>98 



NA TURE 



[September 16, 1922 



Einstein's Theories. 



THE Revue Philosophique (Alcan, Paris), edited 

 * by Proi Levy-Bruhl, has just issued (July- 



\n 1.1I number devoted to the consideration 



of Einstein's theories of relativity. It consists of 

 four articles of exceptional ability and importance, 

 all directed to the philosophical aspects of the problem. 

 That Einstein's theory is established, in the meaning 

 that it is applicable in physical science, is accepted 

 by each of the writers, and their aim is to decide how 

 far it forces upon us a new way of thinking about 

 physical reality. 



The first article is a translation by M. Leon Bloch 

 from the German of Hans Reichenbach, " La significa- 

 tion philosophique de la theorie de la relativite." 

 The philosophical interest centres on the concept of 

 time. Must we give up the absolute meaning of 

 simultaneity ? If we do, the Michelson-Morley ex- 

 periment at once ceases to be incomprehensible, light 

 can have the same velocity in the moving system 

 which it has in the system at rest. But can we give 

 up absolute simultaneity without being involved in 

 logical difficulties and finding ourselves confronted 

 with a pure paradox ? In a very skilful argument 

 the writer concludes that we can and that we must. 

 Relativity is both a logical necessity and an ex- 

 perimental fact. This leads to the consideration of 

 the part played by the velocity of light in Einstein's 

 theory. 



In Nature the electromagnetic waves play a 

 unique part and are of greater importance than 

 any other phenomenon which serves us as a signal. 

 They alone (if we set aside gravitation) transmit an 

 action across empty space. Now as the forces which 

 the individual particles of matter exercise on one 

 another are of the same nature as electromagnetic 

 forces, it follows that all propagation of material 

 action resolves itself ultimately into an electrical 

 transmission. 



The essential ideas of the theory of relativity were 

 forestalled by Ernst Mach forty years ago, and 

 Einstein is continually reminding us of our indebted- 

 ness to him. The idea that movement as a spatial 

 phenomenon can be recognised only in relation to 

 other bodies was much older — we have it, for ex- 

 ample, in Descartes and in Leibniz — but what dis- 

 tinguishes Mach's point of view is the idea that 



movement must have not only kinematic but also 

 dynamic relativitv, that what we call the forces of 

 inertia must be bound up with the presence of other 

 bodies. In Mach, however, the relativity of theory 

 of knowledge is not distinguished from physical 

 relativitv ; it remained for Einstein to show that the 

 actions of movements, or what we name forces, can 

 be reduced to a difference between the distributions 

 of masses. 



The second article, by M. G. Cerf, "Pour Intelli- 

 gence de la relativite," deals particularly with the 

 exact meaning we are to give to the terms employed 

 in the theorv, and the writer draws largely on the works 

 of Henri Poincare. 



The third article, " Einstein et la metaphysique," 

 is by M. Edmond Goblot, the distinguished logician 

 and philologist. He finds considerable amusement 

 and no little instruction in the popularisers of Einstein, 

 more especially those who competed for the Higgins 

 prize of the Scientific American Publishing Co. In 

 his conclusion he says : " Je resume et precise ma 

 question : Einstein est-il mathematicien, physicien ou 

 metaphysicien ? Mathematicien il l'est. Physicien il 

 Test aussi. S'est-il borne a cela, ou s'est-il abandonne 

 anx debauches de metaphysique inconscient qu'on 

 nous fait lire en son nom ? Dans les deux cas, il 

 est grand temps d'exorciser tous ces fantomes." 



The fourth and concluding article, by M. Richard- 

 Foy, " Le temps et l'espace du commun sens," is a 

 very clear statement of the whole problem to meet 

 which the new principle is required. It deals mainly 

 and sympathetically with M. Painleve's protest 

 against the refusal to allow any place for the concept 

 of an absolute in phvsics. The rejection of time 

 and space as absolutes is not irreconcilable, he argues, 

 with such a position. To say that time, space, and 

 movement are not absolutes means that, instead of 

 being realities which impose their laws on phenomena, 

 they are only abstractions, necessary to express those 

 laws, but capable of assuming the most diverse forms. 

 We choose among these forms with the simple aim 

 of discovering the most convenient, but our choice 

 has limits. For example, we cannot define simul- 

 taneity in an}' two points absolutely, y r et we must 

 define it so that it is not possible that my friend has 

 read mv letter to him before I have written it. 



Educational Work of the Ministry of Agriculture. 



THE Intelligence Department of the Ministry of 

 •*• Agriculture and Fisheries has issued a Report 

 on the work of the department for the years 1919- 

 1921, which is published at the price of 5s. by H.M. 

 Stationery Office. The duties of this department are 

 concerned with agricultural education, agricultural 

 research, the agricultural training of ex-officers and 

 men, horticulture, the improvement of live-stock, the 

 destruction of rats, and the diseases of animals. 



Agricultural education is provided through the 

 agency of colleges, including agricultural departments 

 of universities, and by farm institutes, local classes, 

 and lectures. The first group comprises eleven 

 institutions, of which all, except the Harper Adams, 

 the Midland Agricultural College, the University 

 College, Reading, and the Seale Hayne College, are 

 connected with universities in which students may 

 obtain a degree with agricultural science as their chief 

 subject. In most cases the agricultural department 



NO. 2759, VOL. I 10] 



is actually part of the university organisation, and it 

 is recognised that in many respects this is an ideal 

 arrangement. Agricultural students thereby obtain 

 the intellectual stimulus that is associated with 

 intercourse with students in other faculties. Future 

 teachers, scientific workers, and agricultural experts 

 all gain by the indefinable atmosphere which per- 

 vades a university course. On the other hand, it 

 has been found that although, theoretically, expendi- 

 ture should be saved bv taking advantage of the 

 courses in general science which a university provides, 

 in actual practice it has proved necessary to provide 

 special teaching even in preliminary scientific subjects 

 designed for agricultural students. 



By means of this special teaching an agricultural 

 flavour is imparted to chemistry, botany, zoology, or 

 whatever the fundamental science may be, and thus 

 from the very beginning the student's interests are 

 awakened and stimulated. Against such an arrange- 



