Paul Langevin

“ 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. ”

“ 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. ”

“ The mass, which was measured by inertia, an essential attribute of matter, was regarded as an essentially invariable element, characterizing a given portion of matter. This notion now vanishes and merges with that of energy: the mass of a piece of matter varies with the internal energy of it, rises and falls with it. A piece of matter which radiates, loses its inertia in an amount proportional to the energy radiated. It is the energy which is inert; matter can resist the change in velocity only in proportion of the energy it contains. ”

“ 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 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. ”

“ 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. ”

“ We must choose: if we want to maintain an absolute value of the equations of rational mechanics and of space and time corresponding to them, we must consider as false those of electromagnetism, and reject the admirable synthesis that I mentioned above, and introduce for example an optical emission theory with all the difficulties it entails and which were dismissed over fifty years ago. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ All living beings have the capability of internal and spontaneous expansion, that is even greater when they are better adapted to the milieu in which they arose. When, as a result of this expansion, an encounter takes place between individuals or species, there may be mutual adaptation, or, if accordance is impossible, a conflict resulting in the survival of the fittest, which generally assimilates the substance of the other and imposes it a new form that life seems to have judged better. ”

“ As the Earth, in its annual motion, possesses a translation velocity which is constantly varying by amounts of up to sixty kilometers per second for the relative velocity corresponding to two diametrically opposed positions of the globe in the orbit, it was hoped that at least at certain times of the year, observers on Earth and their equipment will move in relation to the aether with a velocity of this order and might be able to demonstrate their motion. ”

“ Causality, whatever its nature, must not propagate with a velocity exceeding that of light, there should be no messenger or signal which can travel at more than three hundred thousand kilometers per second. ”

“ But within a material system, other experiments can be tried which involve electromagnetic or optical phenomena. Electromagnetic theory involves a medium for its explanation, the aether, which transmits the electrical and magnetic actions and in which electromagnetic disturbances (light in particular) propagate with a determined velocity.
It was hoped that if a material system moves with a uniform translation in respect to this medium, electromagnetic or optical experiments inside the system permit the demonstration of this translation. ”

“ At the end of two centuries the news of the encounter of the projectile with the star will come to Earth, that marks the beginning of the return voyage. The arrival of the traveler will occur two days afterwards during which the Earth will see him living two hundred times more quickly than usually, and it will see him achieving the gestures of another year to find him only two years aged at the return. During these two last days, to receive news of him, the Earth will have to use a reception antenna two hundred times shorter than the antenna of the traveler. ”

“ If we now take any two events in the life of our portion of matter, then their time interval measured by observers in non-uniform motion who will constantly monitor the portion of matter, will be, by integrating the previous result, shorter than for the reference system in uniform motion. ”

“ The attention of the physicists was recently led towards the fundamental notions of space and time, as new experimental facts obliged us to alter them; nothing can better demonstrate the empirical origin of these notions than their progressive adaptation, still not finished, with the increasing subtlety of human experiments. ”

“ Rational mechanics was created for the interpretation of the phenomena of visible motion and it succeeded admirably. All the scientific effort of the eighteenth century and much of the nineteenth was devoted to extend this capability to explain all physical phenomena by applying these laws to the motions of various invisible material particles or fluids. ”

“ If we agree to call with Minkowski the world line of a portion of matter that may be in motion relative to the reference system, all events that occur in that portion of matter, then the shape of a body at a given moment is determined by the set of simultaneous positions on the world lines of various material points which constitute this body. ”

“ We shall not forget, that it was only the atomistic conception that was often made responsible for the failure of mechanics, today however it is definitively established by indisputable experimental facts, and its association with the electromagnetic theory proved its remarkable fertility in the last fifteen years. What really seems to be questionable, is the application of mechanical laws to the invisible motions, which were at first established for visible motions, and even for them it represents only a first approximation, albeit it is excellent. ”

“ This introduces, from the point of view of time, an asymmetry between the two events, as the first occurs before the passage of the light signal whose emission coincides in space and time with the second event, while the second occurs after the passage of the light signal which accompanies the first one. A causal connection may exist at least by light between the two events, so the second has been informed of the first, and this requires that the order of succession has an absolute sense, and cannot be reversed by any change of reference system. ”

“ What we know about the spontaneous evolution of radioactive materials allows us to say that if these samples are kept in the laboratory, they will lose both their activity the same way over time and their activities remain continuously equal. ”

“ This remark provides the means for any among us who wants to devote two years of his life, to find out what the Earth will be in two hundred years, and to explore the future of the Earth, by making in his life a jump ahead that will last two centuries for Earth and for him it will last two years, but without hope of return, without possibility of coming to inform us of the result of his voyage, since any attempt of the same kind could only transport him increasingly further. ”

“ It is also difficult for our brain to get used to these new forms of thought: reflection is particularly delicate and can only be aided by the formation of an adequate language.
This is the task, for facilitating the evolution of mankind, for which today philosophers and physicists have to work together. ”

“ We can say then that no action at a distance, as gravitation for example, can propagate faster than light and we know that this condition is not refuted by the astronomical results currently established. ”

“ It is the same for our physical theories: some are particularly well established, have succeeded brilliantly in the interpretation and the grouping of a category of experimental facts, to which they impose a form ”