assumptions, Einstein[1] recently arrived at results, which are in agreement with those of Lorentz concerning the consequences accessible to observation, though in which the previously mentioned difficulties of epistemological kind have been avoided. Einstein introduced the principle of relative motion, at least as regards translations, as a postulate. He thus places the theorem at the top, that physical phenomena observable in any rigid system, must be independent from whether the system (together with the observer) is moving relatively to any other system. From that, by an application to the propagation of light, a new definition of time and the concept of "simultaneity" for two spatially separated points immediately follows. Namely, for an observer in system moving relatively to another normal system , two events happening at different points are simultaneous when two quantities become equal (for an observer resting in ) which are formally identical with "local time" introduced by Lorentz. Furthermore, when according to the previous postulate, all properties (including the geometric dimensions) remain unchanged for a co-moving (i.e. at rest in ) observer, then for an observer at rest in , the "simultaneous" measurements of a structure viewed from , must just be altered in a way, as it was introduced by Lorentz as a theoretical fundamental assumption. Furthermore, the total reciprocity of all laws found, i.e. the exchangeability of systems and (the "resting" and the "moving" system) will be demonstrated. The results accessible to observation are thus the same with respect to both authors; however, while Lorentz only shows that his hypotheses lead to the desired result without excluding that the same can also be achieved in another way, it is shown by Einstein, that when the desired result, namely the principle of relative motion, is placed at the top of the whole of physics, then the kinematics of the rigid
- ↑ A. Einstein, Ann. d. Phys. (4) 17, p. 891. 1905.
