Current Problems in Physics

E. V. Shpolsky

I.

The rapid development that physics has undergone over the last fifty years has placed this science in an entirely exceptional position.

Modern physics, in its development, rests on an unusually powerful experimental base. Physical investigations are not infrequently carried out on a large technical scale and require for their execution not laboratory instruments, but large machines and entire structures weighing tens, hundreds, and even thousands of tons.

Matter is subjected to pressures of the order of 100,000 kg/cm², or is cooled to temperatures close to absolute zero, or placed in electric and magnetic fields of extremely high intensities. Vanishingly small light intensities are measured, corresponding to several tens of photons per cm² per sec., or electric currents in which one or two dozen electrons per second pass through the cross-section of a conductor, and so on.

Automatic registration and recording are widely used, ensuring complete objectivity and accelerating the pace of work to such an extent that, thanks to this alone, qualitatively new possibilities are already being created. Work in the field of cosmic rays is carried out by numerous expeditions equipped with a multitude of delicate instruments and installations, which, however, operate reliably under unusual and difficult expeditionary conditions.

Equally remarkable is the intensity with which new physical methods and theoretical constructions are being introduced into sciences adjacent to physics. Chemistry, and more recently biology as well, may serve as examples. But especially remarkable is the speed with which the achievements of physics are used in practice and give rise to whole new branches of technology. If formerly almost half a century passed between a physical discovery and its use in technology, then now, in

before the eyes of a single generation, from a laboratory experiment there grows a large technology with multimillion capital investments and with tens of thousands of people employed. At the same time, the development of technology also raises the physical experiment to a higher level, creating new possibilities for it. The best example may be the development of radio engineering, which emerged from physical research and, in turn, placed at the disposal of physicists entirely new experimental means, revolutionizing the whole technique of physical experiment.

Of course, in a short article written by one author there is no possibility of outlining, with any completeness, the problems confronting physics. One can only try to indicate some of the directions in which development must proceed in the coming years.

II.

The study of the atomic nucleus is the central problem of contemporary physics. The outstanding successes achieved in this field in recent times are well known. However, despite these successes, the study of nuclear phenomena is not only far from complete but, in essence, is still at the very beginning of its development. The nature of nuclear forces remains to this day an unsolved problem. There is no need to say much about the fact that the utilization of nuclear energy—this grandiose task of the technology of the future—is still only taking its first steps. There is reason to think that the coming years will bring a number of new major discoveries. Indeed, in recent times attention has been drawn to the intensive development of new types of accelerators and the improvement of old ones. What is at issue is the transition from particles with energies of millions (or tens of millions) of electron-volts to energies of hundreds of millions of electron-volts. It is probable that in the near future particles with energies of half a billion electron-volts will be obtained. When this new “atomic artillery” is put into operation, it will undoubtedly reveal a multitude of new facts which, in particular, will also facilitate the further advancement of theory. In this respect the present moment in the development of nuclear physics recalls the time between Rutherford’s first work on the transmutation of elements (1919 and the following years) and the beginning of the rapid development of artificial nuclear reactions; this, too, was a period of searching for new experimental means, ending with the discovery of the cyclotron and its subsequent fruitful applications.

The great successes achieved in the separation of stable isotopes are laying the foundation for a new chemistry and physics of pure isotopes.

In chemistry, the synthesis of a number of organic compounds has already been accomplished with ordinary atoms replaced by their rarer isotopes (\(\mathrm{N}^{15}\) instead of \(\mathrm{N}^{14}\), deuterium instead of hydrogen). In physics one may point, as a curious example, to the obtaining of the pure isotope of mercury \(\mathrm{Hg}^{198}\) (pu-

CURRENT PROBLEMS OF PHYSICS

by means of a nuclear reaction of neutrons with gold \( \mathrm{Au}^{197} \) and the subsequent transformation of the isotope \( \mathrm{Au}^{198} \) thereby obtained into \( \mathrm{Hg}^{198} \)) in a quantity sufficient to fill a discharge tube to a pressure of the order of \(10^{-3}\) mm Hg. The spectrum of the pure isotope \( \mathrm{Hg}^{198} \), obtained when a current is passed through this tube, provides a length standard which, in its qualities, considerably surpasses—owing to the complete absence of hyperfine structure and to the very small Doppler broadening, in view of the considerable mass of the carrier—the usual standard, the red cadmium line.

The possibility of obtaining radioactive isotopes, sometimes in appreciable quantities (\(\mathrm{C}^{14}\), \(\mathrm{Na}^{24}\)), opens up a great variety of paths for the development of applied nuclear physics. In chemistry, the method of radioactive tracers makes it possible to study the mechanism of exchange reactions, to determine vapor pressure or the solubility of substances that are difficult to investigate by other means. The “synthesis” of new elements 43, 85, 93, 94, 95, 96, which fill empty places in the periodic system or extend it, and the investigation of the chemical properties of these new elements, which in particular led to a clarification of the structure of the periodic system itself, constitute a most important contribution of nuclear physics to the development of chemistry. In metallurgy, the use of the same radioactive tracers for studying diffusion processes in metals opens up an enormous number of new possibilities.

Biological studies of metabolism in plants and animals with radioactive tracers and with stable isotopes as “tagged” atoms have become widely known. Finally, the new means and methods of nuclear physics are finding application in medicine. This entire new field of the broadest applications of nuclear physics must receive intensive development in the coming years.

Closely connected with the study of the atomic nucleus is the problem of cosmic rays—this remarkable phenomenon, in which processes are observed that have not yet been realized in the laboratory. In cosmic rays particles with energies up to \(10^{10}\) electron-volts are observed; the velocity of electrons possessing such energy already amounts to 0.99999999870 of the speed of light. There is no doubt that the investigation of cosmic rays, apart from the extraordinary attractiveness of this problem itself, has already played and will continue to play a major role in the development of our ideas in the field of the atomic nucleus. It is enough to recall that two elementary particles—positrons and mesons—were discovered in cosmic rays, and to recall the role played by mesons in modern theories of nuclear forces.

III.

The study of the nature and properties of solids is a problem whose significance for the knowledge of the world around us and for the development of technology is difficult to estimate. Since the discovery of X-ray structural-

analysis, which has placed in the hands of researchers a powerful method for studying the internal structure of solids—especially crystalline solids—this field has made rapid progress.

Substantial advances have been achieved in understanding processes of great importance for technology, such as, for example, plastic deformation or phase transformations in metals. Nevertheless, the number of problems still to be solved is very great here as well.

In the field of studying the mechanical properties of metals, for example, further study of plasticity and strength is coming to the fore; in particular, the study of the nature of plastic deformation and the fracture of materials, and investigation of the properties of imperfect elasticity in metals (elastic after-effect and hysteresis), which have so far been studied very little, and so on.

Among the problems of solid-state physics, the problems of amorphous bodies—simple and high-molecular—have acquired special interest in recent times. Amorphous bodies are plastics and elastic materials, glasses, enamels, resins, etc., materials that in recent times have found exceptionally wide application in technology. The investigation of the physical properties of these extremely interesting and, from the scientific point of view, distinctive kinds of matter, which developed successfully in the USSR in previous years, will be continued in the coming five-year period as well. Researchers face the absorbing task of developing the theoretical foundations for obtaining materials with prescribed properties.

Quantum theory has provided a general picture of the mechanism by which electricity passes through conductors, semiconductors, and dielectrics. Semiconductors have attracted especially great attention in recent years. From the theoretical point of view, the investigation of the properties of semiconductors, which occupy an intermediate position between metals and dielectrics, is extremely interesting, since this investigation at the same time illuminates the nature of metals and dielectrics. The practical importance of semiconductor research is connected above all with the fact that the majority of bodies in nature belong to this class: a very large number of compounds of metals with metalloids are semiconductors and, what is especially interesting, so are some alloys of metals. Further, as is known, semiconductors are used in modern rectifiers, barrier-layer photoelements, and thermoelements. The remarkable properties of semiconductor photoelements, which possess enormous sensitivity—covering, in different photoelements, the region from the middle ultraviolet (300 mμ) to the infrared—are of interest not only from the point of view of measurement technology, but also offer hope for the possibility of using them for the direct conversion of light energy into electrical energy. At the present time the efficiency of this conversion is still very low. Precisely for this reason, further work on improving the properties of semiconductor photoelements, and in particular on increasing their efficiency, is very substantial. Approximately

the same may also be said of semiconductor thermoelements, which make it possible to convert thermal energy directly into electrical energy: their efficiency is still low, and work to increase it is a further task.

The development of high-voltage technology, both for the purposes of transmitting energy over long distances and for modern high-voltage scientific installations, requires work on the study of the properties of dielectrics. In modern high-voltage technology, not only solid and liquid bodies, but also compressed gases, have recently found application as insulators. The search for gases possessing increased electrical strength, the clarification of the possibilities of using gases with high electrical strength in high-voltage technology—are an urgent task. Equally urgent, of course, is the further study of the electrical strength of solids. From the theoretical point of view, it is important to study the connection between the structure of a substance and its electrical strength. This study should in the future help in finding substances possessing the necessary dielectric properties on the basis of a clear picture of the mechanism of the process, and not by empirical selection.

Recently, attention has again been attracted by a class of substances called ferroelectrics. These substances behave in electric fields in a manner quite analogous to the way ferromagnetics behave in magnetic fields. Ferroelectrics, in particular the recently studied barium titanate, possess enormous values of dielectric permittivity. At the same time, as in ferromagnetics, they exhibit a dependence of dielectric permittivity on field strength and hysteresis. The high values of dielectric permittivity in ferroelectrics open up possibilities for their use as electrical insulating materials, in view of which the further study of these peculiar bodies is of great practical interest.

Up to now the discussion has concerned solid bodies—polycrystalline or amorphous. The study of single crystals has always been and remains one of the most important tasks of solid-state physics. Among the problems facing researchers in this field, one should first of all mention the further application of X-ray and electron-diffraction analysis to the establishment of the structure of particular crystals. Further, the study of physical properties—piezoelectric, mechanical, optical—is important not only from the scientific, but also from the practical point of view. In this latter respect it is sufficient to recall the varied applications of piezoelectric elements and of various crystal-optical devices.

The investigation of the properties of matter at temperatures close to absolute zero has outstanding fundamental significance. Under these exceptional conditions the quantum properties of matter manifest themselves: superconductivity and superfluidity. The further development of the successes,

of the results already achieved in this direction, will surely yield many valuable results that deepen our knowledge of the nature of matter.

Until recently, liquids were usually brought closer to gases. This view naturally followed from van der Waals’ theory of the continuous transition between the liquid and gaseous states. Undoubtedly, at high temperatures—especially at temperatures close to the critical ones—and at low pressures, such a rapprochement of a liquid with a gas fully corresponds to reality. On the other hand, the profound difference between solids and liquids seemed all the more obvious because the characteristic feature of a liquid is its fluidity, whereas solids are characterized by high values of shear elasticity.

However, it has long been known that precisely this characteristic difference is by no means qualitative, but only quantitative, since fluidity is to some degree inherent in solids as well, while liquids possess a definite shearing stress. Therefore, bringing the liquid state closer to the solid is just as legitimate as bringing a liquid closer to a gas. In fact, the liquid state, as an intermediate state, under various conditions must reveal all stages of transition from the properties of a solid to the properties of a gas. If, at temperatures close to the critical ones, a liquid should be similar to a gas compressed to a small volume, then at temperatures close to the crystallization temperature a liquid should display features of similarity to a solid.

Recent experimental data, especially those discovered with the aid of X-ray analysis of the so-called “sibotactic regions” in a liquid, i.e. essentially randomly distributed submicroscopic regions preserving a crystalline structure, emphasize the closeness between the liquid and solid states. The idea of the closeness of liquids to solids, expressed already by Maxwell and long consigned to oblivion, has in recent years become widely accepted and has proved extremely fruitful both for theoretical constructions and for new experimental work. As always happens, the revival of any idea at a new theoretical and experimental level immediately opens up great possibilities that were formerly difficult to foresee. Therefore, the study of the liquid state is one of the attractive, though difficult, problems of modern physics.

IV.

In the field of physical optics, the study of photoluminescence has acquired great importance in recent years. This beautiful phenomenon has long attracted the attention of physicists, but until recently this interest was purely theoretical. Twenty-five years ago it was still difficult to imagine that photoluminescence could ac—

acquire practical importance. However, in recent years the applications of luminescent materials have attained rare breadth. Crystalline phosphors have found use in new luminescent lamps, which in the coming years will probably displace incandescent lamps. Luminescent screens in cathode oscillographs, television devices, electron microscopes, and image converters, X-ray intensifying screens, and screens for fluoroscopy are likewise made from crystalline phosphors. Finally, fluorescence underlies a rapidly developing, highly sensitive method of analysis, with the broadest applications—from problems of qualitative and quantitative analysis in inorganic, organic, and biological chemistry to criminology.

All these diverse applications of luminescence pose before physicists an enormous number of problems, both practical and purely scientific in character. Alongside this practical aspect, the phenomena of luminescence continue to attract the attention of researchers as a path toward solving problems concerning the structure of molecules, the redistribution of absorbed energy in molecules, and the exchange of energy between excited and unexcited molecules. This latter direction in the study of fluorescence is of great importance for disciplines adjacent to physics, especially for photochemistry.

Another path for investigating the structure of matter from the standpoint of physical optics is opened by the study of anomalous dispersion of light. This method, which has reached a high degree of perfection thanks to the classical works of D. S. Rozhdestvenskii, is a powerful means of studying the interaction of light and matter. It provides important information that permits one to judge the properties and behavior of individual atoms and molecules. The development and application of studies of anomalous dispersion and absorption of light are therefore among the important tasks confronting our physicists.

A third path for studying the properties and structure of matter by means of optical phenomena is opened by the investigation of the scattering of light. As is well known, the study of combination scattering has provided enormous material concerning the structure of the molecules of the most diverse chemical compounds. Many hundreds of organic and inorganic compounds have been investigated in this way. The study of the so-called “wings” of Rayleigh scattering has led to the difficult problem of studying the nature of the liquid state of matter. This problem may also be solved by the combined study of molecular scattering (classical and combination), infrared absorption, and ultrasonic vibrations. Finally, the investigation of double refraction in a high-frequency electric field also yields valuable data on the nature of the liquid state.

Optical methods for analyzing the composition of matter have acquired exceptionally wide application. These methods are characterized by high sensitivity, accuracy, and speed, placing them beyond competi-

...interference by the usual methods of analysis. There are frequent cases when optical methods make it possible, within a few minutes, to carry out an analysis which, by ordinary chemical methods, requires days and weeks, and is sometimes altogether impossible. Among these methods, the method of emission spectral analysis has so far become most widespread; it has entered extensively into the practice of mass analyses in scientific and factory laboratories. Alongside emission analysis, absorption analysis is acquiring ever greater importance; it is successfully applied to the analysis of multicomponent mixtures of hydrocarbons by their infrared absorption spectra and to the analysis of biologically important substances (vitamins, hormones) by their ultraviolet spectra. Luminescence analysis has already been mentioned above. The development of these methods raises numerous complex problems connected both with the investigation of spectra and with the design of apparatus, the use of automatic recording, the development of light sources, and so on.

V.

Objective methods of optical analysis owe their success to the development of electronic devices—photoelectric cells and amplifier tubes. Of course, however, this is only one, and perhaps a small, aspect of the gigantic significance which electronic devices have acquired in modern science and technology. Radiotelegraphy and telephony, radar, television, and electron microscopy have achieved their truly fabulous successes precisely thanks to the improvement of electronic devices and the development of electron optics. The role is therefore clear of studying those physical phenomena on which these devices are based—the photoelectric effect, thermionic emission, and field emission.

Electromagnetic oscillations and waves in the radio-frequency range confront physics with a large number of interesting and important problems. As is known, in recent times electromagnetic oscillations of ultrahigh frequency (decimeter, centimeter, and millimeter waves) have acquired special significance. The development of radar and radiogeodesy, television, and various practical and scientific applications of radio connected with the use of ultrahigh-frequency waves put this field forward as one of the most urgent. In particular, the problem of generating powerful undamped oscillations in the centimeter–millimeter range and even of shorter waves, the problem of radio-wave propagation, closely connected with the problem of the ionosphere, and increasing the accuracy of measuring the propagation velocity of radio waves, which is of fundamental importance for radiogeodesy—these are a brief and far from complete list of the tasks facing this field.

Closely connected with the problems of radiophysics is the theory of nonlinear oscillations, which has very general and broad significance and which was developed with such success by the school of the late L. I. Mandelstam. To what extent

are broad and varied. From the applications of this theory one may judge that among its immediate tasks requiring solution, alongside questions of the generation and reception of ultra-high frequencies, are the problems of applying the theory of nonlinear oscillations to the dynamics of machines (electrical and steam), to the motion of relativistic particles in connection with the development of accelerators that make it possible to obtain particles with energies of the order of hundreds of millions of electron-volts (the betatron, etc.). Finally, the methods of this theory can be successfully applied to clarifying the nature of the forces acting at small distances (of the order of \(10^{-6}\) cm) from the free surface of solid or liquid bodies, i.e., to problems connected with the nature of mechanical friction, electrical contact, and so on.

Modern acoustics has grown into a large field, interesting and important from both the scientific and the practical points of view. The introduction into acoustical research of new experimental methods connected with the development of radio apparatus, on the one hand, and the improvement of the classical mathematical methods used in acoustics, on the other, have given a powerful impetus to its development. The problems facing physicists here are extremely broad. The transmission and reproduction of human speech and music (telephony, radio broadcasting) lead to investigations that are also of fundamental importance for the physiology of hearing and speech; the propagation of sound in water is of great military importance (underwater sound communication and reconnaissance); finally, architectural acoustics is of special importance in connection with the planned five-year plan and the enormous scale of construction—residential and industrial. A large number of acoustical problems is also posed by the construction of large public buildings (concert halls, auditoriums) and, in particular, by the construction of the Palace of Soviets.

VI.

Alongside the concrete problems briefly outlined above, there are, of course, general theoretical problems still awaiting solution. The task of constructing relativistic quantum mechanics still attracts the attention of theorists; serious difficulties in quantum electrodynamics have still not been overcome.

A characteristic feature of twentieth-century physics is that its theories have raised a number of important philosophical problems. The theory of relativity raised the problem of space and time; quantum mechanics—the problem of causality. Soviet physicists and philosophers face the important task of generalizing the theoretical results of modern physics on the basis of dialectical materialism, in opposition to the idealistic interpretations which still quite often appear in foreign literature.

In his speech at the meeting of voters on February 9, 1946, Comrade Stalin set forth the task of building scientific-research in-

institutes and the development of science as one of the three most important state tasks of our country. “I have no doubt,” said Comrade Stalin, “that if we give our scientists the necessary assistance, they will be able not only to catch up with, but also in the near future to surpass the achievements of science beyond the borders of our country.” This historic assignment of Comrade Stalin places special responsibility upon Soviet physicists, in view of the role that physics has acquired in the development of technology and in the defense of the country. Soviet physicists must exert all their strength in order to fulfill, in good time and with honor, the enormous tasks set by the Party and the Government before the scientists of our country.