Finally, for sound waves we might substitute light signals or radio signals. This was the method most generally employed by classical science; it permitted greater accuracy than sound signals and was of wider applicability than determinations in terms of the earth’s rotation. When, for instance, the astronomer wished to determine the present position of a comet, the optical method was the only one possible. Accordingly, he was compelled to take into consideration the time required for the comet’s light to reach him. In other words, simultaneity was determined by optical signals. In much the same way, the Eiffel Tower in Paris sends out a radio message at noon every day, and ships at sea can regulate their clocks accordingly. (Needless to say, all this has nothing to do with the Einstein theory.) Nevertheless, while recognising that a determination of simultaneity via optical signals was the most precise method possible, classical science had certain misgivings as to its theoretical accuracy. The effect of the ether drift, generated by the earth’s motion, was assumed to produce an anisotropy in the propagation of light, much as the wind would do in the case of sound, causing it to proceed more rapidly in one direction than in another. Inasmuch as the velocity of this ether drift was unknown, there was no means of introducing the necessary corrections. Now, what Einstein urges us to recognise, as a result of the negative experiments in electromagnetics, is that these misgivings of classical science were unfounded. Whether it exist at all or whether it be a myth, the ether drift exerts no effect on the propagation of light. The space of a Galilean frame is isotropic so far as the propagation of light is concerned; that is to say, light proceeds with the same speed in every direction. These results having been established by the most accurate experiments known to science, there is every reason to accept them until such time at least as their validity is disproved by further experiments still more precise in nature. Under the circumstances, the optical method of fixing simultaneity, which was already the one followed prior to the theory of relativity, becomes, in view of Einstein’s discovery, the most perfect method at the command of science. From this rather long discussion we see that in the determination of simultaneity Einstein’s theory does not suggest any essentially new method. With either science, the measure of time and the determination of simultaneity in different places can be arrived at only by an appeal to physical processes of one sort or another. If we wish to discard physical determinations, and define things in the abstract, the concept of simultaneity in different places degenerates into a mere verbal expression. The following passage quoted from Einstein, in his Princeton lectures, may clarify the matter: