Albert Einstein

“ However, already in the twenties, the Russian mathematician Friedman showed that a different hypothesis was natural from a purely theoretical point of view. He realized that it was possible to preserve hypothesis (1) without introducing the less natural cosmological term into the field equations of gravitation, if one was ready to drop hypothesis (2) . Namely, the original field equations admit a solution in which the "world radius" depends on time (expanding space) . In that sense one can say, according to Friedman, that the theory demands an expansion of space. ”

“ We see from (12) that the imaginary time co-ordinate , enters into the condition of transformation in exactly the same way as the space co-ordinates , , It is due to this fact that, according to the theory of relativity, the "time" , enters into natural laws in the same form as the space co ordinates , , .
A four-dimensional continuum described by the "co-ordinates" , , , , was called "world" by Minkowski, who also termed a point-event a "world-point." From a "happening" in three-dimensional space, physics becomes, as it were, an "existence" in the four-dimensional "world." ”

“ If the sun (S) were not present, a star which is practically infinitely distant would be seen in the direction , as observed front the earth. But as a consequence of the deflection of light from the star by the sun, the star will be seen in the direction , i.e. at a somewhat greater distance from the centre of the sun than corresponds to its real position.
In practice, the question is tested in the following way. The stars in the neighbourhood of the sun are photographed during a solar eclipse. ”

“ In point of fact, astronomers have found that the theory of Newton does not suffice to calculate the observed motion of Mercury with an exactness corresponding to that of the delicacy of observation attainable at the present time. ”

“ Theories are evolved and are expressed in short compass as statements of a large number of individual observations in the form of empirical laws, from which the general laws can be ascertained by comparison. ”

“ If instead of Newton's law we insert a somewhat different law of attraction into the calculation, we find that, according to this new law, the motion would still take place in such a manner that the distance sun-planet exhibits periodic variations ”

“ It is required by the law of propagation of light, in conjunction with the postulate of relativity, that the transmission of the signal in question should take place — as judged from — in accordance with the corresponding formula
or,
In order that equation (10a) may be a consequence of equation (10) , we must have
Since equation (8a) must hold for points on the -axis, we thus have . It is easily seen that the Lorentz transformation really satisfies equation (11) for ; for (11) is a consequence of (8a) and (9) , and hence also of (8) and (9) . We have thus derived the Lorentz transformation. ”

“ Obviously it is immaterial whether the axes of be chosen so that they are spatially parallel to those of . It is also not essential that the velocity of translation of with respect to should be in the direction of the -axis. A simple consideration shows that we are able to construct the Lorentz transformation in this general sense from two kinds of transformations, viz. from Lorentz transformations in the special sense and from purely spatial transformations. which corresponds to the replacement of the rectangular co-ordinate system by a new system with its axes pointing in other directions. ”