The Principle of Relativity · Albert Einstein
P′P′/Q′Q′ = OD/OC′ = OD′/OC = QQ/PP
Lorentz called the combination _t′_ of (_t_ and _x_) as the _local time_ (_Ortszeit_) of the uniformly moving electron, and used a physical construction of this idea for a better comprehension of the contraction-hypothesis. But to perceive clearly that the time of an electron is as good as the time of any other electron, _i.e._ _t_, _t′_ are to be regarded as equivalent, has been the service of A. Einstein [Ann. d. Phys. 891, p. 1905, Jahrb. d. Radis. ... 4-4-11-1907.] There the concept of time was shown to be completely and unambiguously established by natural phenomena. But the concept of space was not arrived at, either by Einstein or Lorentz, probably because in the case of the above-mentioned spatial transformations, where the (_x′_, _t′_) plane coincides with the _x_-_t_ plane, the significance is possible that the _x_-axis of space some-how remains conserved in its position. We can approach the idea of space in a corresponding manner, though some may regard the attempt as rather fantastical. According to these ideas, the word “Relativity-Postulate” which has been coined for the demands of invariance in the group G, seems to be rather inexpressive for a true understanding of the group G_{_c_}, and for further progress. Because the sense of the postulate is that the four-dimensional world is given in space and time by phenomena only, but the projection in time and space can be handled with a certain freedom, and therefore I would rather like to give to this assertion the name “_The Postulate of the Absolute world_” [World-Postulate].
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