro-dynamical view of the nature of gravity. If not true, it is at least not obviously false. Through its subtlety it escapes direct confutation. And the method of exclusions, by eliminating competitors, has left it in virtual possession of the field.