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:
“The theory of relativity is often criticised for giving, without justification, a central theoretical rôle to the propagation of light, in that it founds the concept of time upon the law of propagation of light. The situation, however, is somewhat as follows: In order to give physical significance to the concept of time, processes of some kind are required which enable relations to be established between different places. It is immaterial what kind of processes one chooses for such a definition of time.[60]
“It is advantageous, however, for the theory, to choose only those processes concerning which we know something certain. This holds for the propagation of light in vacuo in a higher degree than for any other process which could be considered, thanks to the investigations of Maxwell and H. A. Lorentz.
“From all these considerations, space and time data have a physically real, and not a mere fictitious significance.”
Let us return to Whitehead’s criticisms. It must be admitted that from the standpoint of experiment and observation, optical determinations are amenable to greater accuracy than all other methods of observation based on the propagation of sound, etc. Not only are they easier to observe, but they are also less contingent on variations in surrounding conditions (e.g., velocity of wind). Now, as we have remarked previously, when the critic attacks the theory of relativity on the ground that it defines simultaneity by optical methods, it must be presumed that he attributes the strangeness of Einstein’s theory to its exclusive use of light signals. Were this opinion correct, it would certainly appear rather arbitrary to found a theory of the universe on one particular choice of time determinations, when other methods would have yielded our classical results. Why, indeed, define time in terms of light rather than of sound or moving bodies? But, as mentioned elsewhere, the opinion is not correct, for regardless of the type of propagation to which we appeal, exactly the same determinations of simultaneity would ensue under ideal conditions of observation. Every experimental test which has verified the anticipations of the theory proves this last statement indirectly.
In order to make this fundamental point clearer, let us consider the following concrete example: Suppose we stand at a midpoint between two guns. According to Einstein’s optical criterion, the two guns would have been fired simultaneously were we to perceive the two flashes at the same instant of time. Classical science would have dissented from this view on the ground that the ether drift would have vitiated our results in a minute degree; but even classical science would have granted that had the physical effect of the ether drift been proved inexistent, there was nothing further to dissent from in Einstein’s definition. If, then, with Einstein, we assume that the negative experiments have proved the irrelevance of the ether drift, both classical science and relativity would recognise that a simultaneity in the perception of the two flashes would have entailed a simultaneity in the hearing of the two reports (assuming no wind was blowing). Also, both bullets would pass us at the same instant (assuming the guns, charges and bullets were identical).
It follows from this that Einstein’s theory, since it has started out by claiming the effect of the ether drift to be nil, must inevitably lead to the conclusion that however we elect to define simultaneity (whether by the optical, acoustic or bullet method), exactly the same simultaneity determinations will ensue. Dark, cloudy nights or the accidental circumstance that we are gifted with eyesight have therefore nothing to do with the matter; and blind men would not be debarred, since they could always synchronise their clocks by means of sound signals.
We see, then, that the strangeness of Einstein’s theory arises not from his use of optical signals, but from his fundamental premise that the ether drift is incapable of affecting the course of physical phenomena. Those who wish to dissent from the theory should therefore concentrate their attention on the ether-drift problem. Einstein’s preference for optical signals is a mere side issue which could perfectly well be discarded in favour of sound transmission were it not for reasons of practical convenience and accuracy. No change in the theory would ensue on this score. We have established this point in the case of simultaneity, but in view of its importance we may consider the problem in a more general way.
As we have noted, once the irrelevance of the ether drift is admitted and the equations of electromagnetics accepted as accurate, we are led to those space and time transformations known as the Einstein-Lorentz transformations. When we assert that duration might just as well have been defined in terms of moving bodies or sound transmissions (under ideal conditions of observation), our statement is equivalent to asserting that the Lorentz-Einstein transformations, though obtained from a study of electromagnetic and optical phenomena, are yet of universal applicability to all manner of phenomena. Here the critic may object that this is a gratuitous assumption. Maybe it is, but we must understand that were its validity to be contested, were the classical transformations to hold for mechanics and acoustics, while the Lorentz-Einstein ones were valid for electromagnetic processes, we should be faced with insufferable contradictions. Thus, according to the Lorentz-Einstein transformations, as applied to electromagnetics, a ray of light proceeding in vacuo will pass any two different Galilean observers with the same speed
