14 



NATURE 



[September i, 192 i 



way we shall have to travel if the new ideas are 

 to be translated into everyday physics in all their 

 original force. It also prompts the query whether 

 it is worth while. 



The special theory of relativity disturbed the 

 generally accepted views about the aether by giving 

 equal value to co-ordinate systems moving with 

 uniform velocity with reference to one another. 

 We had always thought of an aether at rest, 

 through which the sun and the planets moved, and 

 in which our ultimate system of co-ordinate axes 

 was at rest. The most straightforward interpreta- 

 tion of the special theory of relativity is to give 

 each planet, each moving electrical charge, its own 

 aether, and at the same time to remove all substan- 

 tiality from the very great number of aethers thus 

 postulated. The plain man wants to know, if the 

 light comes from the sun, in which aether it 

 travels, the sun's aether or the earth's, and if there 

 can be wave motion without a medium. This 

 question of the aether has been discussed so fully, 

 and there are so many different views, that it will 

 be passed by here with the suggestion that pos- 

 sibly the special theory of relativity makes too 

 great demands when it asserts that all moving 

 systems have equal value. The system that the 

 inhabitants of the earth are moving with possesses 

 a special value for them and their physical 

 theories, because they are moving with it. We 

 only ask of Boyle's law, for example, that it should 

 hold for the temperatures that we can produce in 

 the laboratory, not for impossibly high tempera- 

 tures that we can never attain. In the same way 

 it is asking too much of the wave theory of light 

 in the form we use it that it should be equally 

 useful (and true) for us and the possible inhabi- 

 tants of Mars. It is dangerous to attribute uni- 

 versal validity to theories which can be tested only 

 in a limited class of cases. Consequently the 

 aether moving with the earth is the aether. Again, 

 with reference to the apparent unsubstantiality 

 conferred on the aether by the principle of rela- 

 tivity, it is forgotten that it confers some unsub- 

 stantiality on everything else as well, even the 

 water that water waves travel in. 



The general theory of relativity required that 

 light should be deflected on passing close to the 

 sun's surface, and, as is well known, this deflec- 

 tion has been verified experimentally by the obser- 

 vations made by the 1919 solar eclipse expeditions. 

 On the relativity theory the space in the sun's 

 gravitational field is non-Euclidean, and the de- 

 flection Is caused simply by the properties of space. 

 The fact of the deflection is so much simpler than 

 the explanation that it seems probable that the 

 physicist will ignore the latter. One wonders if 

 it is possible to treat the deflection geometrically 

 in a simpler manner directly from the postulate of 

 parallels. There has been an unsuccessful attempt 

 to explain the deflection by an emanation of 

 matter from the sun and a consequent increase 

 of refractive index in its neighbourhood. New- 

 ton's emission theory gives a deflection of exactly 

 half the required amount ; so also does the electro- 



NO. 2705, VOL. 108] 



magnetic theory, if we make the unusual assump- 

 tion that ordinary mass is associated with the 

 energy of the wave, and that this mass is acted 

 on by gravity. At present there seems no satis- 

 factory alternative to a non-Euclidean geometrical 

 optics and wave theory, but it is probably better 

 to wait and in the meantime to suspend judgment. 



The existence of the quantum was discovered 

 theoretically in Planck's celebrated theory of radia- 

 tion. It will be advantageous to give an account 

 of this theory here, because an important modi- 

 fication of it has strengthened the view that there 

 is nothing in the quantum phenomena inconsistent 

 with classical mechanics or electrodynamics. 

 This modification came too late to be noticed in 

 certain widely read descriptions of the theory pub- 

 lished in this country, and it has consequently re- 

 ceived little attention. 



If a hollow vessel is maintained at a uniform 

 temperature, and radiation allowed to issue from 

 a small hole in its side, the intensity of the radia- 

 tion and the spectral distribution of its energy are 

 independent of the material of which the vessel is 

 made. The rays are reflected forwards and back- 

 wards inside the vessel before they issue, and any 

 initial difference in intensity is evened out by the 

 successive reflection. In order to derive a theo- 

 retical value for the spectral distribution of the 

 radiation issuing from such an enclosure at dif- 

 ferent temperatures — "black " radiation, as it is 

 called — Planck assumed that there were in the en- 

 closure a great number of oscillators or vibrators, 

 small Hertzian doublets, all of the same frequency, 

 and in a state of equilibrium, radiating and 

 absorbing energy. The total energy of the system 

 remained constant, but the energy of the different 

 oscillators was not the same; there were always 

 some gaining and some losing energy. Moreover, 

 this exchange took place solely by scattered radia- 

 tion ; there was nothing in the nature of corpus- 

 cular radiation or characteristic radiation taking 

 place. The distribution of the energy among the 

 different oscillators occurs according to the laws 

 of probability, and by using a general definition of 

 temperature the temperature of the system can 

 be derived from this distribution. of energy. Then 

 the density of the radiation in the enclosure can 

 be calculated for the particular frequency in ques- 

 tion. In order to obtain the correct value, 

 namely : 



It was necessary to assume that the emission of 

 energy took place discontlnuously in whole mul- 

 tiples of the quantum, the quantum being defined 

 by e = /?v, where v is the frequency of the radia- 

 tion and h a universal constant, Planck's constant. 

 This emission of radiation in quanta was opposed 

 to all previous ideas. 



The criticism which the experimental physicist 

 naturally passes on Planck's proof as outlined 

 above, and as described in his " Vorlesungen iiber 

 die Theorie der Warmestrahlung " (second edi- 



