The Revolution in Physics

BOOKS OF THE DAY

By Dr. J. D. COCKCROFT PROBABLY no developments in physics have excited so much general interest as the frequently quoted statements that

Determinism has vanished from the physical world. Those

who wish to know precisely what is meant by this statement Nvill find in the translation of Ernst Zimrner's book a most

thorough description of the revolutionary changes which have occurred in physics since the beginning of this century. The author begins by stressing the attainments of classical physics. By the end of the eighteenth century the development of physics had completely transformed the mediaeval view of the world. The triumph of the Newtonian law had led to the belief that the law of causality was absolute ; that the world of matter was free of all chance ; that in the words of Laplace, " a spirit who knew at a given moment all the forces existing in Nature and the relative positions of all

existing things or elements composing it would be able to coniprehend in a single formula the motion of the greatest heavenly body and of the lightest atom."

The discoveries of the nineteenth century established this view with still more certainty. The work of Faraday, Ampere, Maxwell and others developed the sciences of electricity

and. magnetismto a point where their practical application became of enormous consequence. The work of Joule,

Thompson, Carnot, Boltzmann, laid the foundations of thermodynamics and with it the modern age of mechanical engineering. At the end of the century, however, the new discoveries of X-rays, of radio-activity and of the electron, introducing the new era of atomic physics,- were at the same time leading inevitably to far-reaching modifications of the classical laws.

First came the discovery of Planck that to explain the relative proportions of energy of different wave-lengths, red and blue, for example, emitted from a luminescent body, the

energy could be emitted only in multiples of an indivisible unit, the .quantum, symbolised by h. Reinforeing.this came

experiments showing that the energy of light could be absorbed only in multiples of the same unit ; that light behaved in these experiments like a stream of particles rather than as the waves which the experiments of classical physics required for their interpretation. Here for the first time appeared the dualism of waves and particles which now dominates physics. Next in order came-Bohr's successful explanation of Ruther- ford's picture of the atom in which. electrons 'moved like members of a planetary system round a central heavy charged nucleus. Here the laws of classical mechanics were applied with two fundamental restrictions ; first that the energies of

the electrons must be multiples of the quantum so that only a few stable orbits were possible ; second, that electrons could not emit radiations whilst moving in a stable orbit—a postulate equally at.variance with classical physics.

With these -postulates, the Bohr theory could explain beautifully the remarkably complex spectra of radiations

emitted from atoms ; could explain too the general chemical properties of elements and the ordered structure of the Mendeleeff table. In spite of thek succeists,'ho*ever, the theory was fundamentally unsatisfactork:7. There were no logical grounds for its fundamental postulates ; its picture of electrons moving in plane orbits seemed unconvincing, and worst of all it failed to explain many finer details of experimental results. As one eminent physicist remarked : the Bohr atom was more a device for calculation than a " reality of existence."

In the search for a way out of these difficulties, workers in theoretical physics were led by the apparent dual wave and particle aspects of light to see whether material particles such

The Revolution in-Physics: By Ernst Zimmer. (Faber and Faber. 12a. 6d.) as electrons might not also have similar dual properties ; whether an electron might in the interior of the atoin,- for

example, behave like a wave. This new point of view proved immediately successful. If an electron inside an atom is represented by a wave, the wave must join up after one

revolution, and so we find at once a convincing explanation- of

Bohr's empirical postulate that only certain orbits and energies are possible. The theory obtained a direct proof by experiments which showed that electrons fired through thin foils have_ on this occasion predominantly wave properties. Thus, instead of being diffused over all angles as particles should be, they are found to be deflected only at a limited number of angles behaving in this respect very similarly to X-rays which have long been considered to be waves. If, then, the electron is a wave extending over a large volume in space, how can we say where it is ? To this the empirical theoretical physicist 'replies that the extent of the wave motion tells us only the probability that a particle is in a given

place. We can no longer fix its position with certainty. It appears, in fact, that it is no longer possible to predict. the future track of a-particle with complete precision even though we have a precise knowledge of its position and• motion at any instant. There is a fundamental uncertainty in our deter- minations of its position and speed which are closely related to the magnitude of the ubiquitous quantum. - A physical explanation of this point of view comes when we realise that to predict the future' course of a. Particle we

must make an observation on it. rr the Iiiht 'used ler observation is reflected from a planet it makes little difference to its motion ; if it is reflected from an electron the dis- turbance to its path may be appreciable and gives just the uncertainty predicted. The new point of view thus leadi to the position that only the average path of many particles is determinate ; _ the future of the individual particle Can only be approximately determined.

This new principle lends strong support to the positivism which has reappeared in Physics—the point of view that modern atomic physics is not .concerned with the true nature and structure of atoms but rather with the processes which occur when we observe atoms. On this point of view it is meaningless to talk of an electron revolving in an orbit round

an atom, for the principle of uncertainty tells us that experi- ments can only show the electron as somewhere within a certain region ; the exact position is indeterminate. The true business of theoretical physics is therefore to develop; empirically or otherwise, theories which will lead to new

experiments. The theories of the physicist are transient, to be discarded when they have served this fundamental purpose—the discoveries of the experimenter are the only permanencies in physics.

As the author points out, this view, whilst characteristic of the younger school in physics, is not universally held. Thus the founder of the Quantum Theory, Max Planck, believes that the main problem of physics lies in under-

standing the real world. The physicist may in his view be compared to the cave-dweller of Plato, imprisoned in his cave and able to perceive what is happening outside the cave only by means of shadows falling upon its walls through a small opening ; however he may strive he can never attain to complete understanding of the outside world ; each suc- cessive picture, however, captures a greater part of the truth.

This new summary gives a very thorough and brilliant discussion of these changes in physics. It will appeal to those who have enjoyed Eddington's Nature of the Physical World and Darwin's New Conceptions of Matter. It should prove invaluable to those' interested in the philosophical aspects of the new developments.