25 OCTOBER 1930, Page 8

Science : Yesterday and To-day

The following article is the second of a series not mainly intended to convey knowledge of particular conclusions that are being reached in various sciences—this will only be incidental—but rather to give some conception of the new modes of thought and changes of method t hat are being developed with the extension of scientific knowledge, in a manner which is comprehensible and interesting to the lay reader. This week Professor James Rice, University of Liverpool, writes on Physics. Next week Professor William McDougall of Duke University,

N.C., will write on Psychology.]

Physics: Old and New


THE phrase " Science yesterday and to-day " has a very definite significance in relation to Physics. What we now call " Classical Physics " was born to all intents and purposes in the seventeenth century, when little groups of educated men ceased to be content with speculations based on the mere observation of the natural phenomena of daily experience and began to question Nature by making experiments, so as to find order, uniformity and law where there appeared to be only a mass of unto-ordinated events. It was the genius of Newton which foresaw the lines along which this new " Natural Philosophy " was destined to develop. He supplied the methods which were to be used in attacking fresh problems and discovered the principles which were to guide the investigator in the chains of reasoning which each new experimental result pro- voked. Among those principles his laws of motion occupy a pre-eminent position. The accuracy with which they were found to apply to planetary motions and to the movements of bodies on our earth was so great as to be positively sensational in an age accustomed to be satisfied with the vaguest explanations of natural occur- rences. Little wonder is it that they captured the imagination of the educated classes of the day, and that they were regarded as ideal examples of that clarity and conciseness of expression which is the mark of all great scientific generalizations.

Through the eighteenth century men patiently pursued the quest for experimental knowledge, and the broad divisions of Physics into the study of the mechanical properties of matter, Heat, Light, Magnetism and Elec- tricity were marked out. Within each division was embraced a large body of related facts, but the con- nexions between the facts of one branch and those of another were not as yet clearly perceived. The synthesis of these parts into a really unified body of knowledge was the achievement of the nineteenth century. This unification was based on the belief that all the phenomena of Nature are just broad manifestations to our senses of the motions of ultimate particles of matter. That these motions were subject to the laws that Newton had dis- covered for planets and ordinary terrestrial bodies was accepted as an axiom, and the immediate results of that assumption were so amazingly accurate that men began to believe that they were within reach of finality in our knowledge of the material universe. The successes of Physical Science were so marvellous that other sciences such as Biology appeared lamentably " inexact " in comparison, and the efforts of biologists to capture some of the " exactness " of the physical sciences by imitating their methods gave rise to the famous quest for a mechanistic interpretation of life. It was during the nineteenth century that the explanation of light as an undulation in an all-pervading material ether was com- pleted. The layman is probably very familiar with the idea. What he probably does not realize is the extreme simplicity of the hypothesis ; for it is little short of miraculous that mathematical reasoning based on the assumption of a medium possessed of only two simple mechanical properties, elasticity and inertia, should give such a precise formulation of the wealth of experi- mental results obtained by extremely refined optical instruments, whose precision of measurement can hardly he realized by one not trained to handle them. During the same period the effects of electricity in motion were discovered and brought within the general scheme, and Clerk Maxwell by making a bold step in the mathe- matical treatment of electrical oscillations in conductors predicted the existence of electromagnetic waves, a pre- diction which requires no justification in these days of universal broadcasting. Thus at the close of the nine- teenth century, physicists were wont to assert without fear of contradiction that their science possessed a body of laws equal to all the demands that had been made or were likely to be made on them.

And yet, not quite ! In a famous address at the close of the century Lord Kelvin remarked that there were two small " clouds on the horizon." Strangely enough, it was not the recently-discovered Röntgen rays and Radioactivity, nor the sensational splitting of the atom just achieved' by Sir J. J. Thomson and his pupils at Cambridge, that troubled Lord Kelvin's mind. Apparently these new achievements would be embraced in the classical formulation when in due course a sufficient body of experimental knowledge was available. There was no reason to suspect that these new ultimate particles, the electrons torn from the general body of the atom, would not conform in their behaviour to the Newtonian laws. What appeared somewhat inexplicable to Lord Kelvin and other leading physicists was that in some very refined measurements on the propagation of light in different directions an American physicist, Michelson, had recently failed to observe the predicted result, and that in some equally delicate observations on the spectroscopic analysis of the radiation emitted by white-hot carbon certain German investigators had obtained results in flat contradiction with the predictions of the classical principles. Lord Kelvin's clouds soon developed into a veritable tempest. The Michelson experiment was the starting-point for Einstein's Principle of Relativity, which not only modified one very important postulate of the Newton dynamical principles, viz., the invariability of mass, but also completely altered our ideas as to the nature of gravitation. The general public has by its interest in Einstein's general ideas instinctively felt that something unusual and " revolutionary " was in the air. But even more revolutionary, though less widely appreciated outside professional scientific circles, has been the outcome of the second cloud; for the solution of that difficulty has been the direct cause of such a formulation of new principles, entirely foreign to the spirit of Classical Physics, that when we speak of Modern Physics or the Physics of to-day we Mean the body of experimental knowledge concerning the atom and its parts which has been gained in the past thirty years and the body of laws which we have had to create about these ascertained facts.

When it was discovered that all atoms consist of two definite types of electrical particles, elettrons and protons, it was quite natural to physicists accustomed to picture all phenomena in terms of mechanical models to make mental models of the new atom. The sun and planet model offered the most ready help for most purposes, and so Atomic Physics became a kind of microcosmic astronomy, and it seemed that all we needed was a skilful application of the familiar mathematical methods of the astronomer to this world in little in order to explain all the observed facts. This expectation was completely unfulfilled. The Newtonian laws of motion proved entirely inadequate to meet the case. Had they been satisfied, the nature and quantity of the output of energy from an atom when disturbed by external influences would have been quite different from what is actually observed. The Newtonian principles necessitate a certain continuity and " smoothness of working " in exchanges of energy between different portions of matter. That continuity is evident enough in the behaviour of matter in bulk ; but in the interchanges of energy between atoms there is a discontinuity and " jerkiness." which can find no explanation in classical laws of motion. Furthermore, on Newtonian principles every atom would in time have radiated away all its energy and a physical world as we know it would have been a sheer impossi- bility. Some new theory adequate to account for this inherent, non-dynamical stability of the atom and its discontinuous exchanges of energy with its environment was called for. This theory is in course of construction. It is called the " Quantum Theory." Its mathematical formulation has proceeded very rapidly indeed, but unfortunately, being quite unmechanical, it offers little opportunity for formulation in models or pictorial images which have been so helpful to the layman in the past when mathematical knowledge could not be appealed to.

These new mathematical principles do not of course contradict the Newtonian methods when effects on matter in bulk are considered, for in such cases their application is simplified and they contain the classical principles as what is termed " a special case." But whatever be the outcome, and even if some easier method of expounding the new ideas than the mathematical one be ultimately forthcoming, we must for ever surrender the hope of building a model world in our minds whose parts will obey the old mechanical principles and at the same time be a true picture of the happenings in our physical universe.