November 4, 1920] 



NATURE 



325 



tionat experiments, such as the work done by Madame 

 Montessori and the founders of the Little Common- 

 wealth. 



(6) The hygiene and technique of mental work is 

 itself being based upon scientific investigation. Of 

 the numerous problems in the conditions and character 

 of mental work generally, two deserve special men- 

 tion — fatigue and the economy and technique of 

 learning. 



But of all the results of educational psychology. 



perhaps the most valuable is the slow but progressive- 

 inculcation of the whole teaching profession with » 

 scientific spirit in their work, and a scientific attitude 

 towards their pupils and their problems. Matter 

 taught and teaching methods are no longer exclusively 

 determined by mere tradition or mere opinion. They 

 are being based more and more upon impartial ob- 

 servation, careful records, and statistical analysts — 

 often assisted by laboratory technique— of the actuaL 

 behaviour of individual children. 



Popular Relativity and the Velocity of Light.* 



By Sir Oliver Lodge, F.R.S. 



IN using the phrase "popular relativity" I 

 indicate that what I am criticising is not 

 f Einstein's equations— which seem to have justified 

 themselves by results — but some of the modes of 

 interpreting them in ordinary language. Especially 

 do 1 attack that proposition which asserts that to 

 every observer the velocity of light will not only be 

 constant in reality, but will also superficially appear 

 constant even when he ignores his own motion through 

 the light-conveying medium — a proposition or postu- 

 late or a.\iom which has been shown to lead to 

 curious and, as 1 think, illegitimate complications, 

 threatening to land physicists in regions to which 

 they have no right of entry, and tempting them to 

 interfere with metaphysical abstractions beyond their 

 proper ken. 



Not that a physicist's proper ken is limited to what 

 he immediately observes ; he is entitled, and indeed 

 required, to interpret appearances rationally by taking 

 * into account every relevant adventitious circumstance, 

 including complications due to his own unobserved, 

 and p<"rnaps unobservable, travel through space. 



In a relativity discussion at the Physical Society 

 recently a member is reported to have asked the 

 pertinent Question : " Does an observer merely ob- 

 serve, or does he think as well? " If he thinks, I 

 urge that he can allow for changes in his measuring 

 instruments and any other consequences of possible 

 motion, and can refrain from making deductions about 

 spare and time on the strength of experiments on 

 matter. 



He will know that his senses are material senses, 

 and that all his experiments are made ultimately by 

 their aid. He will know that he can only experiment 

 even on the sether of space indirectly by means of 

 matter, for he has no other means of getting a grip 

 on it. Possibly he may be unable to grip it even 

 thus, but matter gives him his only chance ; he cer- 

 tainly cannot experiment on abstractions like space 

 and time. 



On the basis of material experiments he may be 

 able to make deductions or draw inferences about the 

 ather, because that certainly has some inter-relations 

 with matter; but it is probably illegitimate, on the 

 basis of matpri.nl experiments, to make deductions 

 about spare and time at all ; they are not likely to 

 be afTpcted by anything that matter can do, and it is 

 only m.Ttter with which we can directly deal. 



The relation between space and time that represents 

 the velocity of licht gives us directly one property of 

 the aether, viz. the product of its electric and mag- 

 netic constants, both of which separately are at present 

 inknown. Every student who accepts the a-ther of 

 l>:i(e as a reality is probably ready to admit that the 

 velocity ot light through free Kther Is an absolute 



> Th« lu^MUDce -f ihtft conirovvrswl Koic vas communkaitd toScctioQ A 

 '■f lb* llniifth A*»r> i»iH r ai lh« i. »i(l fi n>*eiiDg gn Augtitl 97. 



NO. 2662, VOL. 106] 



constant, not dependent on anything that either the- 

 observer or the source is doing, has done, or may do. 



But this admission has been erected into a fetish 

 by the theory of relativity, at least when expressed, 

 in ordinary words, and is interpreted as requiring 

 that to every observer, whatever he may be doing, 

 the velocity of light in every direction will appear the 

 same. 



That is not only a different, it is a contradictory,, 

 proposition. Given the constancy of the real velocity 

 of light — if an observer travel to meet it, it must 

 appear to arrive more quickly than if he travel away 

 from it, provided he has any means of making the 

 observation at all. He may be unable to make the 

 observation, but suppose he can make it, say by the 

 aid of Jupiter's satellites, and detected a discrepancy, 

 he need not infer any real change in the velocity of 

 light ; because, if he thinks, he can attribute any ob- 

 served difference to his own motion, and thereby 

 emerge with clear and simple views. H he sets out 

 with the gratuitous notion that he can never become 

 aware of his own motion, or that his own motion 

 has no meaning, he will indeed encounter a puzzling 

 universe, and will presently long for a Copernicus to 

 unravel the subjective complexities of observation. 



But it may well be extremely diflScult for an 

 observer to measure the velocity of light through the 

 aether except with the aid of some return signal which 

 the aether likewise has to transmit in the opposite 

 direction ; and in that case he may find that the 

 to-and-fro pair of journeys take exactly the same 

 time in every direction. 



This, as everyone knows, has been done for a to- 

 and-fro journey of a beam of light. .And the timing 

 is exact, not only to the first order of small quantities, 

 as might readily be expected, but to the second order 

 also — an exactitude which, if rigid unchangeable 

 materials could be used, would not be expected, and' 

 ought not to occur. But if the dimensions of the 

 material object used as the foundation-stone of his 

 apparatus are subject to change by reason of motion, 

 and if the changes are in accordance with the elec- 

 trical theory of matter, as suggested by FitzGerald 

 and elaborated by Lorentz, then everything becomes 

 clear again until we come to astronomical and gravita- 

 tional applications, and the precisely negative result 

 of Micholson and Morley is precisolv explained. 



A mathematical doctrine of relativity m.iy be based 

 upon this experimental result, and may be convenient 

 for reasoning purposes, but no such doctrine is 

 required by the facts. The facts are patient of the 

 doctrine ; they do not compel it, nor do they justify 

 it. Any comprehensive mathematical expression is 

 liable to permit other modes of interpretation, as well 

 as the simplest and truest or the one most directly 

 applicable to the problem in hand. It |g devisicd to- 

 cover one set of facts, but in its generality it is apt 



