Norman Robert Campbell

“ All the difficulties and apparent paradoxes of the Principle of Relativity will vanish it the attention is kept rigidly fixed upon the quantities which are actually observed. If anyone thinks he discovers that that Principle predicts some experimental result which is incomprehensible, let him dismiss utterly from his mind the conception of reality. ”

“ Suppose that the disturbed system consists of two parts, A and B, each of which, regarded as a complete system, is quiet: let A be the part which contains the observer and his instruments for measuring , and let the relative velocity of A and B be . Then if A and B formed together a quiet system, the known laws for quiet systems would state a relation between the time and the coordinates of the various parts of the system on the one hand, and some quantities P, Q, R, representing the physical state of the system on the other (forces, for example) . ”

“ It is sometimes objected that the Principle "has no physical meaning," that it destroys utterly the old theory of light based on an elastic æther and puts nothing in its place, that, in fact, it sacrifices the needs of the physical to the needs of the mathematical instinct. That the statement is true there can be no doubt, but the absence of any substitute for the elastic æther theory of light may simply be due to the fact that the Principle has been developed so far chiefly by people who are primarily mathematicians. ”

“ In place of the elaborate calculations which have hitherto been necessary in dealing with moving systems, all that we have to do now is to solve the problem under consideration for the limiting case of infinitesimal velocity, and then effect a mere algebraical transformation. The only objection that seems likely to be raised is that the Principle proves too much, that it appears impossible that such far-reaching conclusions can be drawn from such simple assumptions: the only difficulty, in fact, is that the thing is too easy. ”

“ Now since no two human beings have ever, within historic times, moved relatively to each other with a uniform velocity of cm./sec. and subsequently compared their experiences, and since, on the other hand, we do not expect to detect divergencies from the laws of a quiet system or from the laws predicted by the Principle of Relativity until the relative velocity reaches at least cm./sec, the evidence for all these propositions is extremely precarious. ”

“ They not only inquire whether the real rate changes with the velocity, a question which, as the real rate is defined as that function which does not change with the velocity, is utterly trivial, but they actually give a negative answer. They see that the expression for the real rate contains explicitly and rush to the absurd conclusion that the real rate changes with the velocity. ”

“ The word is intimately associated with the philosophic doctrine of realism, which holds that the most important thing that we can know about any body is not what we observe about it, but its "real nature," which is something that is independent of observation. Now, of course, a quantity which is wholly independent of observation cannot play any part in an experimental science, but there are quantities which are independent of observation in the more limited sense that they are observed to be the same by whatever observer the observation is made. ”

“ It seems to me incredible that anyone, who understands what he is doing, will really propose to reject definitely a proposition which he may hope to prove in the near future in favour of one for which there is never likely to be the smallest direct experimental evidence. ”

“ Consequently, if the theory is true, it will tell us how to change from the form suitable for a quiet system to that suitable for a disturbed system not only the laws which it is the special business of the theory to investigate, but any laws whatsoever. Instead of working out, as heretofore, the transformation necessary for each special law, we shall arrive at a transformation which is valid for all laws. ”

“ So long, that is, as all distances are measured relatively to axes fixed in the earth, all times are measured on clocks fixed in the earth, and the phenomena considered are those of charged bodies fixed in the earth in magnetic fields produced by instruments fixed in the earth, or those of sources of light fixed in the earth observed by instruments fixed in the earth, no conclusions have ever been obtained which are not consistent with that theory. ”

“ But, if Newtonian dynamics are true, is not this relative velocity, but the velocity of the electron relative to the centre of mass of the electron and the instrument. Since the mass of the electron can conceivably become infinite, the distinction, negligible in practice, is of great importance theoretically. ”

“ If, then, anyone prefers one theory to the other it must be either on the ground of differences in the laws not contemplated originally which are predicted respectively by the two theories, or because of some general grounds independent of experimental considerations. ”

“ It is felt that the universal importance attributed to the velocity of light is strange, when it is proposed to apply the principle to laws which have nothing to do with optics. Why. it may be questioned, do we drag in the velocity of light rather than that of sound or of the trains on the twopenny tube? Some part of this uneasiness may arise from the unfortunate way in which Einstein introduces the Second Postulate in his paper: he seems almost to try to deduce it from the First Postulate. ”

“ Nor does it seem in the least likely to become less precarious: so far as I know, nobody has made the faintest suggestion as to how a relative velocity of more than between two human beings might be attained in such a way that they could perform delicate measurements. ”

“ However, some people, who in their eagerness to escape the reproach of being metaphysicians have adopted without inquiry the oldest and least satisfactory metaphysical doctrines, are so enamoured of the conception of "reality" that they refuse to give it up. ”