Note 7 (page [57])
Einstein had become tired of assumptions. He had no particular objection to the “ether” theory beyond the fact that this “ether” did not come within the range of our senses; it could not be “observed.” “The consistent fulfilment of the two postulates—‘action by contact’ and causal relationship between only such things as lie within the realm of observation [see [Note 2]] combined together is, I believe, the mainspring of Einstein’s method of investigation.…” (Prof. Freundlich).
Note 8 (page [59])
That the conception of the “simultaneity” of events is devoid of meaning can be deduced from equation ([2]) [see [Note 4]]. We owe the proof to Einstein. “It is possible to select a suitable time-coordinate in such a way that a time-measurement enters into physical laws in exactly the same manner as regards its significance as a space measurement (that is, they are fully equivalent symbolically), and has likewise a definite coordinate direction.… It never occurred to anyone that the use of a light-signal as a means of connection between the moving-body and the observer, which is necessary in practice in order to determine simultaneity, might affect the final result, i.e., of time measurements in different systems.” (Freundlich). But that is just what Einstein shows, because time-measurements are based on “simultaneity of events,” and this, as pointed out above, is devoid of meaning.
Had the older masters the occasion to study enormous velocities, such as the velocity of light, rather than relatively small ones—and even the velocity of the earth around the sun is small as compared to the velocity of light—discrepancies between theory and experiment would have become apparent.
Note 9 (page [67])
How the special theory of relativity (see [Note 4]) led to the general theory of relativity (which included gravitation) may now be briefly traced.