Now, by further developing the aether concept in an epistemological way, one suddenly arrived at a dualistic view of aether and matter. The aether was interpreted as something that exists separately from matter, and thus one was faced with the questions, as to whether the aether is moving along with matter, or whether it is at rest. It was recognized soon, that the hypothesis of the stationary aether is the simpler one, and Lorentz made it to the foundation of his older theory of electrons. In this theory, the aether occurs as "quasi-matter". Thus the aether must be included in a dynamic system, in order to maintain the validity of the third axiom of Newton. A uniformly moving charge per se exerts another force upon a resting , as upon . The great inner improbability of this assumption caused difficulties for the older theory; even more the inner contradiction consisting in the fact, that a reference system shall be defined by the aether, although it represents an infinitely extended homogeneous medium.
However, the demonstration was most disastrous, that no influence of annual motion of Earth through the aether could be found in the optical phenomena, which was contrary to the requirement of the theory. Although this theory was occasionally further elaborated mathematically, it was definitely clear for the physicists, that further progression can only be made on the basis of the principle of relativity. The requirement was to be made, that at uniform motion of two bodies and relative to each other, their interaction shall be independent from whether or are to be assumed as being at rest or in motion.
The path to such a theory was paved by Lorentz in the year 1904. He showed, that by a suitable transformation of time and coordinates, the influence of uniform motion upon the optics of moving systems in Maxwell's equations, is vanishing, and that all observations known up to then, are in agreement with the additional consequences of this new theory. The deformation which the body's are undergoing by their motion, is characteristic for this theory. All dimensions, which coincide with the direction of motion, are contracted in the ratio , where means the ratio of the body's velocity to the speed of light. Then it was shown by Einstein, that one arrives at the exact same experimental consequences, when "local time" as introduced by Lorentz, is defined as time per se, and at the same time the space coordinates in Maxwell's equations are so transformed, that they are in agreement with this definition of time. The relativity principle is clearly emphasized in Einstein's version. While in Lorentz's version, the deformation and the kinetic energy are still definitely localized, this localization becomes relative according to Einstein. Einstein and Planck alluded to the important consequences following from the relativity principle. I recall, that the equations of motion assume the classical from of the Lagrangian equations after some transformations; and that one, starting from the principle of least action, arrives at important conclusions concerning the entropy and temperature of moving bodies. Also the extension of the concept of mechanical mass (which is dependent on velocity and energy content, and which also has relative character) is highly remarkable, namely the law of the constancy of mass is now logically connected with the law of the conservation of energy. The law of the conservation of the center of gravity becomes widened, by extending it to radiating systems as well. This is because electromagnetic radiation is connected with an emanation of mass. That this theory will also be fundamentally important for astronomy, and that it is destined to achieve a particular agreement with the astronomical observation by extension of Newton's law, shall also be mentioned.
The concept of the aether experiences a remarkable transformation. Because when a pure translatory motion of the system doesn't influence the phenomena arising in it, then properties must ascribed to the aether as the mediator of these processes, which are incompatible with the previous concept of the aether. The previous dualistic view of aether and matter must be replaced by a monistic one.
Thus, the relativity principle presents itself as a far-reaching and surprisingly unifying principle.
This principle peremptorily requires a direct experimental test. – It was clear from the outset, that only such phenomena can be used as confirmations of the validity of the competing theories, where the bodies are moving with high velocities. For that, measurements of Becqerel rays are suitable, and W. Kaufmann underwent the difficult task, to conduct experiments in this direction. Kaufmann's method is known to all of us, and also that Kaufmann drew the conclusion with certainty,
