Kinetic Theories of Gravitation/Tait, 1864

The professor of natural philosophy in the University of Edinburgh, P. G. Tait, has expressed himself with a cautious moderation on the probable origin of gravity, but with a sufficient distinctness to indicate his inclination to a kinetic hypothesis. In an able though somewhat prejudiced and partisan review of "The Dynamical Theory of Heat," published in the North British Review for February, 1864, after the very distinct affirmation of the great truth that "natural philosophy is a,u experimental and not an intuitive science: no a priori reasoning can conduct us definitely to a single physical truth ;" the reviewer thus proceeds to suggest his inductive conclusions :

"In the physical world we are congnizant of but four elementary or primordial ideas, beside the inevitable Time and Space. They are Matter, Force, Position, and Motion. Of these, motion is simply change of position; and force is recognized as the agent in every change of motion. Till we know what the ultimate nature of matter is, it will be premature to speculate as to the ultimate nature of force; though we have reason to believe that it depends upon the diffusion of highly attenuated matter throughout space."

Indefinite as the statement is, the indication that "force" probably depends rather on "highly attenuated matter" than on ordinary gross or sensible matter would appear to be derived from a somewhat metaphysical reason to believe. If the conception of a material connecting-link throughout space may be supposed to rest on a perception of physical fitness or necessity in such a transmitter of energy, this gives no physical reason to believe the origin of force resident in the one form of matter rather than in the other. For whether this "highly attenuated matter throughout space " is supposed to act statically or kinetically, and whether its function be to give or to receive impulses, we are [259] correctly informed that "force is recognized as the agent in every change of motion ;" and therefore we are no nearer the source of this agency after the acceptance the hypothesis than we were before it.

Still more recently, Professor Tait, in an evening lecture on "Force," delivered September 8, 1876, before the British Association at its Glasgow meeting, has recurred to his kinetic hypothesis. " Why two masses of matter possess potential energy when separated, in virtue of which they are conveniently said to attract one another, is still one of the most obscure problems in physics. I have not now time to enter on a discussion of the very ingenious idea of the ultramundane corpuscles, the outcome of the life-work of Le Sage, and the only even apparently hopeful attempt which has yet been made to explain the mechanism of gravitation. The most singular thing about it is that if it be true, it will probably lead us to regard all kinds of energy as ultimately kinetic And a singular quasi-metaphysical argument may be raised on this point, of which I can give only the barest outline. The mutual convertibility of kinetic and potential energy shows that relations of equality (though not necessarily of identity) can exist between the two; and thus that their proper expressions involve the same fundamental units, and in the same way. Thus as we have already seen that kinetic energy involves the unit of mass and the square of the linear unit directly, together with the square of the time unit inversely, the same must be the case with potential energy ; and it seems very singular that potential energy should thus essentially involve the unit of time, if it do not ultimately depend in some way on energy of motion."

This is the unavoidable inference of the kinetic system of force, if consistently maintained. But if there be any induction impregnable, as the generalization of a life-long, a continuous, and an unvarying experience, it is that potential energy does not "involve the unit of time." The carbon that has lain protected in the bowels of the earth for untold ages (certainly for many millions of years) has precisely the same relation to oxygen as the carbon prepared from last year's wood, and holds stored in the same mass the same exactly measurable potential energy. The stone ball that may have lain a thousand years undisturbed on the brink of a precipice has during that time lost no fraction of its static tension, but will fall with absolutely the same dynamic effect as if thrown up to its seat by a cannon but a moment before. The familiar case of a wound-up clock or watch, with the pendulum or the balance-wheel at rest, is equally irreconcilable with any scheme of kinetic action or of a force involving as a function any " unit of time." As Professor Maxwell points out : " In both cases, until the clock or watch is set agoing, the existence of potential energy, whether in the clockweight or in the watch-spring, is not accompanied with any visible [260] motion. We must therefore admit that potential energy can exist in a body or system all whose parts are at rest."

The degree of accordance between the logical conclusion and the unreasoned fact, is a measure of the value of the " singular quasi-metaphysical argument" by which it is deduced. " In dealing with physical science, it is absolutely necessary to keep well in view the all-important principle that nothing can be learned as to the physical world save by observation and experiment, or by mathematical deductions from data so obtained."