Forty years have passed since the mechanical explanation of the universe reached its definite and perfect form. It dominates physics under the name of the theory of kinetic energy. The minds of men in our own time are so strongly impregnated with this idea that most scientists of ordinary culture get no glimpse of the world of phenomena but by means of this conception. And yet it is only an hypothesis. But it is so simple, so intuitive, and appears to be so thoroughly verified by experiment, that we have ceased to recognize its arbitrary and unnecessarily contingent character. Many physicists from this standpoint consider the kinetic theory as an imperishable monument.
However, as in the case of H. Poincaré, the most eminent physicists and mathematicians are not the dupes of this system; and without failing to recognize the immense services which it has rendered to science, they are perfectly well aware that it is only a system, and that there may be other systems. Certain among them, such as Ostwald, Mach, and Duhem, believe that the monument is showing signs of decay, and at present the theory is opposed by another theory—namely, the theory of energy.
The theory of energy is usually considered and presented as a consequence of the kinetic theory; but it is perfectly independent of it, and it is, in fact, without relying on the kinetic theory, without assuming the unity of physical forces, which are combined in molecular mechanics, that we shall expound the general system.
This is not the point at issue for the moment. It is not a question of deciding the reality or the merit of this or that mechanical explanation; it is a question of something more general, because upon it depends the idea of matter. It is a question of knowing if there are any explanations other than mechanical. The illustrious English physicist, Lord Kelvin, does not seem willing to admit this. “I am never satisfied,” he said, in his Molecular Mechanics, “until I have made a mechanical model of the object. If I can make this model, I understand; if I cannot, I do not understand.”
This tendency of so vigorous a mind to be content only with mechanical explanations, has been that of the majority of scientific men up to the present day, and from it has arisen the scientific idea of matter.
What is matter, in fact, to the student of mechanics? It is mass. All mechanics is constructed of masses and forces. Laplace said: “The mass of a body is the sum of its material points.” To Poisson, mass is the quantity of matter of which a body is composed. Matter is therefore confused with mass. Now, mass is the characteristic of the motion of a body under the action of a given force; it defines obedience or resistance to the causes of motion; it is the mechanical parameter; it is the co-efficient proper to every mobile body; it is the first invariant of which a conception has been established by science.
In fact, the word matter appears to be used in other senses by physicists, but this is only apparently so. They have but broadened the idea of the mechanicians. They have characterized matter by the whole series of phenomenal manifestations which are proportional to mass, such as weight, volume, chemical properties—so that we may say that the notion of matter does not intervene scientifically with a different signification from that of mass.
Two kinds of Matter. Ponderable and Imponderable.—In physics we distinguish between two kinds of matter—ponderable, obeying the law of universal attraction or weight, and imponderable matter or ether, which we assume to exist and to escape the action of that force. Ether has no weight, or extremely little weight. It is material in so far as it has mass. It is its mass which confers existence on it from the mechanical point of view—a logical existence, inferred from the necessity of explaining the propagation of heat, light, or electricity.
It may be observed that the use of mass really comes to bringing another element, force, to intervene, and we shall see that force is connected with energy; thus it comes to defining matter indirectly by energy. The two fundamental elements are not therefore irreducible; on the contrary, they should be one and the same thing.
Energy is the only Objective Reality.—This fusion into one will become more evident still when we examine the different kinds of energy, each of which exactly corresponds to one of the aspects of active matter. Shall we define matter by extension, by the portion of space it occupies, as certain philosophers do? The physicist will answer that space is only known to us by the expenditure of energy necessary to penetrate it (the activity of our different senses). And then what is weight? It is energy of position (universal attraction). And so with the other attributes. So that if matter were separated from the energetic phenomena by means of which it is revealed to us—weight or energy of position, impenetrability or energy of volume, chemical properties or chemical energies, mass or capacity for kinetic energy—the very idea of matter would vanish. And that comes to saying that fundamentally there is only one objective reality, energy.