Paul Langevin

“ Already all views, not only of the ether, but of matter, source and receiver of luminous waves, obtain an immediate interpretation which mechanics is powerless to give, and this mechanics itself appears to-day as a first approximation, largely sufficing in all cases of motion of matter taken in mass, but for which a more complete expression must be sought in the dynamics of the electron. ”

“ Just as the growth effort of living beings is facilitated by organic syntheses already realized in other beings that were absorbed, the new theory conserves and uses more or less completely the body of facts already established by the theories over which it triumphed.
We are now witnessing a conflict of this kind between two conceptions of the universe that are of particular importance and beauty: the rational mechanics of Galileo and Newton on the one hand, and on the other hand the electromagnetic theory in the advanced form as it was given by Maxwell, Hertz, and Lorentz. ”

“ Without any other hypothesis than that of its electric charge, the electron is found to have inertia defined as capacity for kinetic energy, but with a particular law of variation of this as a function of the velocity, and this inertia appears to approach infinity as the velocity approaches that of light.
The behavior of this law depends very little on the hypothesis made as to the form of the electron and the distribution of the electric charge which it carries. In all cases it is found to be impossible to give the electron a velocity equal to that of light, at least permanently. ”

“ We also know that if the acceleration of the collective motion is communicated to the system by external actions that are exerted, contrary to what happens with gravity, only on parts of the system, we have many other means to demonstrate it, for example, strains within the system through which the acceleration is transmitted from portions of the system, that suffer external actions, to other portions that do not undergo them. ”

“ Thus the asymmetry - which occurred because only the traveler, in the middle of his journey, has undergone an acceleration that changes the direction of his velocity, and which brings him back to the starting point on Earth - results in the fact that the traveler sees the Earth moving away and approaching during times that are equal to one year, while the Earth, which is informed of the acceleration only by the arrival of light waves, sees the traveler moving away during two centuries and returning during two days, i.e. during a time forty thousand times shorter. ”

“ Although still very recent, the conceptions of which I have sought to give a collected idea are about to penetrate to the very heart of the entire physics, and to act as a fertile germ in order to crystallize around it, in a new order, facts very far removed from one another. ”

“ We know, indeed, by the example of α-particles of radium, i.e. the material atoms of helium with a velocity of just 20,000 kilometers per second, can follow a perfectly straight trajectory and cross through other atoms without leaving any trace of their passage other than increased conductivity, and our projectile has per unit mass a kinetic energy a hundred thousand times greater than α particles. It would be an extraordinarily penetrating radiation. If we want to avoid these difficulties, we have to find a way to slow its motion gradually as it approaches Earth. ”

“ The limited charge which we can extract from a gas thus modified, the velocity, finite and easily measured, with which they move under the action of an electric field, their progressive recombination, are interpreted in an admirable manner on the hypothesis that the radiations, as well as the intense heat agitations in a flame, dissociate a certain number of the molecules of the gas into electrified parts carrying charges of opposite kinds. ”

“ The world is the ensemble of all events: an event consists in the fact that it happens, or there is something in a certain place at a certain moment. If a reference system is given, that is to say a system of axes connected with a certain group of observers, any event is determined in terms of its position in space and time by four coordinates measured in this reference system, three for space and one for time. ”

“ It is the fundamental principle of the conservation of electricity, which will perhaps absorb the principle of the conservation of matter, as we cannot have matter without electricity. It is, however, probable that electricity alone is not sufficient to constitute matter.
We have actually no very precise information of the relative displacement of charges and of the ether, of electrified centres in an immovable medium, no tangible form under which we can conceive it. ”

“ We can suppose that this continual radiation, much more important naturally when the atom, as the result of external shock, is displaced from its most stable equilibrium, is a cause of decay to the atomic structure and which at the end of a certain length of time ought necessarily to give the structure a fundamental rearrangement, as a top falls when its rotation has sufficiently diminished in velocity. A condition of instability is thus reached, the consecutive rearrangement being accompanied by a violent projection of certain electrified centres from the atom. ”

“ We can define time by all the events that follow one another at one point, for example in the same portion of matter in relation to a reference system, and define space by all the simultaneous events. ”

“ But if the accuracy of experiment is not sufficient to determine completely the law, the agreement with the equations, obtained by supposing the mass to be entirely electromagnetic, is so good that we can reasonably conclude that cathode particles constituting the β rays have no mass other than that due to their electric charges or the train which they carry with them in their motion through the ether. ”

“ However, if we want to keep electromagnetism, we must adapt our minds to new concepts that are required for space and time and consider rational mechanics as having no more than the value of a first approximation, but it is largely sufficient when it comes to motions which velocity does not exceed a few thousand kilometers per second. Electromagnetism, or the laws of mechanics admitting the same transformation group as it, alone permits to go further and take the important place of rational mechanics. ”

“ It is difficult, in fact, to conceive how, on the hypothesis that the charges are distributed in a continuous manner in space, a mass of gas electrically neutral could furnish a limited quantity of electricity of each kind, decreasing with the time by progressive recombination if one delays the establishment of the electric field in the gas. ”

“ We must therefore admit that an event cannot act instantly as a remote cause, that its impact can only be felt immediately on the position or the point where it took place, and subsequently at increasing distances, and increasing with at most the velocity of light. In this view, the new concept plays the same role that was played in the old concept by the infinite velocity, and represents the velocity limit at which causality can propagate. ”

“ The small mass of the cathode corpuscle, and the possibility of separating from matter electrified centres a thousand times smaller than the smallest atom, is confirmed by the mobility of the negative ions in flames. We obtain enormous mobility compared to that observed in gases at ordinary temperatures, and the methods of the kinetic theory of gases permits us to calculate, by means of this experimental mobility, that the movable negative centres in flames have a mass about a thousand times smaller than the hydrogen atom, and should consequently be identical with the cathode corpuscles. ”

“ The concept of energy itself loses its absolute sense: its measurement varies with the reference system to which the phenomena are related, and physicists are currently, in the expression of the laws of the universe, looking after the real elements that possess an absolute sense, i.e. the elements that remain invariant when changing from one reference system to another, and which will play the role in the electromagnetic conception of the world, that which was played by time, mass and energy in the mechanistic synthesis. ”

“ If the acceleration is periodic, on the contrary, as in the case when the electron revolves around an electrified centre of opposite sign to itself, the acceleration is periodic, and the radiation emitted constitutes a light-wave whose length is determined by the period of revolution of the electron. ”

“ The theory permits to calculate the work that the Earth should spend to launch the projectile, to transmit the kinetic energy corresponding to its enormous velocity. Assuming the mass of the projectile is only equal to one ton, it can easily be calculated if one wants to spend only one year to launch it, for example by turning it at the end of a sling before releasing it, then it should function nonstop for that year by four hundred billion horsepowers, and to produce them it must burn at least a thousand cubic kilometers of coal. ”

“ Electrons, analogous to the cathode particles, possess almost all the known properties of matter by the fact alone that these centres are electrified. We shall see within what limits this conception can be considered sufficiently known, and if it is necessary to superimpose other properties on those which result from electrically charged centres in order to obtain a satisfactory representation of matter; the ether alone, on the contrary, never contains any electricity. ”

“ Why do we usually not admit that two events that are simultaneous for a certain group of observers, may not be simultaneous for another group moving relative to the first, or, equivalently, why don't we admit that a change of reference system permits to reverse the order of temporal succession of two events? ”

“ The negative results can be explained, as we shall see, if all the internal forces of matter are of electromagnetic origin; but gravitational force, alone different, can be superimposed on them without introducing an appreciable modification of this result, for its intensity is extraordinarily small compared to electromagnetic actions, even if there is no mutual compensation between them, and in all the experiments in question, interference of light or equilibrium of an elastic system, the gravitational forces play no appreciable role. ”

“ We therefore have hold on the aether through accelerations, and acceleration has an absolute sense as determining the production of waves from matter that has undergone a change in velocity, and the aether manifests its reality as the vehicle, as the carrier of energy transported by these waves. ”

“ The consequences extend through all parts of the old physics; especially in electromagnetism, in optics, in radiant heat; they throw a new light even on the fundamental ideas of the Newtonian mechanics, and have revived the old atomistic ideas and caused them to be lifted from the rank of hypotheses to that of principles, owing to the proper relation which the laws of electrolysis have established between the discontinuous structure of matter and that of electricity. ”

“ Finally it is necessary to abandon the perfectly rigid body of mechanics in which we could find a way to signal immediately at a distance and to establish a causal connection propagating faster than light. Nothing of what we know about real rigid bodies is opposed to this, that any action or any wave must be propagated less fast than light; elastic waves in the most rigid solids, in fact propagate with a much lower velocity. ”

“ In extreme cases where the electric charges appear to be most completely separated from their material support, as in the case of the cathode rays for example, the experimental fact of the granular structure of these rays and the complete indestructibility of their charge, the fact finally that cathodic particles are charges possessing the fundamental property of matter, inertia, and experiencing acceleration in the electromagnetic field, these facts do not allow us to distinguish their charge from the so-called free charge of ordinary electrified matter. ”

“ We are going to see rapidly and definitely from our idea of the electron, how it is sufficient to represent at the same time the inertia of matter, its dynamic properties, also how it can emit and absorb the radiations which the ether transmits. ”