May 2^, 1918] 



NATURE 



■35 



I apologies, therefore, for presenting to-night one of the 



L great unsoh-ed problems of mouern physics, nor lor 



y leaving it with but the vaguest of suggestions towarus 



i a solution. 



7 The newest of the problems of physics is at the 



same time the oldest. For nothing is earlier in the 

 experiences either of the child or ot the race than the 

 sensation of receiving light and heat from the sun. 

 But how does light get tg us from the sun and the 

 stars through the empty interstellar spaces? The 

 Greeks answered this querj^ very simply and very satis- 

 factorily from the point of view of people who were 

 content with plausible explanations, but had not yet 

 learned perpetually to question Nature experimentally 

 as to the vaUdity or invalidity of a conclusion. They 

 said that the sun and all radiators of light and heat 

 must shoot off minute corpuscles the impact of which 

 upon the eye or skin produces the sensations of light 

 and warmth. 



This corpuscular theory was the generally accepted 

 one up to A.D. 1800. It was challenged, it is true, about 

 1680 by the Dutch physicist Huygens, who, starting 

 with the observed phenomena^ of the transmission of 

 water waves over the surface of a pond or of sound 

 waves through the air, argued that -light might be 

 some vibratory disturbance transmitted by some 

 medium which fills all interstellar space. He postu- 

 lated the existence of such a medium, which was called 

 the luminiferous or light-bearing aether. 



Partly no doubt because of Newton's espousal of 

 the corpuscular theory, the aether or wave theory 

 gained few adherents until some facts of interference 

 began to appear about 1800, which baffled explanation 

 from the point of view of the corpuscular theory, but 

 were easily handled by its rival. During the 

 nineteenth century the evidence became stronger and 

 stronger, until by its close tlie corpuscular theory had 

 been permanently eliminated for four different reasons : 

 (i) The facts of interference were not only found in- 

 explicable in terms of it, but also completely 

 predicted by the wave theory. (2) The fact that the 

 speed of propagation of light was exi>erimen tally found 

 to be greater in air than in water was in accord with 

 the demands of the aether theory, but directly contrary 

 to the demands of the corpuscular theory. (3) Wireless 

 waves had appeared and been shown to be just 

 like light weaves save for wave-length, and they had 

 been found to pass over continuously, with increasing 

 wave-length, into static electrical fields such as could 

 not possibly be explained from a corpuscular point of 

 view. (4) The speed of light had been shown to be 

 indef>endent of the speed of the source as demanded by 

 the jether theory and denied by the corpuscular theory. 

 By 1900, then, the asther theory had become appar- 

 ently impregnably entrenched. A couple of years later 

 it met with some opposition of a rather ill-considered 

 sort, as it seems to me, from a group of extreme 

 advocates of the relativity theory, but this theory is 

 . now^commonly regarded, I think, as having no bear- 

 ing whatever upon the question of the existence or non- 

 existence of a luminiferous aether. For such an aether 

 was called into being solely for the sake of furnishing 

 a carrier for electromagnetic waves, and it obviously 

 stands or falls wnth the existence of such waves in 

 vacuo, and this has never been questioned by anyone so 

 far as I am aware. 



Up to 1903, then, the theory which looked upon an 

 electromagnetic wave as a disturbance which originated 

 at some point in the aether at which an electric charge 

 was undergoing a change in speed, and was propagated 

 from tha*^ point outward as a spherical wave or pulse, 

 the total energy of the disturbance being always spread 

 uniformly over the wave front, had met with no serious 

 question from any source. Indeed, it had been* extra- 



NO. 2534, VOL. lOl] 



ordinarily successful, not only in accounting for all the 

 known facts, but also in more than one instance in pre- 

 dicting new ones. The first difficulty appeared after the 

 discovery of the electron and in connection with the 

 relations of the electron to the absorption or emission 

 of such electromagnetic waves. It was first pointed 

 out in 1903 by Sir J. J. Thomson in his Silliman lec- 

 tures at Yale. It may be stated thus : — 



X-rays unquestionably pass over, or by, all but an 

 exceedingly minute fraction, say one in a thousand 

 billion, of the atoms contained in the space traversed 

 without spending any energy upon them or influencing 

 them in any observable way. But here and there they 

 find an atom from which, as is shown directly in 

 C. T. R. Wilson's photographs (Figs, i and 2), 

 they hurl a negative electron with enormous speed. 

 This is the most interesting and most significant char- 

 acteristic of X-rays, and one which distinguishes them 

 from th.e a and /3 rays just as sharply as does the 

 property of non-deviability in a magnetic field; for 



Fig. I. — Tracks of /3 particles 

 ejected by X-rays from mole- 

 cules of air. 



Fin. 2.— Tracks of 

 o ray s in air. 



neither a nor /? rays ever eject electrons from the atoms 

 through which they pass with speeds comparable with 

 those produced by X-rays, else there would be new 

 zigzag lines branching out from points all along the 

 paths of the a and ^ particles shown in the Vvilson 

 photographs. 



But this property of X-rays introduces a serious 

 difficulty into the aether theory. For if the electric 

 intensity in the wave front of the X-ray is sufficient 

 thus to hurl a corpuscle" with huge energy from one 

 particular atom, why does it not at least detach cor- 

 puscles from all the atoms over which it passes? 



Again, when ultra-violet light falls on a metal it, 

 too, like X-rays, is found to eject negative electrons. 

 This phenomenon of the emission of corpuscles under 

 the influence of light is called the photo-electric effect. 

 Lenard {Ann. d. Phys. {^], vol. viii. [1902], p. 149) first 

 made the astonishing discovery that the energy of 

 ejection of the corpuscle is altogether independent of 

 the intensity of the light which causes the ejection, no 

 matter whether this intensity is varied by varying the 

 distance of the light or by introducing absorbing 



