1941: Physics

Effects of the War.

The history of physics during 1941 is a startling and a fascinating one. Sir Edward Grey at the onset of the first World War created one of the classic sayings of history, by speaking of the lights going out all over Europe. The lights of physics have mostly gone out so far as the general public can see, though actually they are now burning with great luster in laboratories to which the world at large is denied all access. The process of apparent extinction began even before the outbreak of formal war: the present writer, being in England in July of 1939, heard the comment 'If this state of affairs continues, the only people in England left to work in pure physics in another few months will be the refugees.' In the United States the process began within a month after the fall of France and a year and a half before Pearl Harbor: as early as that did the 'mobilization of physics' commence. By October 1941, more than half of the outstanding physicists of the United States were engaged in what then was still called 'defense work.'

Of the character of this busy and hidden work, nothing may yet be said. Hereafter there will be much to say; in the meantime, one must be content with such statements as that of President Conant of Harvard, 'This is a physicists' war,' and that of Sir William Bragg addressing the Royal Society of London in November 1940: 'The RAF could not carry out its operations without the knowledge resulting from the studies of cathode rays and electrons made by our physicists, which is equivalent to saying that by this time we might well have lost the war.'

This is not to imply that the journals of physics suddenly ceased to appear, as though cut off with a knife, in June of 1940 or at any subsequent date, although something nearly as drastic befell France. But in the English-speaking countries the journals have continued to come out, with a gradual shrinking in content. If number of published pages were a measure of the number of novel ideas, one would certainly have to say that 1941 was a year of greater vitality in 'uncensored' physics than any of the years before the first world war. This, however, is very far from being the case; and valuable as the recent work has been, one must characterize a large part of it as being a checking-in-detail of the remoter and more intricate consequences of theories already long established, while another large part consists in gropings after theories for phenomena which have not yet yielded up their secrets. The lack of sensational novelties is certainly due in the main to the withdrawal of so many of the leaders of physics from their familiar fields, but one should also realize that periods of rapid advance have a way of alternating with periods of quiet consolidation, and a 'quiet period' of the latter kind was already overdue in 1941.

Nuclear Physics.

The experimental side of nuclear physics had evidently reached in 1940 a stage of temporary completeness. Every element had been transmuted into at least one other, every element had been obtained by transmutation of at least one other, every element had been produced in one or more radioactive forms. There were very plausible reasons for affirming that the 'atom-smashers,' defying the name with which the journalists had dubbed them, had already managed to synthesize practically every type of atom-nucleus capable of existence either permanent or extending over a detectable period of time. A great deal of delicate work remained to be done in weighing the atoms both old and new, in measuring the lifetimes of the unstable or radioactive ones, and in measuring the energies of the particles and the wavelengths of the waves which these unstable ones emit: also, in choosing the best ways for producing the particularly interesting ones among these, and magnifying the necessary apparatus. Work of these latter types accounted for most of the recent publications in the field; and one may say that experimental nuclear physics, when it passed behind the veil in 1941, had happily closed its first period of great discovery and entered prosperously upon its first period of routine measurement. Meanwhile the next period of great discovery was being hopefully anticipated from the greatest of all cyclotrons, commenced at Berkeley late in 1940 and to be finished (circumstances permitting) in 1943, able presumptively to deliver the equivalent of more than 100,000,000 volts. The hitherto-unconquered problem of producing electrons of comparable energies has just been mastered by Kerst, whose 'rheotron' involves a variable magnetic field (not a constant one, like that of the cyclotron): the electrons revolve in this field with a steadily-rising velocity, a particular relation being maintained between the magnetic flux through their orbit and the fieldstrength at the orbit itself in order to ensure that they shall remain in the same circular path throughout. Recent newspaper reports indicate the attainment of energies equivalent to what 20,000,000 volts of direct potential would produce. X-rays of unprecedented power may be expected from such an apparatus, and the medical and biological applications of these and of the new radioactive substances produced by transmutation will probably be less hampered by the war than most fields of 'pure' physics.

Theoretical nuclear physics remains in an inchoate and perplexing state, awaiting perhaps some totally new idea which may come suddenly or never. The nucleus continues to be regarded, as for the past nine years, as a clump of particles of two kinds, neutron and proton. These are held together by very strong short-range forces analogous to cohesion, but also they appear to have a nearly-irreducible volume: thus the contemporary model of the atom-nucleus is quaintly similar to Newton's image of a solid body, to wit, a clump of hard incompressible particles cohering together by a specific attraction. It is well known that corpuscles of light originate in atoms and are absorbed in other atoms, by virtue of the electrical forces which operate within the atoms and between them. It is similarly conjectured that to this intra-nuclear cohesion correspond other corpuscles which are emitted and absorbed by the nuclear particles. This, the so-called 'meson theory of nuclear forces,' is still under development and incessant revision, and the time for judging of its value has not come.

Investigation of Cosmic Rays.

Investigation of cosmic rays has survived better into the war period than has many another field of physics, but without showing as yet any sign of completing its task of interpretation. It is now generally agreed that the great majority of the particles which are actually observed, be they at sea-level or at great heights in the stratosphere, are not the 'primary' particles which rain down on the earth from interstellar space. The electrons, the mesons or mesotrons, the photons or light-corpuscles of high energy, and many more massive charged particles, which produce phenomena of such bewildering variety, are born in the atmosphere itself; one, two, three or countless stages of transformation may intervene between them and the primaries. The primaries themselves, which a score of years ago were thought to be photons and then were considered to be electrons, are now held to be neither: the most-widely held idea is, that they are protons. The mesons (generally believed to be the very particles which figure in the theory of nuclear forces aforesaid) are held to originate in the upper atmosphere; they are radioactive, and most of them expire by self-conversion into an electron and a neutral particle before they reach sea-level. The fast electrons are presumed to be in part the offspring of mesons, in part they are believed to have existed as bound electrons in the molecules of the air and to have been driven out of these by impacts of mesons. The magnificent phenomenon of the 'cosmic-ray shower' in which literally thousands of electrons (and sometimes mesons and heavier particles also) spring from a small volume of dense matter, has been abundantly studied. Assuming that the energy of all the particles in such a shower was originally borne by a single primary particle, one arrives at quite fantastic figures for the energies of these last, a billion times as great as that of the most energetic particle which has ever yet issued from a cyclotron.

Chemical Physics.

In the field of chemical physics, the present stage may be described as a stage of replacement: that of the replacing of the older concepts of the forces holding atoms together (formerly called by such names as 'cohesion' and 'chemical attraction') and the forces keeping atoms apart (formerly described sometimes as 'repulsive forces,' and sometimes merely by speaking of incompressible or slightly-compressible atoms) by the newer concepts based upon quantum mechanics. In these the electrons form, as it were, the cement which holds the atoms together, whether in simple diatomic molecules such as those of oxygen, or in massive pieces of sodium or silver. This is, of course, what would be expected, but it must be remembered that classical mechanics would forbid any such conclusion, would in fact require that any system of electrons and nuclei be intrinsically unstable. Not only does quantum-mechanics permit molecules and blocks of metals to be stable by virtue of the electrical forces acting among their component particles — it actually prescribes the amount of this stability, i.e., the amount of energy required to dissociate the molecule or vaporize the metal. For many molecules and a few metals the calculated values agree admirably with the observed ones. For many molecules and most metals the calculations are impossibly hard to make, but nevertheless the new way of visualizing the structure is superseding the old way, and the 'valence-bond' so familiar to students of chemistry as a line drawn between the symbols of two atoms, is changing into a group of two or more electrons rapidly moving about in the region between the atom-nuclei. Progress in this field may still continue, as chemists have so far been less diverted from their normal occupations than have physicists.