One definite outcome of the bold generalisations or challenges of relativity is that energy, which since its conservation was estab lished has always been recognised as protean in form, now includes not electric charges alone but matter itself as one of its forms, so that atoms of matter might be turned into other forms of energy, if only we knew how. The idea already has some cosmo logical significance, for it is believed by Jeans to be the cause or mechanism of the radiation of the sun and other stars. The atom of matter is electrically constituted; it is composed of equal amounts of positive and negative electricity in its normal condi tion. High temperature signifies great activity or immense speeds among the atoms. They can be dissociated or broken up by col lision, and their opposite ingredients may occasionally clash, lose their identity, or at least their locality, and pass quickly away as a quantum of radiation. The interior of stars is known to be at an exceedingly high temperature, and accordingly their mass is believed to be gradually decreasing, their matter dying and passing away as radiation into the depths of space. Whether there is ever or anywhere a recuperative action is at present unknown. When, if ever, such action is discovered—as is not unlikely—it will have a cosmological and philosophical bearing.
It is interesting, as illustrating Newton's exceptional foresight and speculative skill, that at the end of a disquisition on the changes and interchanges of known forms of energy (though the term "energy" as a dominating physical term was not adopted till long afterwards), he should conclude thus: "And among such vari ous and strange transmutations, why may not Nature change bodies into light, and light into bodies?" Interaction of Matter and Radiation.—Meanwhile strange reactions have been discovered between matter and radiation. Radiation is generated by electrons, and accordingly itself suffers discontinuity. Maxwell calculated that it must exert pressure : it is now found to exert pressure after the same bombarding fashion as the molecules of a gas exert it. Radiation is generated and absorbed not continuously but in quanta, and these quanta exert a bombarding action as if they were particles or corpuscles; yet they must be waves, because they exhibit interference and dif fraction phenomena—a study of which, in optics, began with New ton himself. Wave quanta with extra high frequency of vibration are more efficient projectiles, and eject such suitable electrons as they encounter with more energy than do those with a more ordi nary rate of vibration. This connection between energy and fre quency is a development of Planck's quantum, which, like much else. is due to Einstein's bold and forcible conceptions. The out
come of all this is that the old barrier between the corpuscular theory and the wave theory is being removed. There is truth in both. High speed particles are accompanied by waves, somewhat akin to the waves at the bows of a steamer, and the apparently sharp distinction between a wave and a corpuscle is subject to a compromise which includes both.
We have long studied matter by means or by the aid of radi ation or waves; we are now beginning to see that momentum, which seemed a property of matter, is possessed by waves, and that the familiar thing which we call matter is after all a mani festation or localization of ether-energy, in a form not as yet com pletely known or understood. Ether energy—the one fundamental existence—seems to manifest itself alternatively, or even as it would seem indiscriminately, sometimes as what we might call a particle and sometimes as a wave. De Broglie seems to have initiated this idea, which has been taken up and elaborated by SchrOdinger and others. The result is that, whereas we once thought that we knew a great deal about an electron, its size and its speed, we are now confronted with some uncertainty. It is all very well to speculate and theorize, but all theories must be veri fied and brought to the test of experiment ; and experiment—even indirect observation—on this minute scale is barely possible.
The difficulty can easily be rendered intelligible. We have to use some physical means to examine the behaviour of anything. We cannot see a thing unless it is illuminated, or unless it emits light itself ; i if we are examining something very minute, the illu mination itself, which consists of quanta with a definite momen tum, may be altering the conditions of the very thing we want to observe. How then can we determine the position and speed of an electron? Heisenberg has shown that we cannot do both. If one determination is accurate, the other is vague. We are not even sure now of the size of an electron, for if it carries waves with it, those waves must occupy a certain region, which, though perhaps small compared with an atom, is far greater than any thing we can call the electric nucleus. Indeed a highly accel erated electron, like one of those in Bohr's inner atomic orbits, seems to spread like a sinuous disturbance over the whole orbit, so that only in the outer and larger orbits is there any close correspondence between ordinary dynamics and actual fact. That there is a correspondence at all is satisfactory, but there is much detail in the ultimate minutiae which at present escapes us.