Forty Years of Soviet Physics

E. V. Shpolsky

“The scientific sowing will sprout for the harvest of the people.”

D. I. Mendeleev

I. Physics in Russia before the Revolution and the Beginning of the Organization of Soviet Physics

The Great October Socialist Revolution opened a new epoch in the development of physics in Russia. On the memorable days of the fortieth anniversary of the world-historic date of October 1917, it is interesting and instructive to compare, against the background of the development of world science, the scale and general character of pre-revolutionary physics in Russia with the physics of the Soviet Union in our own day.

Approximately sixty years ago there began in physics that profound revolution which unfolded in the following decade and which determined the direction and character of the development of this science for many years, perhaps for centuries to come. Let us recall the basic facts and the most important defining features of this revolution. In 1895 X-rays were discovered, and in 1896—radioactivity. Although the existence of the electron had already been foreseen in the mid-seventies and in the eighties of the nineteenth century, exhaustive proof of its reality and the determination of its charge and mass were given in 1898—an event for that time probably no less staggering than the discovery of X-rays and radioactivity. Indeed, by this the existence of an “atom of negative electricity” was experimentally proved, with a mass almost 2,000 times smaller than the mass of the lightest of the elementary particles then known—the hydrogen atom. The study of Brownian motion at the beginning of the twentieth century gave the concept of atoms and molecules of matter a reality just as “tangible” as the reality of the macroscopic objects around us. Finally, the discovery of the interference of X-rays in 1912 confirmed from a new side, in the most vivid manner, the reality of atoms and opened the way to the study of the structure of crystals. At the same time, the investigation of the structure of the atom was begun—at first in the form of a static model, abandoned after Rutherford’s discovery in 1912 of the atomic nucleus.

On the basis of all these and many other remarkable experimental discoveries, at the turn of the century and in the first years of the twentieth century there arose those most important physical theories which determine the entire character of physics in our day and for many years to come. These are precisely quantum theory, the theory of relativity, and physical statistics.

Let us now consider what was happening in Russia in that remarkable epoch in the history of physics to which, to an even greater degree than to the well-known literary movement, the name of a period of “storm and stress” in physics may be applied. Let us note, first of all, that in the sciences contiguous with physics—in chemistry and mathematics—in the nineteenth and early twentieth centuries discoveries were made in Russia which, in their significance and in their transformative, revolutionary influence, extended far beyond the bounds of their own sciences and were in no way inferior to the discoveries listed above in the field of physics. It is enough to recall Mendeleev’s discovery of the periodic system of the elements, Butlerov’s discovery of the structure of organic compounds, and Zinin’s organic syntheses—in chemistry; and, in mathematics, the most important works in mathematical analysis by Chebyshev, Lyapunov, and Steklov, not to mention Lobachevsky’s brilliant discovery of geometry.

The position of physics in tsarist Russia was far less favorable. Of course, we can name a number of outstanding physicists of whom we are proud. These—to limit ourselves, in keeping with what has preceded, to the end of the nineteenth and the beginning of the twentieth century—are the names of Stoletov, Lenz, Umov, Golitsyn, Lebedev, and Eichenwald. However, with the exception of Lebedev, who by heroic efforts created in Moscow the first Russian school of physicists, the others were solitary scholars. All the greater, then, must be our respect for their outstanding work. The fact of the extremely unfavorable position of physics in pre-revolutionary Russia, due to the tsarist government’s failure to understand the role of science in the fate of the state and to the weak development of technology and industry, which to a considerable extent was in the hands of foreign capitalists, is emphasized with extraordinary vividness in the quotation given below from an article by one of the outstanding physicists of that time—N. A. Umov. In the article “The Physical Institute of Moscow University”*), arguing for the importance of creative, and not merely instructional, activity at the University, Umov wrote in 1898: “The political significance of a nation can be secure on the condition that its cultural level corresponds to its political rise. In our time, arms and courage are not the only factors ensuring success in the struggle of nations for their development and existence.” Having noted that the most important characteristic of a country’s cultural level is its “contributions to the realm of knowledge,” Umov goes on to write: “If we turn to our country, we realize that, unfortunately, up to now we have for the most part been taking over and borrowing, and have been contributing very little to the cultural life of mankind. And if we look at what has been done among us for the development of knowledge, it will become clear to us that efforts have been directed toward making us study, and people were satisfied if we studied well... Museum, cabinet—these are terms characterizing the old view: study; institute—this is the new view: study and create.”

And here is testimony relating already to the beginning of the twentieth century, from the still living outstanding Soviet physicist Acad. A. F. Ioffe: “When I began working in Petersburg (this was in 1906),” writes A. F. Ioffe, “the traditions of the nineteenth century, and even rather of its middle, of the school of F. F. Petrushevsky, were still strong there. The teaching of physics in higher education proceeded along the line of so-called measuring physics—the methods of measurement as the basis of exact knowledge.

In all the higher schools of St. Petersburg, the first course was devoted to the description of measuring instruments, and only from the second course were the laws from the fields of heat, electricity, magnetism, optics, and acoustics set forth.

*) N. A. Umov, Collected Works, vol. III, p. 142, Moscow, 1916.

Theoretical—or, more precisely, mathematical—physics in the universities was reduced to the phenomenological formulation of laws and the solution of partial differential equations from the fields of heat conduction and electrostatics.

Professors and instructors at institutions of higher learning possessed extensive erudition, but paid little attention to creative activity. The scientific works of those who remained at the university often amounted to a repetition of published works*).

However, it was precisely then that a group of brilliant young physicists appeared in Petersburg, a group destined to play an important role in the organization of Soviet physics. D. S. Rozhdestvenskii was at that time carrying out, entirely independently, his classical work on anomalous dispersion in sodium vapors at Petersburg University. A. F. Ioffe, as his master’s dissertation, presented a brilliant work on the “Elementary Photoelectric Effect,” in which, by direct experiment on metallic dust particles suspended in Millikan’s condenser, he demonstrated the electronic nature of the photoelectric effect from metals and the statistical independence of elementary acts of the photoelectric effect. Thus, along with Millikan’s work, experimental proof was given of the existence of electrons and light quanta.

To this same brilliant band of young physicists belonged D. A. Rozhanskii, whose dissertation (an investigation of the spark) attracted general attention by the freshness of its physical ideas**).

Finally, a great influence was exerted by the outstanding representative of theoretical physics P. S. Ehrenfest, then working in St. Petersburg***), who subsequently occupied the chair of H. A. Lorentz in Leiden (obviously not only because of the honor associated with occupying a chair whose activity had attracted the attention of the whole world, but also because promotion under the conditions of tsarist Russia was hopeless for P. S. Ehrenfest).

Such was the physics of St. Petersburg. More favorable conditions at the beginning of the century arose in Moscow, where, thanks to the activity of one of the most remarkable Russian physicists, P. N. Lebedev, the first Russian school of physicists was created, and where, alongside Lebedev, worked his childhood friend, the brilliant physicist and teacher A. A. Eikhenvald. Already in 1900, in his classical works, Lebedev gave impeccable proof of the existence of light pressure on solid bodies, and in 1908 he reported to the Congress of Russian Natural Scientists and Physicians his record-breaking, in terms of difficulty, work on the pressure of light on gases. Eikhenvald completed his important works proving the existence of the magnetic field of moving charges and displacement currents. His lectures and public reports, brilliant in both pedagogical and oratorical form, which struck the imagination of listeners by the brilliance and effectiveness of their demonstrations, attracted large audiences of young people and greatly increased interest in physics. But even so, the conditions under which this important work was carried out were extremely unfavorable. For Lebedev’s laboratory in the building of the Physics Institute of Moscow University, space was found only in the basement. What we now call the “technical base” of a laboratory was in fact absent, since this “base” was represented by one small (though precise) lathe and one mechanic (Aleksei Ivanovich Akulov, who helped Lebedev greatly in his

) A. F. Ioffe, Soviet Physicists and Pre-Revolutionary Physics in Russia, UFN 33*, 454, 1947.

**) A. F. Ioffe, cited article, p. 465.

) See the articles by G. Jollenbeck and A. F. Ioffe, UFN *62, 367 (1957).

difficult experimental work). Therefore each of those who worked in Lebedev’s laboratory was obliged to make with his own hands all the necessary parts of his apparatus; and in order that the collaborators (“practicants,” as they were then called) should be prepared for this, they first had to pass through a “course of apprenticeship” in the private workshop of P. I. Gromov and pass an “examination” in the form of making some simple instrument. Incidentally, these practicants held no regular staff positions and received no salary: most of them devoted their leisure hours to scientific work, earning their living by teaching in secondary schools or, as an exception, in higher educational institutions. Yet even these modest working conditions were denied Lebedev when, in 1911, together with all the progressive professors, he considered it his civic duty to leave the University as a protest against the repressions against the University undertaken by the reactionary minister of education.

One small fact, preserved for posterity by Lebedev’s pupil T. P. Kravets*), vividly shows the full depth of the misunderstanding, on the part of representatives of the big Russian bourgeoisie, of the needs of science and of the role of physics in the development of industry. After Lebedev’s departure from the University, his pupils, concerned with creating for Lebedev the possibility of continuing his research work, appealed for help to one of the most prominent representatives of the Moscow bourgeoisie. The latter, after graciously hearing out the young scientists, promised to think it over. The next day he telephoned Lebedev and informed him that he would be given the right to work in the physics room of the so-called Practical Academy—a secondary educational institution maintained by the funds of the Moscow merchant class. In this school physics room it was proposed that a scientist with a world reputation carry on scientific work after completing experiments, record-breaking in their difficulty, on the light pressure on gases!

However, apart from this event so tragic for Russian science and culture—the rout of the oldest Russian University by the reactionary tsarist government—even then, in 1911, the need was clearly felt for the creation of scientific-research institutes not connected with teaching tasks and provided with a sufficiently serious technical base. Outstanding scientists—P. N. Lebedev, K. A. Timiryazev, N. A. Umov, M. A. Menzbir (zoologist)—appeared in print with articles proving the necessity of creating, alongside the universities, research institutes—national laboratories, as Lebedev called them.

The tsarist government remained deaf to these appeals. Nevertheless, the public movement that had arisen did not pass in vain. Scientists succeeded in convincing representatives of the big Russian bourgeoisie of the necessity of developing physics in Russia, and with the private donations collected, as a matter of public initiative, the “Moscow Society of the Scientific Institute” was created; it set itself the task of assisting in the organization of “national research institutes,” first and foremost a Physics Institute. Unfortunately, P. N. Lebedev did not live to see the realization of his dream, since he died at the age of 46, one year after leaving the University.

His work was continued in Moscow by his pupil and closest assistant P. P. Lazarev, around whom there gathered a small group of young people, among them S. I. Vavilov, B. V. Il’in, P. N. Belikov, S. N. Rzhevkin, N. T. Fedorov, A. S. Predvoditelev, A. G. Kalashnikov, E. V. Shpolsky,

*) T. P. Kravets, “The Creative Path of P. P. Lazarev,” UFN 27, 17 (1945).

T. K. Molodyi, G. S. Landsberg, V. V. Shuleikin, and others. Another group of physicists was concentrated around the Moscow Physical Society (A. A. Eikhenvald, Yu. V. Wulf, V. K. Arkad'ev, A. K. Timiryazev, A. B. Mlodzeevskii, S. A. Boguslavskii, A. I. Bachinskii, and others). In Petrograd, just before the revolution, two groups of young scientific workers had taken shape: one, connected with A. F. Ioffe’s seminar and including P. L. Kapitsa, N. N. Semenov, Ya. I. Frenkel, P. I. Lukirskii; and a second group of enthusiasts in scientific and applied optics headed by D. S. Rozhdestvenskii (A. I. Tudorovskii, A. L. Gershun senior, I. V. Grebenshchikov, and others). These groups of Moscow and Leningrad physicists formed the centers around which the major Soviet institutes arose and grew, institutes created in the first years of the revolution.

The organization of scientific research work on a large scale was undertaken by the Soviet government from the first days of the revolution. While supporting the already established university scientific centers and developing with extraordinary intensity a network of new higher educational institutions, the Soviet government at the same time focused attention on developing a network of large scientific research institutes in the most diverse specialties. This intensive development of scientific research institutes followed logically from the principle steadily pursued by the Soviet government, by virtue of which scientific research work was made the basis of the country’s industrial development, the development of its agriculture, public health, and the development of its culture. The dream of the outstanding Russian scientists of the late nineteenth and early twentieth centuries was realized precisely by the Communist Party and the Soviet government. Science was recognized as a necessary element of state construction.

As for physics, the beginning of the planned creation of scientific institutes dates back to 1918. In that harsh time, at the height of the struggle against counterrevolution and intervention, amid the numerous economic difficulties that the revolution inherited from the First World War and the tsarist government, the largest institutes were created, which quickly raised science in the USSR to a new, higher level. The pioneers in this historically important undertaking were the outstanding Soviet scientists P. P. Lazarev, A. F. Ioffe, and D. S. Rozhdestvenskii. The first of them, P. P. Lazarev, organized in Moscow the Institute of Physics and Biophysics, which included in its program a wide range of problems in physics, biophysics, and geophysics. A. F. Ioffe and D. S. Rozhdestvenskii created in Leningrad the Physico-Technical and Optical Institutes. Their first staff members were young scientists who at that time had grouped themselves around their leaders. Subsequently, many of them themselves created their own scientific schools, which by the present time already comprise two generations of scientists. In December 1918, the First Congress of Russian Physicists met in Leningrad; it played the role of the founding congress of Soviet physics. Leningrad at that time was experiencing difficult days because of various economic hardships. But the enthusiasm of the comparatively small group of scientists of different ages who had gathered, and who ardently discussed the scientific problems to be solved, was so great that it made the participants completely forget these everyday difficulties.

In the following years, a number of major physics institutes were organized in Moscow, Leningrad, and other cities of the USSR. Such was the P. N. Lebedev Physical Institute of the Academy of Sciences of the USSR, originally founded on the basis of a small physics laboratory of the Academy of Sciences, but, beginning in 1934, with the Academy’s move to Moscow, thanks to the energy

and, through the initiative of its director S. I. Vavilov, turned into a major scientific center*). Let us also note the Institute for Physical Problems of the Academy of Sciences of the USSR, founded by P. L. Kapitsa, which has become famous for its remarkable work in the field of low-temperature physics, theoretical physics, and other areas. Ukraine has a number of large institutes, including the Physico-Technical Institute in Kharkov and the Physics Institute of the Academy of Sciences of the Ukrainian SSR in Kiev. In Tomsk the Siberian Physico-Technical Institute is being created; in Sverdlovsk, the Institute of Metal Physics. In Belorussia, Georgia, Armenia, Kazakhstan, and other national republics, scientific centers in the field of physics are being established. It is not the task of this article to survey all the institutes and scientific-research physics laboratories organized after the Revolution and carrying on intensive activity at a high modern scientific and technical level. The outstanding activity of the President of the Academy of Sciences of the USSR, S. I. Vavilov, played a major role in the development and strengthening of the physics institutes of the Academy of Sciences of the USSR, its branches, expeditions, and bases.

It is especially necessary to note the growth that nuclear physics has undergone in the Soviet Union in the postwar years, in connection with the exceptional importance of work in this field. As is known, work in nuclear physics places especially high demands both on the creative activity of scientists and on the scientific and technical level of institutes and laboratories, which in turn makes unprecedentedly high demands on the general level of technology in the country. In the Soviet Union the organization of work in this field was undertaken on a broad scale. A number of laboratories equipped according to the latest achievements of technology were established. Among them are powerful institutes: the Institute of Atomic Energy and the Institute of Nuclear Problems of the Academy of Sciences of the USSR. On the basis of the latter institute and the Electrophysical Laboratory of the Academy of Sciences of the USSR, in 1956 the international “Joint Institute for Nuclear Research” was organized, of which 12 states are members.

In the very latest years, in view of the sharply increasing role of semiconductors in physics and technology, a special Institute of Semiconductors of the Academy of Sciences of the USSR has been created, headed by A. F. Ioffe.

Alongside the creation of institutes specially engaged in scientific research work, the old university centers were also considerably expanded; among them the largest is the Physics Institute of Moscow University, which played a most important role in the development of physics in the USSR, mainly thanks to the work of L. I. Mandelstam’s school in the field of physical optics and the theory of oscillations, and which in recent years, in its new building on the Lenin Hills, has acquired scientific laboratories unique in their wealth. Much work has also been carried out within the walls of the Physics Institute of Leningrad University, and at Odessa, Kiev, Tomsk, and a number of other universities.

II. SURVEY OF THE WORKS OF SOVIET PHYSICISTS

In what follows we shall attempt to give a survey of the most important works carried out by Soviet physicists over the past forty years. It goes without saying that to give in a single article, written by one author, anything like a complete and even survey of the development of all branches of physics is unthinkable. We have tried to note the most important achievements and the crea-

) For the history of this institute see the article by S. I. Vavilov: “The Physics Cabinet—the Physics Laboratory—the Physics Institute over 220 Years,” UFN*, vol. 28, no. 1, 1946. See also the article by D. V. Skobeltsyn and I. M. Frank in this issue of the journal, p. 503 ff.

...we have deliberately limited ourselves to the principal branches of physics, declining to consider borderline areas such as geophysics, astrophysics, biophysics, chemical physics (including photochemistry, closely connected with physical optics), and also problems of molecular physics that belong mainly to physical chemistry.

Theoretical Physics

Theoretical physics as an independent part of physics arose as a consequence of the extraordinary development of physics in the twentieth century. In earlier times every physicist was both a theorist and an experimenter. An example among scientists of the nineteenth and early twentieth centuries is J. J. Thomson, who began his career as a mathematician, continued Maxwell’s work on the theory of the electromagnetic field, but alongside this experimentally proved the reality of the electron, built the first mass spectrograph (the parabola method), and experimentally discovered the isotopes of nonradioactive elements. L. Boltzmann’s profound theoretical works are widely known, but perhaps not everyone knows that he also carried out delicate experimental work on the dielectric constants of gases. In Russia, all physicists—A. G. Stoletov, N. A. Umov, N. N. Schiller—were at the same time theorists and experimenters; their activity belonged either wholly or mainly to the nineteenth century. Gradually, however, both mathematical and experimental methods became so complicated that physicists had to solve a peculiar problem of “choosing a profession”—whether to become a theorist or an experimenter.

Since the object of a theoretical physicist’s work may be any physical problem, the concept of “theoretical physics,” strictly speaking, embraces all of physics. In our survey we place in a special heading “theoretical physics” those works that concern the most general problems and theories of modern physics: the theory of relativity, quantum mechanics, and statistics. Alongside this, references to the work of theorists will be given in practically all the subsequent headings. It hardly needs to be said that in a survey such as ours, covering all of physics, profound and difficult works in theoretical physics can be characterized only very briefly and superficially—almost by title alone.

Before the Revolution in Russia, theoretical physics, in the sense in which we use the term here, was weakly represented. This was despite the fact that by the beginning of the Revolution the theory of relativity, special and general, Planck’s quantum theory and Bohr’s atom with its problems of quantization of various systems, and, finally, classical statistics already existed. However, under the new conditions of scientific work created by the Soviet government, in the newly organized research institutes young theorists already appeared at that time—Ya. I. Frenkel, Yu. A. Krutkov, V. A. Fock, A. A. Friedmann, and several others—who formed the nucleus for the subsequent intensive quantitative and qualitative development of theoretical physics in the USSR.

Among works on the most general problems of theoretical physics, one should first of all mention the widely known work of A. A. Friedmann on the general theory of relativity. In this work Friedmann showed that, alongside the stationary solutions of the general theory of relativity that underlay Einstein’s relativistic cosmology, there also exists a nonstationary solution compatible with a change in the radius of curvature of world space with the passage of time. It is interesting to note that

Einstein initially disputed the results of Friedmann’s work, but later agreed with them. This work served as the basis for a new direction in relativistic cosmology.

Another fundamental work on the general theory of relativity and the theory of gravitation was the work of V. A. Fock, devoted to an approximate solution of the \(n\)-body problem in Einstein’s theory of gravitation. Without touching on the mathematical side of the work, one may note as its important general result the proof that, in a system of material points, from the equations of the theory of gravitation there follows not only Newton’s law of mutual attraction between masses, but also Newton’s law of motion of each of the bodies of the system under the influence of the others.

In quantum theory, after the success of quantizing the generalized model of the hydrogen-like atom (elliptic orbits), the problem arose of finding general principles for the quantization of systems with many degrees of freedom. This problem was solved with the aid of Ehrenfest’s hypothesis of “adiabatic invariants.” This theory received its fullest and deepest development in the extensive work of Yu. A. Krutkov, published as the monograph Adiabatic Invariants and Their Application in Theoretical Physics.

Modern quantum mechanics, or, as it was called at first, the new quantum mechanics (meaning by the old the theory of Bohr—Sommerfeld), was created in the mid-twenties in a rapidly unfolding sequence of works by L. de Broglie, E. Schrödinger, W. Heisenberg, and P. A. M. Dirac. From the moment the basic ideas and equations of quantum mechanics were formulated, Soviet theorists took the most active part in the development of approximate methods for solving the Schrödinger equation and in the applications of the methods of quantum mechanics to the solution of a great variety of particular problems. In this section, as has been said, we dwell only on the results of works of the most general character.

Among such works are those of V. A. Fock, devoted to the development of an approximate method for solving the Schrödinger equation for many bodies. As is known, the Schrödinger equation in principle makes it possible to solve any problem of quantum physics. However, for a many-body system the problem quickly becomes so complex that an exact solution is practically impossible. Yet there is no need for an absolutely exact solution of such problems, since the results of measurements always have limited accuracy. Therefore good approximate methods are of the utmost importance. In quantum mechanics such a reliable method is the so-called Hartree—Fock method, first proposed by Hartree but so radically improved by V. A. Fock that it is now often called simply Fock’s method. The basis of this method is the model of the so-called “self-consistent field,” in which, for example, in solving the problem of a many-electron atom, the motion of each electron is considered as motion in the field of the nucleus plus the averaged field created by the remaining electrons. The fundamental improvement introduced by V. A. Fock leads to equations for the wave functions of the individual electrons which contain, in addition to the terms entering the former equations, also terms corresponding to the interaction between electrons, namely, the exchange interaction. Solving these equations makes it possible, in the case of an atom, to calculate the energy levels and intensities of spectral lines. With the aid of this method, V. A. Fock and M. A. Petrashen, for example, solved the problem of the sodium atom; the value of the principal term and the ionization potential of sodium were found with an accuracy of up to 2%. It should not be forgotten that all these calculations

P. P. LAZAREV

were carried out in the twenties, when modern calculating machines did not exist.

Among the fundamental works in which general problems of quantum physics were solved is the work of L. I. Mandelstam and M. A. Leontovich on the behavior of a quantum particle in the presence of a potential barrier in space. This work contained the foundations of the theory of “tunnel transitions”—a quite distinctive phenomenon which, as is known, plays an extremely important role in innumerable processes on the atomic or nuclear scale, i.e., in processes that belong among the basic phenomena of atomic physics and electronics.

In recent years the efforts of theorists have been directed toward the development of relativistic quantum mechanics and quantum electrodynamics, or, more precisely, quantum field theory in general (meaning not only the electromagnetic field, but also meson fields). In these most complex problems, connected with the use of the most refined mathematical methods, Soviet theorists feel “at home” and are making substantial contributions (the works of N. N. Bogolyubov, L. D. Landau, I. E. Tamm, M. A. Markov, I. Ya. Pomeranchuk and their students). Without any claim to give even a remote idea of the results obtained here, we shall mention the approximate method, developed by I. E. Tamm, for solving the equations of quantum mesodynamics; it differs from the ordinarily used perturbation theory (the so-called Tamm–Dancoff method; an analogous method had earlier been applied by V. A. Fock to the solution of certain problems of quantum electrodynamics).

In the field of general problems of thermodynamics and statistics, the works of L. D. Landau on the thermodynamic theory of phase transformations of the second kind were of great importance; these include certain transformations in alloys, transformations of ferromagnets into paramagnets, and, in general, transformations associated with “Curie points.”

Fundamental works on the foundations of statistical mechanics were published by N. N. Bogolyubov and M. A. Leontovich. Works by B. I. Davydov and others were devoted to the statistical theory of irreversible processes.

In our own day the study of new unstable particles ($K$-mesons, the so-called $\tau$–$\theta$ problem) has brought a new surprise in addition to the surprises that twentieth-century physics has already more than once brought in the theory of relativity and in quantum mechanics.

The experimentally established apparent contradictions in the behavior of $K$-mesons were explained by Lee and Yang as a violation, in the decay of $K$-mesons, of one of the most important conservation laws, namely the law of conservation of “parity.” The essence of the principle of conservation of parity, speaking pictorially, is expressed in the requirement of “right-left” symmetry of an object or phenomenon; in other words, in the requirement that the object and its mirror image coincide with each other, or, finally, in the requirement that the laws of nature be invariant with respect to mirror reflection. The violation of the principle of conservation of parity, first advanced as a hypothesis to explain a special case—the decay of $K$-mesons—was soon shown quite convincingly by the experiments of Wu and a group of collaborators at the National Bureau of Standards of the USA, using the $\beta$-decay of oriented Co$^{60}$ nuclei.

In connection with this problem, L. D. Landau put forward an extremely interesting principle, according to which right-left asymmetry is associated with electric charge. This means that if a particle with positive charge, for example a proton, is characterized by a spatial asymmetry of a definite type—say, a right-handed screw—then an antiproton, having-

were carried out in the twenties, when modern calculating machines did not exist.

Among the fundamental works in which general problems of quantum physics were solved is the work of L. I. Mandelstam and M. A. Leontovich on the behavior of a quantum particle in the presence, in space, of a potential barrier. This work contained the foundations of the theory of “tunnel transitions”—a phenomenon of quite special character which, as is known, plays a most important role in innumerable processes on an atomic or nuclear scale, i.e., in processes belonging among the fundamental phenomena of atomic physics and electronics.

In recent years the efforts of theorists have been directed toward the development of relativistic quantum mechanics and quantum electrodynamics or, more precisely, of quantum field theory in general (having in view not only the electromagnetic field, but also meson fields). In these most difficult problems, connected with the application of the most refined mathematical methods, Soviet theorists feel “at home” and are making substantial contributions (the works of N. N. Bogoliubov, L. D. Landau, I. E. Tamm, M. A. Markov, I. Ya. Pomeranchuk, and their pupils). Without any claim to giving even a remote idea of the results obtained here, let us mention the approximate method, developed by I. E. Tamm, for solving the equations of quantum mesodynamics, distinct from the perturbation theory usually employed (the so-called Tamm–Dancoff method; earlier an analogous method had been applied by V. A. Fock to the solution of some problems of quantum electrodynamics).

In the field of general problems of thermodynamics and statistics, great importance attached to the works of L. D. Landau on the thermodynamic theory of phase transitions of the second kind, which include certain transformations in alloys, transformations of ferromagnets into paramagnets, and, in general, transformations connected with “Curie points.”

Fundamental works on the foundations of statistical mechanics were published by N. N. Bogoliubov and M. A. Leontovich. The statistical theory of irreversible processes was the subject of works by B. I. Davydov and others.

In our own day the study of new unstable particles ($K$-mesons, the so-called $\tau$–$\theta$ problem) has brought a new surprise in addition to those surprises which twentieth-century physics had more than once already brought in the theory of relativity and in quantum mechanics.

The experimentally established apparent contradictions in the behavior of $K$-mesons were explained by Li and Yang as a violation, in the decay of $K$-mesons, of one of the most important conservation laws—namely, the law of conservation of “parity.” The essence of the principle of conservation of parity, speaking graphically, is expressed in the requirement of “right–left” symmetry of an object or phenomenon, or else in the requirement that the object and its mirror image coincide with one another, or, finally, in the requirement that the laws of nature be invariant with respect to mirror reflection. First advanced as a hypothesis to explain a special case—the decay of $K$-mesons—the violation of the principle of conservation of parity was soon demonstrated quite convincingly by experiments of Wu and a group of collaborators at the National Bureau of Standards of the USA with the $\beta$-decay of oriented Co$^{60}$ nuclei.

In connection with this problem, L. D. Landau advanced an extremely interesting principle according to which right–left asymmetry is associated with electric charge. This means that if a particle with positive charge, for example a proton, possesses a spatial asymmetry of a definite type, say, a right-handed screw, then an antiproton, having—

therefore a negative charge, must have the opposite asymmetry characteristic of them, i.e., a left-handed screw. Since the mirror reflection of a right-handed screw is a left-handed screw, particles and antiparticles as it were transform into one another under mirror reflection. Thus, the two asymmetries existing in nature—the asymmetry of electric charge and spatial right-left asymmetry—are combined into a certain new higher type of symmetry, manifested in the principle called by Landau the “principle of combined parity,” according to which the mirror image of any process is also a possible process, provided that all charges are replaced by their opposites. In this fascinating field, where very deep, fundamental laws of nature are manifested, we may be on the eve of new important discoveries.

Atomic Nuclei and Cosmic Rays

In this most important and at the same time most difficult field of modern physics, such great successes have been achieved that they have brought the Soviet Union to one of the foremost places in world science. A merely external indication of the scale of work in this field may be the number—102—of very substantial papers presented by Soviet physicists to the Geneva Conference on the Peaceful Uses of Atomic Energy in 1954. The world’s first atomic power station, the most varied work involving artificially radioactive isotopes, the successfully under-construction “atomic icebreaker,” important investigations in high-energy particle physics carried out on the six-meter synchrocyclotron, and, finally, the world’s largest 10-billion synchrophasotron, launched this year—these are only the most striking achievements in the field of peaceful applications of nuclear energy in recent times.

However, successful experimental and theoretical work in the field of atomic-nuclear physics has been conducted throughout the past 40 years. Let us list the most important of the works carried out during this time. The idea that atomic nuclei do not contain electrons but consist of positively charged protons and chargeless neutrons was briefly formulated in 1932 by D. D. Ivanenko. The presently accepted conception of the origin of nuclear forces as a result of exchange by particles was developed in detail by I. E. Tamm (1934). Although the initial supposition that this exchange is effected by electrons and neutrons leads, as I. E. Tamm’s calculations showed, to the assumption of the existence of forces many orders of magnitude smaller than the real forces that hold the atomic nucleus together, the basic ideas of this theory have remained guiding ones to this day.

A large number of works in the field of nuclear physics and cosmic rays have been carried out by experimental physicists. Numerous works at the first stages of the study of cosmic rays were performed by L. V. Mysovskii. Together with Chizhov he developed (1925) a method of photographic plates for studying fast particles, a method that has gained wide currency in recent years. This method, improved by A. P. Zhdanov, enabled the latter, as well as P. I. Lukirskii and N. A. Perfilov, to discover the “stars” produced in a photographic emulsion by cosmic rays. Among the most important works in the field of cosmic rays are the investigations of D. V. Skobeltsyn, who in 1927 for the first time succeeded in observing showers of cosmic particles in a Wilson chamber. The method employed in these works—a Wilson chamber placed in a magnetic field—has since become widely used.

Extensive work on the study of phenomena arising in the interaction of primary cosmic rays with atomic nuclei was carried out in recent years (D. V. Skobeltsyn, N. A. Dobrotin, G. T. Zatsepin, S. N. Vernov).

One of a number of important results of this work is the discovery of the so-called electron-nuclear showers. This discovery substantially deepened our understanding of the cascade mechanism by which showers arise, since it showed that the initial link of the cascade is not electromagnetic, but electron-nuclear processes at high energies. A large number of important results were obtained at the high-altitude laboratory in the Pamirs, organized by the Physics Institute of the Academy of Sciences of the USSR.

A. I. Alikhanov and A. I. Alikhanian, who organized a high-altitude laboratory for the study of cosmic rays on Mount Alagez in Armenia and built a mass spectrograph for determining the masses of cosmic particles, were pioneers in the study of mesons of different masses.

Of great fundamental significance was the experiment of A. I. Alikhanian, A. I. Alikhanov, and L. A. Artsimovich (1936), in which it was definitely shown that the law of conservation of momentum is fulfilled in electron–positron annihilation. The most exact verification of the fulfillment of the law of conservation of momentum in the annihilation of positrons of the $\beta^{+}$ decay of $\mathrm{Cu}^{64}$ was carried out later, in 1950, by N. A. Vlasov and B. S. Dzhelepov. A number of important works on the experimental study of $\beta$-decay ($\beta$-spectra, internal conversion) were performed by A. I. Alikhanov, A. I. Alikhanian, and their collaborators with the aid of A. I. Alikhanov’s magnetic $\beta$-spectrograph. Important stages in the experimental proof of the existence of the neutrino were the experiments of A. I. Leipunsky, which showed nonconservation of the law of conservation of momentum in the electron–recoil nucleus system.

B. V. Kurchatov, I. V. Kurchatov, L. I. Rusinov, and L. V. Mysovsky discovered (1935) the remarkable phenomenon of nuclear isomerism of radioactive elements. Using bromine isotopes as an example, it was shown that there exist radioactive nuclei which are isotopes and isobars, i.e. have exactly the same composition, but have substantially different decay periods. Thus, for example, the bromine isotope exhibits two periods: 18 min. and 4.4 hr. This phenomenon proved to be very widespread among radioactive nuclei. It was called nuclear isomerism in view of a certain analogy with the phenomenon known in organic chemistry, where isomers are molecules having the same composition but different structure. The cause of nuclear isomerism, however, lies not in differences in the structure of isomeric nuclei, but in the existence, for $\gamma$-radiation, of metastable nuclear levels, transitions from which to the normal state are more or less strongly “forbidden.” As a consequence of the low probability of transition, nuclei that have reached such a metastable excited level will discharge with the emission of $\gamma$-rays over a long interval of time.

After the invention of the betatron, Soviet physicists showed that the “ceiling” for accelerating electrons by this device is due to the fact that the electrons being accelerated must ultimately, by virtue of the laws of classical electrodynamics, begin to lose energy owing to the emission of electromagnetic waves (D. D. Ivanenko, I. Ya. Pomeranchuk, and A. A. Sokolov; later—L. A. Artsimovich and I. Ya. Pomeranchuk). As is known, visible radiation (with a continuous spectrum) was in fact discovered in the USA by Pollock and collaborators. Let us note in passing that this fact of the occurrence of visible radiation when electrons move in a magnetic field along curvilinear trajectories has recently received

an unexpected application in astrophysics. Namely, according to a hypothesis formulated by V. L. Ginzburg and I. S. Shklovsky and confirmed by subsequent polarization measurements, the continuous-spectrum glow emitted by nebulae—the shells of supernovae—is explained precisely by such motions of cosmic electrons in interstellar magnetic fields. According to the hypothesis developed by V. L. Ginzburg and I. S. Shklovsky, the “magnetobremsstrahlung” radiation of charged particles makes it possible to detect the regions where cosmic rays are generated.

The decisive success in the construction of modern high-power accelerators, necessary for the study of nuclear processes, was achieved thanks to the work of V. I. Veksler, who proposed and substantiated the so-called principle of autophasing (1944). Thanks to this principle it proved possible to overcome the limitation in the use of resonant acceleration methods caused by the relativistic dependence of mass on velocity. Instruments built on this principle—synchrotrons, phasotrons (or synchrocyclotrons), and, finally, synchrophasotrons—made it possible to pass from energies measured in tens of millions of electron-volts to energies of many hundreds of millions and even billions of electron-volts. Thus, for example, the synchrocyclotron of the Institute for Nuclear Problems of the USSR Academy of Sciences provides particle beams with energies up to 700 MeV.

The creation of such powerful installations, which make it possible under laboratory conditions to obtain beams of particles of very high energies approaching the energies of cosmic rays, has made it possible actively to develop a new field of physics—the physics of particles of high and ultrahigh energies. This vast field is still quite young—its history numbers barely one decade; its subject is the investigation of the nature and properties of the simplest structural elements of matter—nucleons, mesons, and hyperons. Soviet physicists have made a substantial contribution to this new science, which concerns the deepest properties of matter. Their investigations of the interaction of elementary particles over a broad range of energies proceeded in three directions: the elastic scattering of nucleons by nucleons was studied (the scattering processes \(pp\), \(np\), and \(nn\)), the formation of charged and neutral mesons in collisions of nucleons, and the interaction of \(\pi\)-mesons with nucleons. All these investigations yielded much valuable information about the structure of elementary particles and the nature of nuclear forces. Experimental work was carried out in a number of institutes under the direction of V. I. Veksler, V. P. Dzhelepov, B. M. Pontecorvo, M. G. Meshcheryakov; theoretical work—under the direction of I. Ya. Pomeranchuk and others.

The greatest achievement in the field of obtaining particles of ultrahigh energies is the commissioning (April 1957) of the giant synchrophasotron of the Joint Institute for Nuclear Research. This installation was created by the joint work of a large collective of Soviet physicists and engineers under the leadership of V. I. Veksler, D. V. Efremov, E. G. Komar, of the staff of the radio-engineering laboratory of the Academy of Sciences of the USSR headed by A. L. Mints, and with the participation of a number of other scientific and technical institutions (the P. N. Lebedev Physical Institute of the Academy of Sciences, the All-Union Electrotechnical Institute, and others). The scale and extraordinary precision of the operation of this quite unique installation may be judged from the following data*): the weight of the annular electromagnet of the synchrophasotron is 36,000 tons; the mean diameter of the steel ring reaches almost 60 m; the pressure inside the vacuum chamber, pumped out by 56 powerful pumps, falls to \(10^{-9}\) atm. The protons subjected to acceleration must, in 3.3 seconds, make inside this vacuum

) The newspaper Pravda*, April 11, 1957.

of the chamber 4.5 million revolutions and in doing so traverse a path of a million kilometers. For the proper operation of this installation it is necessary to control the processes of switching on and off the system of particle “injection,” and of switching on the accelerating radio-frequency voltage in individual cases with an accuracy of up to \(10^{-5}\) sec. On this installation it was possible first to obtain protons with an energy of 8.3 billion electron-volts, and then—10 billion electron-volts—the highest energy that physicists had ever succeeded in creating artificially*). As one of the important conclusions following from this great event, it must be noted that the industry of the USSR can now solve the problem of creating a modern and highly complex technical base for the development of research in the field of atomic physics.

The end of the 1930s and the beginning of the 1940s are characterized by the intensive development of work on the study of the fission of the nuclei of heavy elements. A number of important results that played a substantial role in solving the problem of obtaining and using nuclear energy were obtained at that time by Soviet physicists. As is known, the basis of nuclear chain reactions is the process of fission of heavy nuclei under the action of neutrons. A qualitative explanation of this phenomenon from the standpoint of the electrocapillary model was first given in 1939 by Ya. I. Frenkel (simultaneously and independently this idea was developed by N. Bohr and J. Wheeler). In 1940, K. A. Petrzhak and G. N. Flerov showed that the process of uranium fission also occurs spontaneously, though with a very small probability. Following this, Ya. B. Zel’dovich and Yu. B. Khariton showed (1939–1940) that with a small enrichment of the natural mixture of uranium isotopes by the light isotope \(U^{235}\), a chain process is possible using ordinary water as a moderator.

Subsequently, Soviet scientists (I. V. Kurchatov, A. I. Alikhanov, V. S. Fursov, A. P. Aleksandrov, D. I. Blokhintsev, N. A. Dollezhal, and others) created many experimental nuclear reactors for scientific research purposes and carried out numerous investigations on the most important problems of nuclear physics. These works created the foundation for the development of applied nuclear physics. The most important result here was the creation of nuclear weapons, and, in the field of peaceful applications, the already mentioned development of nuclear power engineering; moreover, the experience of operating the first atomic power station built in the USSR made it possible to outline a large program for the development of the nuclear-power industry. The production, with the aid of nuclear reactors, of a large quantity of artificial radioactive isotopes led to the broad development of the method of “labeled atoms” in metallurgy, biology, medicine, and agriculture. Serious investigations were carried out with the aim of mastering a controlled thermonuclear reaction. Here one should note the theoretical work of A. D. Sakharov and I. E. Tamm and the experiments, carried out under the direction of L. A. Artsimovich and M. A. Leontovich, on powerful discharges in rarefied gases. By concentrating the discharge under the action of its own magnetic field into a thin plasma cord, it was possible for the first time to obtain, under laboratory conditions, a temperature of the order of 1 million degrees and to detect the appearance of free neutrons.

The work of physicists and engineers in mastering nuclear energy may rightly be counted among the most serious achievements of Soviet science.

*) It is worth recalling that all calculations of the motion of particles in such an installation were, of course, carried out on the basis of relativistic mechanics. Thus, the start-up of this machine may be regarded as a confirmation of the formulas of the special theory of relativity up to an energy of 10 billion electron-volts.

Optics

The great importance of this branch of physics is due, in particular, to the fact that the optical industry, exceptionally important both from the defense and from the cultural point of view, still maintains a direct connection with it.

In an interesting article written for the fifteenth anniversary of the State Optical Institute, D. S. Rozhdestvenskii characterized the cultural significance of the optical industry in the following vivid words: “The prevalence of optics is a sign of the height of culture. A microscope, a photographic camera, a telescope or binoculars have always distinguished a cultured family. The optical industry is the highest industry—because it is the most delicate and difficult, and because, through the microscope and the telescope, it leads us most quickly to culture, to genuine scientific materialism, and to the dispersal of prejudices. Nations are renowned precisely for their optical industry, however insignificant it may be in size: who does not know the Zeiss firm, the pride of Germany?”*).

What, then, was the level of the optical industry in Russia before the Revolution? On this point we have the authoritative testimony of D. S. Rozhdestvenskii in the same article; it may be briefly characterized in the words: “with respect to 1917, one has rather to note what did not then exist.” Rozhdestvenskii goes on to list the small workshops that existed at that time, with a total number of workers of fewer than 1000 people (this also included the workers of workshops that produced optical instruments for military purposes). “There was not a single designer of optical systems, and no one in Russia was engaged in optical engineering. Therefore the factories understood little of the essence of the matter and could only slavishly copy foreign models. Nowhere were eyeglasses, geodetic instruments, photographic cameras and objectives, cinematographic apparatus, microscopes, or scientific instruments produced”**).

But already in 1933, according to data presented in the same article, the picture had changed sharply. There were already 7 factories of the optical-mechanical industry with 11,000 workers, producing a varied range of optical instruments for military, scientific, and everyday purposes. If in 1917 there had not been a single designer, then in 1933 the SOI already had a computing bureau with several dozen employees, for whom “there were already no more secrets or difficulties in the complex matter of calculating optical systems, right up to the fastest photographic and microscopic objectives.” Since then another 25 years have passed, and now our optical industry fully supplies the Soviet Army, the Navy, and aviation with all necessary instruments; it produces a variety of entirely modern photographic cameras with excellent lenses—up to the most complex ones—made from Soviet optical glass, as well as various scientific instruments—microscopes, astronomical telescopes, spectral apparatus, etc.

An enormous role in this rapid development of the optical industry was played by the State Optical Institute named after S. I. Vavilov, headed by its founder D. S. Rozhdestvenskii and by his successor S. I. Vavilov.

) D. S. Rozhdestvenskii, “The Fates of Optics in the USSR,” Fifteen Years of the State Optical Institute. Collection of articles edited by S. I. Vavilov, ONTI, 1934, p. 25.
*) D. S. Rozhdestvenskii, cited article, pp. 19–20.

The most important raw material for optical production is optical glass. Before the revolution, during the height of the First World War, Tsarist Russia found itself in an exceptionally difficult situation, since the stocks of optical glass needed to equip military matériel, previously imported from Germany, were negligible and were used up in a few months. “At that time the highly distinctive and difficult production of optical glass constituted the monopoly of only three firms in the entire world and was kept under the greatest secrecy. In Russia it was impossible to find a single person even slightly acquainted with this question, nor in any language was it possible to read a single line devoted to this forbidden subject” *). Attempts to melt optical glass, made in 1916 according to the formulas of the English firm Chance Brothers, yielded an insignificant quantity of glass of only moderately satisfactory quality. One of the first tasks set for itself, upon the organization of the GOI, by the group of enthusiasts of Soviet optics headed by D. S. Rozhdestvenskii, was to develop their own methods for melting optical glass—methods built on the solid foundation of broad scientific physico-chemical investigations of this distinctive process, and not in the form of “something like a cleverly conceived trick, guarded by the strictest secrets” **) as was the formula for melting at foreign firms. The credit for developing and improving these methods belongs to a large collective of scientific workers, in particular I. V. Grebenshchikov, N. N. Kachalov, A. A. Lebedev, and A. I. Stozharov. Control of the process was greatly facilitated thanks to the ingenious method developed by I. V. Obreimov for the rapid determination of the refractive index of glass in the form of a piece of irregular random shape. Thanks to all these works, already in 1925 the Soviet Union was able completely to renounce the import of optical glass.

For the development of applied optics, the creation of the Soviet school of optical calculators played a major role (A. I. Tudorovskii, G. G. Sliusarev, E. G. Yakhontov, and others). Original methods of calculation were developed and auxiliary tables were compiled that facilitated the choice of kinds of glass and the course of calculation. The construction of reflecting objectives (E. M. Brumberg and S. A. Gershgorin) made it possible to create an original type of ultraviolet microscope (E. M. Brumberg). An entirely distinctive design of astronomical telescopes—the mirror-meniscus system—was created by D. D. Maksutov (1941). A number of original designs of photographic objectives were also developed (M. M. Rusinov, D. S. Volosov, and others). Ingenious methods for testing optical systems were proposed by V. P. Linnik; he and A. A. Lebedev also devised a number of original designs of optical instruments. Especially noteworthy is the creation and industrial development of all types of spectral apparatus, which fully supplied the numerous factory laboratories, scientific-research institutes, and educational institutions. Finally, the most important recent achievement was the creation at the GOI, under the direction of F. M. Gerasimov, of high-quality Soviet diffraction gratings.

An important branch of applied optics is illumination engineering and the photometry directly connected with it. In the field of the theoretical foundations of illumination engineering, the theory of the light field developed by Soviet scientists, in particular V. A. Fok, A. A. Gershun, M. M. Gurevich, and N. V. Boldyrev, is of great importance. In this theory the problem

*) I. V. Grebenshchikov and N. N. Kachalov, Collection “Fifteen Years of the State Optical Institute,” pp. 160–161.

**) I. V. Grebenshchikov and N. N. Kachalov, cited article, p. 170.

lighting engineering, i.e., the problem of rational illumination, is solved on the model of a general physical field theory with the introduction of “density of luminous energy,” “luminous vector,” and with the subsequent mathematical development of the theory by means of vector analysis. The significance of these works was vividly formulated by the editor of the English translation of A. A. Gershun’s book The Theory of the Light Field, published in the USA, Parry Moon, who emphasized that the theory set forth in this book by one of its pioneers represents the first important step in photometry since the work of P. Bouguer (i.e., since the middle of the eighteenth century).

In the field of physical optics the works of Soviet physicists are numerous and varied. Mention has already been made above of the classical works of D. S. Rozhdestvenskii on anomalous dispersion in sodium vapor, carried out before the Revolution. In these works Rozhdestvenskii developed the ingenious “hook method,” which made it possible conveniently and rapidly to study dispersion in metal vapors and to extract from these measurements precise values of transition probabilities and intensities of spectral lines. Owing to further improvement of the technique, namely the construction of a fluorite interferometer designed by D. S. Rozhdestvenskii, his pupils V. K. Prokof’ev and A. N. Filippov were able to extend the investigation of anomalous dispersion into the ultraviolet region and obtained a number of valuable results.

In the field of atomic spectroscopy, works by D. S. Rozhdestvenskii, published as early as the 1920s, were of outstanding importance. In these works, using the atom of lithium and other alkali metals as examples, the close similarity of the higher levels of these monovalent atoms with the terms of hydrogen-like atoms was shown. On this basis the so-called model of the radiating electron was clearly formulated. Further, by comparing the spectrum of ionized magnesium with the spectrum of helium, the so-called “spectroscopic displacement law” was established, according to which the spectrum of a singly ionized atom with atomic number \(Z\) is analogous to the spectrum of a neutral atom with atomic number \(Z - 1\).

Two circumstances should be noted in connection with these works of D. S. Rozhdestvenskii. First, although Rozhdestvenskii’s works were based wholly on Bohr’s theory of the atom, the model of the optical electron and the spectroscopic displacement law have fully retained their heuristic significance and serve as a guiding thread for experimental spectroscopists to this day. Second, in the historical aspect these works had special significance for Soviet physics. They were done during the period of the blockade, in the complete isolation of Soviet scientists from foreign science. And although after the blockade was lifted it became known that the same results had been obtained by Sommerfeld, Schrödinger, and other Western scientists, the fact that young Soviet science, in complete isolation from the established foreign scientific schools, was able to pose and solve the most important problems of that time served for us as a source of joy and faith in our strength.

The most important organizational result of these works was the creation around Rozhdestvenskii of a brilliant school of Soviet spectroscopists (A. N. Terenin, S. E. Frisch, E. F. Gross, A. N. Filippov, V. K. Prokof’ev, M. A. Veingerov, and others). Let us note the results of the work of this school that have entered the practice of modern spectroscopy.

The ultimate goal of spectroscopic investigation consists in establishing the system of terms of the atom, i.e., the scheme of its energy levels. This scheme of levels can be checked also by direct experiment, by exciting the atom to a definite upper state and establishing the subsequent downward transitions. The excitation itself may be accomplished

either by electron impact or by optical means, if the atoms are made to absorb quanta of a strictly definite frequency. In the twenties the method of electron impact enjoyed great popularity, having been applied with such success in the classical works of J. Franck and G. Hertz. However, for all the significance of the first works in this direction, subsequent work yielded results that were rather crude in quantitative terms and, qualitatively, by no means always admitted a clear interpretation. Much more refined is the method of optical excitation. It was widely used by A. N. Terenin in a number of his exemplary works. Thanks to various improvements introduced by A. N. Terenin into the experimental technique, he succeeded in studying level schemes in detail and in tracing transitions between various levels of a whole series of atoms—mercury, cadmium, thallium, bismuth, lead, zinc—as well as in studying the so-called stepwise excitation, in which an already excited atom absorbs one more quantum of energy and passes to a higher level. Today all these ideas have entered so thoroughly into the “flesh and blood” of physicists that it is even difficult to imagine how great was the significance of the works described, which made it possible with particular clarity to grasp the stream of new ideas that poured into spectroscopy with the development of the quantum theory of the atom.

An important result of the cycle of works just noted was the discovery by A. N. Terenin and L. N. Dobretsov, alongside the fine and still finer structure of the sodium lines, and by A. N. Terenin and E. F. Gross—the hyperfine structure of the mercury lines (simultaneously and independently of the Soviet researchers the hyperfine structure was discovered by Schuler in Germany). The fundamental significance of this result, at first glance a rather special one, consists in the fact that by studying hyperfine structure it proved possible to establish such properties of the atomic nucleus as its mechanical moment (spin) and magnetic moment. Until comparatively recent times this optical method was the only way of determining these important constants of the nucleus. Extensive investigations of the hyperfine structure of atomic lines were carried out by S. E. Frisch.

From the optical excitation of atoms there is a natural transition to the study of the optical excitation of molecules. In this field the investigations of A. N. Terenin and his collaborators have an important fundamental significance; by means of the spectroscopic study of excited molecules they elucidated the mechanism of the elementary photochemical act. Here it should be noted that, although the study of photochemical processes is as old as mankind itself, before the development of modern spectroscopy, with its perfected experimental methods and clear theoretical premises, even the basic problem of the mechanism of the elementary photochemical act could not be distinctly formulated physically. Meanwhile, in the present case the issue is a quite concrete physical problem concerning the mechanism by which the energy of the electronic excitation of a molecule is transformed wholly or partly into the vibrational energy of atoms, with subsequent dissociation. At the same time the enormous sensitivity of spectroscopic methodology, immeasurably surpassing the usual chemical methods of analysis, not only makes it possible to detect in the most delicate way the very fact of dissociation, but also makes it possible to establish the state in which the products of dissociation are liberated. Indeed, when one of the dissociation products is liberated with sufficient energy, the fragments of the molecule become visible by the radiation they emit, the spectral composition of which indicates their energetic state. In the words of A. N. Terenin, spectroscopic

the technique makes it possible not only to capture decomposition products in statu nascendi, but, what is still more important, in statu luminescendi.

In these works a highly refined technique was employed: a powerful ten-kilowatt discharge hydrogen tube gave the most intense radiation up to the far ultraviolet, where the photon energy is 150–200 kg cal, and for identifying the decomposition products, besides spectroscopy, a variety of other sensitive methods were also used. Numerous works on optical dissociation, predissociation, and induced predissociation were also published by V. N. Kondrat’ev.

In recent years the interest of researchers has shifted from the simplest diatomic molecules toward complex, especially organic, molecules. Let us note the interesting works of B. S. Neporent on the investigation and interpretation of broad absorption bands in vapors of organic compounds.

By contrast, in the solid state, at extreme degrees of cooling, organic crystals and frozen solutions under certain conditions give spectra consisting of sharp lines. Obviously, the significance of studying the spectra of complex molecules precisely under these conditions—when, on the one hand, the individual properties of molecules are manifested, and, on the other, the possibility opens up of tracing the subtle effects of the crystal field on the emitting molecule—is great. The study of the absorption, luminescence, and also dispersion spectra of a number of aromatic hydrocarbons in the crystalline state (I. V. Obreimov, A. F. Prikhot’ko and their collaborators) and in frozen solutions of certain special solvents (E. V. Shpol’skii with collaborators) opens highly promising paths for a profound investigation of these important molecules. The theory of the spectra of molecular crystals developed by A. S. Davydov now serves as the basis for interpreting the results of numerous works on the spectroscopy of complex molecules carried out in the USSR and abroad.

The phenomena of fluorescence in solutions and solids, and the processes of long-lasting emission—phosphorescence—are of great interest from both theoretical and practical points of view. In this field as well Soviet physicists have made a very substantial contribution. In the works of S. I. Vavilov, V. L. Levshin, and their collaborators, fluorescence in solutions was subjected to comprehensive and careful study. Above all, S. I. Vavilov studied the fluorescence yields of dye solutions. It was found that the energy yield of fluorescence in many cases is close to 100%, while the quantum yield and the lifetime of the excited state do not depend on the exciting wavelength. This regularity, which plays an important role in elucidating the mechanism of fluorescence of complex molecules, bears the name of Vavilov’s law.

The remarkable phenomenon of fluorescence polarization, discovered by F. Weigert in 1920, was subjected by S. I. Vavilov and V. L. Levshin, in a series of fundamental works, to comprehensive investigation. The interest of this phenomenon lies in its unquestionable connection with the structure of the emitting molecules and in its unusual sensitivity to all kinds of perturbing influences on the emitting molecule. Thanks to this, one or another degree of fluorescence polarization can serve as a most delicate indication of molecular interactions.

Especially important and interesting proved to be the sharp dependence, discovered by S. I. Vavilov, of the degree of polarization on the exciting wavelength; moreover, for certain wavelengths the polarization of fluorescence even changes sign, i.e., becomes negative. Thanks to the existence of this

dependences opens up the possibility of establishing a new characteristic of molecular properties, just as important as the absorption and emission spectra—namely, polarization spectra, knowledge of which makes it possible to draw conclusions about the structure of fluorescing molecules. The possibilities opened up by knowledge of fluorescence polarization spectra were demonstrated by S. I. Vavilov’s student P. P. Feofilov, who studied and interpreted the polarization spectra of a large number of complex organic molecules. P. P. Feofilov is also responsible for an extensive series of works on the polarization of luminescence in crystals and solutions.

In the works of S. I. Vavilov and his collaborators (I. M. Frank, B. Ya. Sveshnikov), the phenomena of fluorescence quenching of solutions by extraneous colorless salts and the phenomena of self-quenching observed at high concentrations of solutions of fluorescing substances were also studied in detail. S. I. Vavilov gave a complete phenomenological theory of these phenomena.

V. L. Levshin substantiated, on the basis of extensive experimental material, the law of “mirror symmetry” of fluorescence and absorption spectra. In the form originally established by Levshin, this law applies to curves represented on a frequency scale. Recently B. S. Neporent has shown that there exists a whole class of complex organic molecules for which mirror symmetry on the frequency scale is absent, but other relationships between the spectra do occur. A number of theoretical questions relating to the luminescence of complex molecules have been investigated by B. I. Stepanov.

Alongside ordinary fluorescence, characterized by a luminescence duration of the order of \(10^{-8}\) sec, organic substances, in particular dyes in “rigid” media—for example, in solid solutions in boric acid, in sugar candies, or in frozen solutions in organic solvents—give a long-lasting glow with a lifetime measured in seconds. In this case, at low temperatures the emitted region of the spectrum is strongly shifted toward longer wavelengths. This phenomenon of phosphorescence of organic compounds was also studied by Soviet researchers (S. I. Vavilov, B. Ya. Sveshnikov, P. P. Dikun). After the Polish physicist A. Jabłoński gave the correct phenomenological explanation of this long-lasting luminescence as the result of the existence of a metastable level situated between the normal and the first excited levels, A. N. Terenin revealed the physical mechanism of this metastability. According to this explanation, the metastable level is a triplet one, i.e. the spins of one pair of electrons are arranged parallel, whereas the ground level is a singlet one because of the antiparallel spins of its electrons; in view of this, the duration of the luminescence is a consequence of the prohibition of the triplet–singlet intercombination transition. This explanation has recently received direct experimental confirmation in a number of works by foreign investigators (G. N. Lewis, M. Kasha and M. Calvin, D. Evans), who showed that in the “phosphorescent” state the molecule becomes paramagnetic, whereas in the normal state it is diamagnetic. Quite recently it has begun to become clear that such molecules, remaining for a long time in the excited “triplet” state, or otherwise “biradical” molecules, apparently play an essential role in certain very important biological processes.

Also deserving mention is a series of works on the experimental determination of the duration of the excited state (the duration of luminescence) in luminescence. L. A. Tumerman, and subsequently, in a more refined form, A. M. Bonch-Bruevich, constructed

fluorometers—installations by means of which, in the latter variant, the duration of luminescence can be determined down to \(10^{-11}\) sec. A very valuable instrument for rapid determinations of duration (the so-called “taumeter”) was constructed by N. A. Tolstoi and P. P. Feofilov.

The luminescence of crystal phosphors has also been subjected to numerous investigations by Soviet physicists (V. V. Antonov-Romanovskii, V. L. Levshin, and others). Let us note the extensive and extremely careful studies of the decay law of zinc-sulfide phosphors carried out by Soviet physicists. The results of these studies, which are of fundamental importance for judging the mechanism of phosphorescence, have firmly entered into the usage of experimentalists and theoreticians working in this field as the most reliable material *).

The modern theory of the luminescence of crystal phosphors is based on the so-called band theory of crystals. This theory, which underlies modern ideas about the electrical properties of solids, has proved extremely fruitful for understanding the mechanism of luminescence in crystal phosphors. Despite its approximate character, band theory is at present an indispensable working picture for theoreticians and experimentalists engaged in research and technical applications of crystal phosphors. The value of this theory for the theoretical interpretation of the laws of luminescence, in particular for the kinetics of afterglow in crystal phosphors, was demonstrated in a number of works by D. I. Blokhintsev, S. I. Pekar, and E. I. Adirovich.

Experimental and theoretical studies in the field of the luminescence of crystal phosphors, the development of the technology for producing crystal phosphors, and the investigation of luminescence conditions in a gas discharge helped industry master the production of economical “daylight” lamps, which are becoming ever more widely used.

The successes of atomic spectroscopy created a solid basis for the development of qualitative and quantitative atomic spectral analysis. Incomparably faster than the usual methods of chemical analysis, the methods of atomic spectral analysis have found the broadest applications in the metallurgical and machine-building industries, in the analysis of ores and minerals, and in a number of other fields. At present, not one of these industries can do without a spectral-analysis laboratory, and the number of analyses performed annually in geological work is measured in the millions. Credit for the development and introduction of various methods of spectral analysis, and for assistance in the design and construction of all types of modern spectral apparatus and auxiliary instruments for spectral analysis, belongs both to the Moscow (G. S. Landsberg, S. L. Mandelstam, N. N. Sobolev, A. K. Rusanov, and others) and to the Leningrad (A. N. Filippov, V. K. Prokofiev, S. E. Frish, and others) schools of physicists.

Recently, alongside the methods of atomic spectral analysis, methods of molecular spectral analysis (infrared and ultraviolet absorption spectra, combination-scattering spectra, luminescent spectral analysis) have begun to acquire ever greater importance and distribution. For these methods a new and extensive field of applications is opening up: medicine and biology, the petroleum, pharmaceutical, and food industries, and agriculture. Great credit for the development and introduction of the corresponding methods belongs to V. M. Chulanovskii. It is necessary to note the outstanding role of the Commission

) See, for example, N. Mott and R. Gurney, Electronic Processes in Ionic Crystals*, ch. VI, Gostekhizdat, 1950.

in spectroscopy of the Academy of Sciences of the USSR and G. S. Landsberg, its permanent chairman from the moment of the Commission’s creation until his recent death (1957), in uniting workers; coordinating research topics, creating centers in the chief ministries, and stimulating the creation of high-quality domestic apparatus.

Molecular scattering of light is an area in which Soviet physicists have made achievements of the greatest importance. Let us recall first of all that the very fact of the existence of molecular scattering was long subject to doubt. The work of such outstanding scientists as Lord Rayleigh, M. Smoluchowski, and A. Einstein was required in order to clarify the conditions under which molecular scattering of light is possible. The subtle works of L. I. Mandelstam, carried out before the Revolution, brought great clarity to the discussion of this difficult question. In 1920 the French physicist Cabannes first succeeded in reproducing the blue of the sky in the laboratory, i.e., in reproducing and studying in detail true molecular scattering in gases. Soon afterward, in 1927, G. S. Landsberg indisputably proved the existence of molecular scattering in a solid body—in crystalline quartz. Continuing the investigation of this phenomenon, G. S. Landsberg and L. I. Mandelstam in 1928 discovered that in the spectrum of molecularly scattered light, alongside the unshifted exciting spectral lines, there are also lines shifted toward the red and violet sides. The discovery of this phenomenon, which in the USSR is usually called combination scattering of light, is one of the most important and most fruitful discoveries in twentieth-century physics. It served as the stimulus for an enormous number of works, numbering in the thousands and carried out in virtually all countries. The method, based on combination scattering, for experimentally determining the natural frequencies of oscillation of molecules has opened enormous possibilities for physics, physical chemistry, and inorganic and organic chemistry.

As is known, the discovery of combination scattering was made in India, almost simultaneously with Landsberg and Mandelstam and independently of them, by Raman and Krishnan, who published their first communication earlier than the Soviet scientists. In view of this, according to the established tradition, in foreign literature the phenomenon itself is usually called the Raman effect. However, this purely technical circumstance—the date of publication—does not in the least diminish the merit of the Soviet physicists, to whom belongs, besides the very fact of the discovery of the new phenomenon, the great merit of constructing its rigorous theory (L. I. Mandelstam, M. A. Leontovich, G. S. Landsberg, I. E. Tamm).

The fact that molecular scattering of light can be associated with a change in wavelength was not unexpected for Soviet scientists. Considering molecular scattering as the interference reflection of light from Debye elastic thermal waves, L. I. Mandelstam, and also L. Brillouin independently of one another, showed as early as 1918 that, under such scattering in a medium with refractive index \(n\), the exciting wave \(\lambda_0\) must undergo splitting into two waves shifted relative to \(\lambda_0\) by the amount

\[ \Delta\lambda = \pm 2\lambda_0 n \frac{v}{c}\sin\frac{\theta}{2}, \]

where \(v\) is the velocity of sound in the medium and \(\theta\) is the scattering angle. Because of the presence of the factor \(\frac{v}{c}\), the shift \(\Delta\lambda\) is, in order of magnitude, smaller than the shift in combination scattering. However, this more subtle effect too was detected experimentally by E. F. Gross.

Of great interest also are the works of E. F. Gross, M. F. Vuks, and their collaborators, devoted to the study of the so-called “wings” of Rayleigh scattering. These investigations led to the discovery of very slow oscillations (frequencies of the order of \(50\ \mathrm{cm}^{-1}\)), which are attributed to oscillations of entire molecules relative to one another in the crystal lattice. The study of these so-called “Gross” oscillations shed light on the nature of the liquid and crystalline states.

One of the most important discoveries in the field of optics was made in 1934 by P. A. Cherenkov, at that time a graduate student of the Physics Institute of the Academy of Sciences of the USSR. Already in the first observations of the properties of radioactive substances by the Curies it had been found that solutions of certain mineral salts emit a weak glow under the action of radioactive substances. This glow was usually regarded as fluorescence.

However, P. A. Cherenkov showed in 1934 that under the action of \(\gamma\)-rays not only solutions (such as solutions of uranium salts, whose fluorescence Cherenkov studied), but also pure liquids, such as distilled water, xylene, toluene, glycerin, and various alcohols, give a weak glow. In its properties this glow clearly differed from fluorescence: it was not quenched by the strongest “quenchers” of fluorescence (a solution of KI, etc.); its polarization was essentially different from the polarization of fluorescence. S. I. Vavilov, who directed Cherenkov’s work, correctly discerned in this glow a new effect, which he associated not with the \(\gamma\)-rays themselves, but with free electrons liberated in the medium by the \(\gamma\)-rays.

A complete quantitative theory of Vavilov–Cherenkov radiation (in the literature it is more often called “Cherenkov radiation”) was given by I. M. Frank and I. E. Tamm (later, in a more rigorous form, by I. E. Tamm) and was confirmed experimentally in all details by P. A. Cherenkov. Frank and Tamm explained the origin of this radiation from the standpoint of classical electromagnetic theory as a wave accompanying an electron moving uniformly with a velocity greater than the phase velocity of light in the given medium, i.e. greater than \(c/n\) (\(n\) is the refractive index of the medium). The simple condition of coherence of the elementary Huygens waves arising when an electron moves in a medium with velocity \(\beta = v/c\) is

\[ \cos \theta = \frac{1}{\beta n}, \tag{a} \]

where \(\theta\) is the angle formed by the normal to the wave front with the direction of motion. It follows from this that an electron moving with velocity \(v > c/n\) must be accompanied by a V-shaped wave, a vivid analogue of which may be the shock wave in air accompanying the flight of a projectile at a velocity greater than the speed of sound in air, or the bow wave accompanying the motion of a ship when its speed exceeds the speed of waves on the surface of water.

V. L. Ginzburg gave a quantum treatment of the Vavilov–Cherenkov effect, applying to the emission of a photon by a moving particle the laws of conservation of energy and momentum. Ginzburg further showed that Cherenkov radiation should be observed when a charged particle moves near the surface of a dielectric parallel to it. In doing so Ginzburg showed that in this way it is possible to create a source of microwave electro-

magnetic radiation of such wavelengths, which are difficult to obtain by other means.

Recently, on the basis of the Cherenkov effect, fast-particle counters have been constructed, and these have become extremely widespread in nuclear research. Such a counter consists simply of a pure liquid (for example, water) or a cylinder of Plexiglas and a photomultiplier that records the flash of radiation.

Let us note, finally, two fundamentally important experiments that vividly reveal the quantum nature of light. The first experiment, carried out by A. F. Ioffe and N. I. Dobronravov, detected fluctuations in the “hits” of photons of very weak X-radiation on an ultramicroscopic charged bismuth dust particle suspended in a Millikan condenser.

The second experiment, carried out by S. I. Vavilov with collaborators (E. M. Brumberg, Z. M. Sverdlov), detected statistical fluctuations in the number of photons of visible light entering the eye at extremely low intensities (the dark-adapted eye was chosen as the detector because of its unusual sensitivity, exceeding the sensitivity of any laboratory devices). These works, besides their fundamental significance for establishing the nature of light, open up a new path for studying the operation of the eye.

We shall conclude our survey of works devoted to physical optics with works devoted to the extreme regions of the spectrum. The optical nature of X-rays was established by the discovery of the interference of X-rays in crystals. However, classical interference and diffraction experiments are difficult to carry out with X-rays because of their short wavelength. In spite of this difficulty, V. P. Linnik succeeded in carrying out with X-rays Lloyd’s interference experiment, which is a modification of Fresnel’s experiment with two mirrors, and, from the spacing of the interference fringes, directly determining the wavelength of X-rays.

The region of the spectrum lying on the other side of the visible part—namely, the portion of the spectrum between long infrared and short electromagnetic waves—was discovered thanks to the works of A. A. Glagoleva-Arkad’eva and M. A. Levitskaya, carried out entirely independently of one another. Thanks to an ingenious method of exciting rays lying in this region, it was possible to detect them with complete clarity and thus to fill the last gap in the unified scale of electromagnetic waves.

Radiophysics and the Theory of Oscillations

The history of the development of radiophysics in the USSR in many respects recalls the history of the development of optics. In both cases we are dealing with a discipline that has the most important applications for the defense and culture of the country; in both cases, despite their importance, these disciplines in prerevolutionary Russia were at a low level of development.

True, at the beginning of the twentieth century radio engineering did not yet exist as a separate technical science; even the terms “radiophysics” and “radio engineering” did not exist. The branch of physics bearing the name “electromagnetic oscillations and waves,” with practical applications in the form of “wireless telegraphy,” was still very young. Nevertheless, in this field the backwardness of prerevolutionary Russia—a country where A. S. Popov had invented “wireless communication”—was obvious. Before the First World War, there existed in Russia neither

special laboratories, nor departments of higher educational institutions where problems of radiophysics might be developed, and there did not exist any national industry of “wireless telegraphy.” As a positive development one may note the appearance of the first original textbook on radiophysics in Russian—A. A. Petrovsky’s book Scientific Foundations of Wireless Telegraphy (1907).

A certain revival of work in the field of radiophysics arose already during the First World War of 1914–1918, thanks to the activity of M. V. Shuleikin, N. D. Papaleksi, and their colleagues. But, as in the field of optics, the intensive development of radiophysics and radio engineering began after the October Revolution. A major role in the first stage of this development was played by the Nizhny Novgorod Radio Laboratory, founded on the personal instructions of V. I. Lenin under the scientific leadership of M. A. Bonch-Bruevich. It is also fitting to recall with gratitude the activity of V. K. Lebedinsky, who belonged to the group of founders of the Nizhny Novgorod Laboratory and played a great role not so much through his creative work as through his organizational, pedagogical, and literary activity. At approximately the same time, a number of radiophysical and radio-engineering centers arose in other cities, chiefly at newly established departments of higher educational institutions. Such were M. V. Shuleikin’s department in Moscow, at the then Moscow Higher Technical School; the department of L. I. Mandelstam and N. D. Papaleksi in Odessa; and the radio laboratory organized by A. A. Chernyshev at the Leningrad Polytechnic Institute.

Finally, in 1923 a major center was created in Leningrad—the Central Radio Laboratory, in whose work such outstanding specialists as D. A. Rozhansky, L. I. Mandelstam, N. D. Papaleksi, M. A. Bonch-Bruevich, and others took part. It should be noted here that, although because of the enormous practical importance of all types of radio communication and the rapid successes in this field, radio engineering very soon separated from physics as a quite independent large technical science, the close connection between radio engineering and physics was preserved, and certain problems of fundamental significance for radio engineering—such, for example, as problems of the general theory of oscillations or of the propagation of radio waves—are still being solved by physicists. Thus there occurred—very conditionally, of course—a division between radio engineering and radiophysics.

In the field of the theory of oscillations, whose significance for physics and technology extends far beyond radio engineering alone, such considerable successes were achieved that the results obtained in this field may with full justification be counted among the most brilliant achievements of Soviet physics during the past 40 years. An outstanding role in these achievements was played by the works of L. I. Mandelstam, N. D. Papaleksi, and their extensive school. Let us note that these achievements were the fruit of close collaboration among physicists, mathematicians, and engineers.

As is known, the classical theory of oscillations is above all the theory of linear oscillations, i.e., oscillations obeying a very simple, widely known linear differential equation with constant coefficients. Despite the great completeness of this classical theory, to the Mandelstam and Papaleksi school we owe a substantial rise in the scientific level in this field as well. This rise manifested itself in such questions, for example, as the generalization and deepening of what would seem to be so familiar a concept as the concept of resonance. That the concept of resonance must be generalized is clear from an elementary mechanical example of a pendulum, which may be strongly set swinging either

A. F. IOFFE

D. S. ROZHDESTVENSKY.

in the customary way under the action of a periodic external force, i.e. periodic “pushing,” with a period equal to the pendulum’s own period, or by periodic variation of the length of the pendulum. The latter method, which is also used in setting swings in motion, is a clear example of so-called parametric resonance, since resonance here is achieved by periodic variation of parameters, such as the length and the moment of inertia of the swing. Another example of parametric resonance may be the excitation of oscillations in electrical oscillatory systems by periodically changing the capacitance or inductance of the system (without an external emf). This case is especially interesting because its in-depth analysis, carried out by L. I. Mandelstam and N. D. Papaleksi, led them to the discovery of a new method for generating alternating currents, realized in the so-called parametric machine they constructed.

Fifty years ago, the classical theory of linear oscillations practically satisfied all the needs of physics and technology. But with the appearance of such a most important physical and technical device as the electron tube, whose entire operation is based on its nonlinearity, the task of developing a theory of nonlinear oscillations came to the fore.

The merit for the initial development of the theory of nonlinear oscillations belongs to Van der Pol, who created a scientific center on these questions in Holland. However, in the 1930s, thanks to the work of the Mandelstam and Papaleksi schools (A. A. Andronov, A. A. Witt, G. S. Gorelik, S. M. Rytov, S. E. Khaikin, S. P. Strelkov, and others), the center of gravity of work in the field of nonlinear oscillations shifted from Holland to the USSR. It is interesting to note that the mathematical apparatus necessary for the theory of nonlinear oscillations, as it turned out, had already existed for a long time. It was contained in the works of Henri Poincaré, on the one hand, and in the works of the remarkable Russian mathematician A. M. Lyapunov, on the other. However, neither the one nor the other had in mind the theory of nonlinear oscillations in their works: Poincaré developed his mathematical apparatus for solving certain problems of celestial mechanics, while Lyapunov was interested in purely mathematical questions of the stability of solutions of differential equations. The merit of establishing the connection between problems of nonlinear oscillations and the works of Poincaré and Lyapunov belongs to A. A. Andronov, after whose work appeared the intensive development began both of mathematical methods and of physical applications of the theory of nonlinear oscillations.

Subsequently, other mathematical methods adequate to the problems of nonlinear oscillations were also developed. Thus, for example, use was made of the method of expansion in a series in powers of a small parameter—a method developed in connection with problems of celestial mechanics by many creators of classical mechanics, from Euler and Lagrange to Poincaré. The so-called “matching” method proved very fruitful; it consists in approximating the nonlinear dependence entering into a problem by a series of rectilinear segments, which are then “stitched together” with the aid of the corresponding continuity conditions.

Alongside the work of the Mandelstam–Papaleksi–Andronov school, a broad development of mathematical methods and problems of the theory of nonlinear oscillations was also carried out in the works of N. M. Krylov and N. N. Bogolyubov, Yu. B. Kobzarev, B. V. Bulgakov, and K. F. Teodorchik.

The effective methods developed as a result of all these investigations subsequently found numerous applications both in the field of radiophy-

physics, as well as in a number of other fields far removed from radiophysics. Thus, for example, A. A. Andronov and his school extended the theory of nonlinear oscillations to problems of automatic regulation, in particular the regulation of the running of machines; with the aid of the same theory new results were obtained in the theory of clocks and in the theory of the action of an autopilot. In very recent times these methods have been successfully applied to the theory of the action of charged-particle accelerators. A more detailed characterization of this entire field, as important as it is interesting, lies far beyond the scope of the present general survey of the development of Soviet physics. Those interested will find a number of brilliant articles and lectures in the collected works of L. I. Mandelstam and N. D. Papaleksi. The extensive monograph on the theory of nonlinear oscillations, translated into foreign languages and now a classic, was written by A. A. Andronov and S. E. Khaikin.

Another field of radiophysics in which Soviet physicists worked very successfully is the theory of the propagation of electromagnetic waves, a field of the greatest importance for all kinds of radio transmission. As is known, the propagation of radio waves, which makes possible radio communication over large and small distances, is, from the standpoint of physics, determined by two phenomena—diffraction and refraction, whose relative role depends essentially on the distance, or, more precisely, on the ratio of the distance to the wavelength. The mathematical problem of radio-wave propagation is so complex that its solution required efforts on the part of such outstanding representatives of mathematical physics as A. Sommerfeld in Germany and V. A. Fock in the USSR. The occasion for the beginning of V. A. Fock’s work was that, in 1926, he discovered in Sommerfeld’s generally accepted solution, given as early as 1909, a rather substantial error, later acknowledged by the author himself. In its final form, V. A. Fock’s work, correcting a whole series of inaccuracies in earlier works, was published in 1933. A number of other problems, for example the problem of so-called coastal refraction, were solved in the works of M. A. Leontovich, G. A. Grinberg, and E. L. Feinberg.

In investigations of radio-wave propagation, interest was usually confined exclusively to the amplitudes of electromagnetic waves. L. I. Mandelstam and N. D. Papaleksi opened up a new field for fruitful research by concentrating attention on the propagation of the phase of oscillations*). As a result of a profound analysis of the peculiarities of radio-wave interference in comparison with interference in optics, they showed how the speed of propagation can be measured by means of the interference of radio waves, or the distance when the propagation speed is known. In a roughly schematic form the idea of the method is as follows: at some point \(A\) a radio station is installed, emitting in all directions undamped waves of constant amplitude. These waves, having reached point \(B\), are reflected there and return to point \(A\), where they are made to interfere with the waves emitted by station \(A\). The phase shift of the reflected wave relative to the wave emitted at \(A\) is then determined, for which purpose the interference result is observed on the screen of a cathode oscilloscope. Since the phase shift, in essence—

*) It is not without interest to note that attention to the phase of oscillations in quite another field of long electromagnetic waves, namely in the optical part of the spectrum, also led to an important discovery. This concerns phase microscopy, which makes it possible to see and study under the microscope uncolored transparent objects. The possibility of such microscopy escaped the attention of the creators of the modern theory of microscopic imaging—Abbe and Rayleigh—only because they were not sufficiently interested in the phases of the diffracted beams in the microscope.

exists, represents a time interval, measured in special units, spent on the passage of the wave “there and back,” i.e., for traversing the doubled distance \(AB\); thus, if the distance is known one can determine the propagation velocity, or, if the velocity is known, the distance. In practical implementation this scheme encounters a number of complications; for example, instead of reflection at \(B\), as a result of which oscillations arriving at \(A\) would be too weak, the wave propagating “there” controls the action of a generator placed at \(B\), whose waves, transformed in a definite way in frequency, play the role of the reflected waves. But all these complications, in which the favorable features of radio-wave interference are precisely exploited, do not change the essence of the method.

In practice the method was used both for measuring distances and for measuring the propagation velocity. For measuring distances, Mandel’shtam and Papaleksi constructed a special instrument—a radio rangefinder—with which distances of the order of 100 km are measured in 4–5 minutes with an accuracy of up to hundredths of a percent. This instrument found application in measurements at sea and in geodetic surveys. The propagation velocity of electromagnetic waves was measured in a number of expeditions; moreover, in measurements on the Black Sea, results were obtained with an error of 2–3 tenths of a percent. These measurements showed that the propagation velocity of electromagnetic waves over the sea is equal to the speed of light.

The works considered by no means exhaust the investigations of radio-wave propagation carried out in the USSR. Let us mention work on the study of the laws of propagation of waves in the meter range, carried out by B. A. Vvedenskii and collaborators. The most complete and rigorous theory of the diffractional propagation of radio waves was developed by V. A. Fok. A number of other valuable experimental and theoretical works on propagation (M. A. Bonch-Bruevich, B. A. Vvedenskii, A. N. Shchukin, and others) lie outside the scope of the present review.

We shall also confine ourselves to a brief mention of the intensive experimental and theoretical work being carried out in radio astronomy (S. E. Khaikin, I. S. Shklovskii, and others), since these works now for the most part already belong to astrophysics. An important achievement of the most recent years is the work of N. G. Basov and A. M. Prokhorov on the creation of the so-called “molecular generator,” which can be used as an absolute standard of time possessing a high degree of accuracy (according to the authors’ data—not less than \(10^{-9}\)).

Low Temperatures

Research in this field was carried out at two major centers: under the direction of P. L. Kapitsa at the Institute for Physical Problems of the Academy of Sciences of the USSR in Moscow, and at the Cryogenic Laboratory of the Physico-Technical Institute of the Academy of Sciences of the Ukrainian SSR in Kharkov.

P. L. Kapitsa’s work represents a major step forward in the creation of a new type of machine for the technical production of liquid air.

In the Linde machines that were widespread until recent times, cooling was achieved by the performance of internal work by a nonideal gas (the Joule–Thomson effect). P. L. Kapitsa took another path; the design of the machines he built is based on the principle of cooling due to the performance of external work by the gas. This principle was first applied in the helium liquefier he built in 1934,

and then—in a new type of machine for the industrial production of liquid air, where the gas performs external work by rotating, at enormous speed (40,000 revolutions per minute), a highly efficient turbine (turbo-expander). The machine for compressing air built by P. L. Kapitsa on the basis of this turbine operates at a low initial pressure (4–5 atmospheres instead of the usual 200 in Linde’s method); it combines remarkable compactness with high productivity.

Substantial successes were also achieved in solving the important problem of separating atmospheric gases and, in particular, obtaining oxygen directly from air in installations with a high coefficient of efficiency.

From a scientific standpoint, the study of the remarkable properties of helium II, which is an example of a “quantum liquid” (1938), is of outstanding interest. The peculiar behavior of helium II, paradoxically combining superconductivity with negligibly small viscosity, was fully clarified by P. L. Kapitsa’s subtle experiments, which showed that this behavior is explained not by superconductivity, but by superfluidity. This new important effect, discovered by P. L. Kapitsa, was subsequently comprehensively investigated in a number of refined experimental works carried out by P. L. Kapitsa himself and his collaborators (P. G. Strelkov, E. L. Andronikashvili, V. P. Peshkov), as well as in the cryogenic laboratory of the Ukrainian Physico-Technical Institute (A. K. Kikoin and B. G. Lazarev). An explanation of the observed phenomena was given by the theoretical works of L. D. Landau, who constructed the theory of the motion of a quantum liquid (1941–1944), which not only explained all the phenomena observed during its development, but also predicted a number of new phenomena. In particular, an interesting consequence of Landau’s theory concerning the existence in helium II, alongside ordinary or “first sound,” also of so-called “second sound” (thermal waves propagating with a velocity substantially different from that of first sound; E. M. Lifshitz) received full confirmation in the works of V. P. Peshkov.

The phenomenon of superconductivity was investigated with success; here one should especially note the theoretical works of L. D. Landau and the experimental works of A. I. Shalnikov and N. E. Alekseevsky in Moscow, and of B. G. Lazarev with collaborators in Kharkov (UFTI). Landau’s works were devoted to the nature of the transition state between the superconducting and nonsuperconducting states. According to his theory, the intermediate state is a mixture of superconducting and normal layers alternating with one another. This layered structure was fully confirmed by A. I. Shalnikov’s experiments. For the theory of superconductivity, the study of the superconducting properties of impeccably pure, extremely thin metal films proved important. Shalnikov was the first to succeed in obtaining thin metal films with thicknesses from \(5 \cdot 10^{-7}\) cm to \(3 \cdot 10^{-5}\) cm and to investigate their superconductivity (1946).

A major success in the theory of superconductivity is the microscopic theory developed very recently by N. N. Bogolyubov, the foundations of which were laid by Bardeen and Cooper.

Solid-State Physics

Important results relating to the study of the mechanical properties of solids were obtained by A. F. Ioffe and his extensive school.

From the standpoint of modern physics, a solid is a crystal. But real solids, with which we deal in technology and in

social life, in their mechanical properties differ sharply from ideal crystals. An ideal crystalline body, upon cessation of deformation, should immediately return to its original state, but in reality any deformation produces in a solid body only a slowly disappearing trace—an elastic aftereffect. How is this contradiction to be explained? First of all, the solid bodies in which aftereffect, fatigue, and other phenomena are observed are in fact not at all homogeneous crystals. A. F. Ioffe, in one of his early works, showed that in a quartz crystal no true aftereffect is observed. Thus, all phenomena occurring beyond the limit of elasticity are the result of the physical inhomogeneity of the solid body.

At a sufficiently large magnitude of deformation, a solid body begins to flow like a viscous liquid. The mechanism of such plastic deformation was also disclosed by A. F. Ioffe, who for the first time applied X-ray analysis for this purpose, observing on a fluorescent screen the Laue pattern during the stretching of rock salt. It turned out that when the tensile force passes a known limit (the yield point), the spots of the X-ray photograph suddenly split apart, then multiply and, finally, are drawn out into whole tails. This shows that the mechanism of plastic deformation consists in the fact that the integral crystal breaks up into separate small crystallites, which are displaced and rotate relative to one another. These works of A. F. Ioffe gave impetus to the development of an entirely new field (X-ray analysis of plastic deformation), to which hundreds of works carried out in all countries have been devoted. We have no possibility here of characterizing the numerous subsequent works of A. F. Ioffe’s pupils and collaborators in this direction. Let us note only two facts. First, I. V. Obreimov, by means of a refined optical method developed by him, showed that shifts along definite crystallographic planes begin long before the appearance of distortions in the X-ray pattern. Second, A. F. Ioffe discovered, and M. V. Klassen-Neklyudova investigated in detail, a completely new effect: the discontinuity of the deformation process. Under a continuously applied load, deformation proceeds in jumps, repeated at astonishingly equal intervals of time and accompanied by a faint sound reminiscent of the ticking of a clock. This phenomenon was studied by a whole series of Soviet scientists (N. N. Davidenkov, A. V. Stepanov), as well as abroad. Its theory was given by N. Davidenkov and M. Klassen-Neklyudova.

A large number of works, also carried out in other laboratories, were devoted to the mechanical properties of solid bodies. Let us note here the numerous works of V. D. Kuznetsov and his collaborators. They developed convenient methods for measuring hardness and other mechanical constants of solid bodies, studied the influence of various factors on the elastic limit, the surface energy of solid bodies, etc. The results of all these works were brought together by V. D. Kuznetsov in the monograph Physics of the Solid Body.

So significant a number of works by Soviet physicists was devoted to the application of X-ray analysis to the study of the mechanical properties of solid bodies that we have no possibility here either of characterizing these works or of naming their authors. We shall note only a few works that were of a pioneering character. S. T. Konobeevsky and N. E. Uspensky are responsible for the first detailed work in the literature on the application of X-ray analysis to the study of the internal mechanism of metal-working processes (rolling).

N. Ya. Selyakov and G. V. Kurdyumov were the first to show that hardened steel has a crystal lattice different from that of iron. In the physics of metals—physical metallurgy—the application of X-ray structural methods acquired the greatest importance. Particularly varied and extensive investigations in this field belong to G. V. Kurdyumov and his numerous collaborators. These works, important from both the scientific and the practical points of view, were devoted to the structure of alloys, the nature of hardening and tempering, and other problems of the most immediate interest and great significance for metallurgy.

Another direction of work in this field, associated with ideas that received worldwide recognition from N. S. Kurnakov’s school (N. V. Ageev with collaborators, and others), is by its nature closer to physical and inorganic chemistry, and therefore lies outside the scope of our article.

With the aid of X-ray structural analysis the structure of many silicates has been determined and general considerations have been developed concerning the role of close packing in the structure of inorganic compounds (N. V. Belov); new data have been obtained concerning the structure of organic crystals (A. I. Kitaigorodskii); systematic work on the application of X-ray structural analysis to theoretical chemistry has been carried out by G. S. Zhdanov.

One must also note the great educational work done in this field. X-ray diffraction analysis is at present an indispensable support for factory laboratories. An X-ray laboratory at a plant, especially at a metallurgical plant, performs a very important function. Thanks to the development in our country of work in the field of X-ray diffraction analysis, factory X-ray laboratories have received cadres of trained workers, and substantial work has been done in the very organization of these laboratories and in propagating their importance in production.

As an independent method of structural analysis, electron diffraction analysis has undergone great development (V. E. Lashkarev, Z. G. Pinsker, N. A. Shishakov, B. K. Vainshtein, and others), with the aid of which the structures of a number of inorganic and organic compounds have been determined. A detailed survey of Soviet works in this field was given by Z. G. Pinsker in the monograph Electron Diffraction (Moscow, 1949).

The works of A. F. Ioffe on the study of the strength of solids attracted very great attention in the USSR and abroad. The theory of the crystal lattice, developed by Born, made it possible to calculate what stresses are necessary for the rupture of crystals. These calculated values of the stresses proved to be many times greater than those actually observed. Thus, rock salt theoretically should withstand stresses up to \(200\ \text{kg}/\text{mm}^2\), whereas in reality it ruptures under a load of only \(400\ \text{g}/\text{mm}^2\). A. F. Ioffe showed that this discrepancy is explained by the properties of the crystal surface. By immersing a crystal of rock salt in warm water, he achieved a strengthening of it by a factor of 10–12. A. F. Ioffe explained this strengthening by the dissolution of the surface layer and thereby the elimination of surface cracks that greatly lower the strength limit. These works made a great impression throughout the world and provoked lively discussion, as a result of which both A. F. Ioffe’s experimental results and his theoretical premises were confirmed.

Let us note, further, the works of P. A. Rebinder, who showed what an enormous role surface conditions play in the plastic properties of a specimen: the introduction onto the surface of a negligible amount of surface-active substances lowers the yield point many times over.

The works of P. P. Kobeko, A. P. Aleksandrov, E. V. Kuvshinskii, and others, on the basis of extensive experimental material, led to broad generalizations and to a new understanding of the elastic-relaxation properties of high-molecular compounds of the rubber type.

Among works more closely adjoining crystallography, we shall note the work of A. V. Shubnikov, who pointed out the important role of the concept of antisymmetry in the description of the properties of crystals. To him also belongs a number of achievements in the field of the theory and experimental realization of crystal growth. The work of the Institute of Crystallography of the Academy of Sciences of the USSR, headed by A. V. Shubnikov, on the growing of single crystals supplied domestic industry with corundum crystals needed for the production of watches, with piezoelectric, semiconductor, and other crystals.

The highest-precision works of P. G. Strelkov were devoted to the thermal properties of solids. V. V. Tarasov succeeded in finding a valuable formula for the heat capacity of layered lattices.

Physics of Dielectrics

In this field, substantial results also belong to A. F. Ioffe’s school. Already in his early works, carried out jointly with V. K. Röntgen, A. F. Ioffe showed that, although the electrical conductivity of dielectric crystals is chiefly ionic in character, the photoconductivity of X-irradiated or naturally colored rock salt is due to electrons. In this connection, the complicated picture observed when current passes through a dielectric is mainly due to space charges accumulating in different places.

Thus, for example, the decrease in current strength with time, observed in most dielectrics, is explained by the occurrence of a counter electromotive force caused by space charges accumulating near the electrodes. The presence of an insignificant quantity of foreign impurities leads to the accumulation near them of ions that create a space charge and complicate the picture of current passage.

Much attention was devoted to the problem of electrical strength. The theoretical works of V. A. Fock, devoted to the then dominant so-called thermal theory of breakdown, and the works of N. N. Semenov directly connected with them clarified the limits of applicability of this theory and showed that under ordinary conditions, at room temperature, the mechanism of breakdown must be different. In view of this, A. F. Ioffe developed a theory of breakdown based on the idea of avalanche ionization. Although the hopes for the possibility of obtaining “thin-layer insulation,” which arose in connection with this idea and with the study of so-called “high-voltage polarization” in dielectric crystals, were not justified, the extensive work on the study of the dielectric properties of crystals and thin films and of dielectric breakdown yielded many valuable results. Fruitful, in both scientific and practical respects, investigations of the dielectric properties of amorphous bodies were carried out by P. P. Kobeko, A. P. Aleksandrov, S. N. Zhurkov, and others.

Closely connected with the investigation of dielectrics and semiconductors is the study of the so-called ferroelectrics. A typical representative of them is Rochelle salt, distinguished by an anomalously high value of dielectric permittivity and by the presence of hysteresis—properties characteristic of ferromagnetic substances, in view of which Rochelle salt and substances analogous to it have recently often been called ferroelectrics. Thorough experimental and theoret—

... the systematic study of the properties of Rochelle salt was first carried out in 1930–1932 by I. V. Kurchatov, B. V. Kurchatov, and P. P. Kobeko.

In 1945 B. M. Vul and I. M. Goldman discovered a new ferroelectric—barium titanate—which differs in many favorable properties from Rochelle salt itself and from crystals isomorphous with it. This discovery served as an impetus for numerous studies of ferroelectrics of a new type; moreover, it turned out that barium titanate is only one of many ferroelectrics of this type. Recently, ferroelectric materials have been finding ever wider application in various branches of technology.

Physics of Semiconductors

The study of semiconductors has in recent times come to the fore as a most important scientific and scientific-technical problem. From the point of view of electrical properties, semiconductors comprise the broadest class of bodies found in nature. Numerous technical devices are based on the properties of semiconductors—rectifiers, photocells, thermistors, semiconductor triodes (transistors), and a number of others. Interest in the physical properties and technical applications of semiconductors is so great that in recent times this field has begun to compete with nuclear physics, which has attracted the greatest attention. Soviet physicists have a number of achievements in this field. Especially valuable results were yielded by the works of A. F. Ioffe, A. V. Ioffe, Ya. I. Frenkel, V. P. Zhuse, B. E. Lashkarev, S. I. Pekar, S. G. Kalashnikov, and others.

At the basis of the physical understanding of the properties of semiconductors lies—despite all its shortcomings and limitations—the band theory of crystals. Investigation of the nature of current carriers in semiconductors (\(\mathrm{Cu_2O}\), \(\mathrm{Se}\), \(\mathrm{V_2O_5}\), \(\mathrm{Cu_2S}\), etc.) led to the idea of “electron” and “hole” mechanisms of conduction and made it possible clearly to divide the semiconductors investigated into these two classes—“electron” and “hole” semiconductors. V. P. Zhuse, B. V. Kurchatov, B. M. Tohberg, and others studied in detail the mechanism of conduction as a function of various factors.

Also connected with the band theory of crystals are works devoted to the so-called theory of “polarons.” An electron moving slowly in the conduction band of an ionic crystal is in a field whose potential is caused by the polarization of the medium, and in turn the Coulomb field of the electron acts on the polarization of the medium. During the slow motion of the electron, the state of polarization it creates moves together with it, i.e. equilibrium has time to be established between the electron field and the polarization of the medium, while the regions left behind by the electron return to their initial state. Such formations, consisting of the electron and the field in equilibrium with it, moving in an ionic crystal, are called polarons. The theory of polarons was developed in detail by S. I. Pekar. A mathematically more exact solution of the problem of the slow motion of an electron in a polarized medium was given by N. N. Bogolyubov and S. V. Tyablikov.

The study of the processes of absorption of light by crystals led Ya. I. Frenkel to develop the hypothesis of the existence of special states of excitation, moving through a crystal but not connected with the transfer of electric charge. Such states, called by Ya. I. Frenkel excitons, i.e. excitations, are systems consisting of an electron and a positive...

“holes”; in their optical properties they are analogous to the hydrogen atom. This hypothesis proved to be extraordinarily fruitful and is finding ever new applications for explaining the electrical and optical properties of crystals. One of the most striking results of investigations carried out recently is the experimental discovery by E. F. Gross and his collaborators of a “hydrogen-like” spectrum in certain crystalline semiconductors, which is regarded as experimental proof of the reality of excitons.

Processes occurring at the contact of two different semiconductors, or of a semiconductor with a metal, have also been subjected to thorough investigation (A. F. Ioffe, A. V. Ioffe, the theoretical works of T. I. Bliokhintsev, B. I. Davydov, S. I. Pekar), as have phenomena on the surface of a semiconductor. In particular, I. E. Tamm showed that special “surface” energy levels must arise on the external surface of a crystal, playing a major role in the interpretation of various complex phenomena in real semiconductors.

The discovery of so-called cyclotron resonance, which has attracted attention in recent years, was predicted by Ya. G. Dorfman.

Along with achievements of a scientific nature, a number of important practical achievements should also be noted (Institute of Semiconductors of the Academy of Sciences of the USSR and the Physical Institute of the Ukrainian Academy of Sciences): the development of new photocells with a blocking layer (thallous sulfide and silver sulfide), which exhibit high current yields, and the development of rectifiers. Semiconductor thermoelements (Institute of Semiconductors of the Academy of Sciences of the USSR) found application in the construction of thermobatteries, successfully used as current sources for radio receivers. The inverse effect—thermoelectric cooling—is being used with lesser success in semiconductor refrigerators developed by the Institute of Semiconductors, which have shown “the real possibility of creating a thermoelectric refrigerator in an industrial design, more economical than absorption-type refrigerators”*).

Electronics

The study of the external photoelectric effect served as the basis for the development of the quantum theory of light. In view of this, a rigorous test of Einstein’s fundamental photoelectric equation was of great importance in principle. This problem was solved by P. I. Lukirskii and S. S. Prilezhaev with the aid of an ingenious method of the spherical capacitor, developed by P. I. Lukirskii. The same investigators obtained the most accurate value of Planck’s universal constant. The quantum-mechanical theory of the external photoelectric effect was first developed, in 1931, by I. E. Tamm and S. P. Shubin. Soviet scientists also carried out numerous investigations of photocells with the external photoelectric effect—oxygen–cesium ones (P. V. Timofeev) and antimony–cesium ones (P. I. Lukirskii, S. A. Vekshinskii, S. Yu. Lukyanov, N. S. Khlebnikov, N. D. Morgulis, and others).

The photomultiplier, which at present has the broadest applications in measurement technology, was first invented and constructed in the USSR by L. A. Kubetskii.

The internal photoelectric effect in dielectrics and semiconductors also served as the subject of a large number of important works (A. F. Ioffe and

) A. F. Ioffe, A. S. Stilbans, E. K. Iordanishvili, T. S. Stavitskaya, Thermoelectric Cooling*, Academy of Sciences of the USSR Press, 1956, p. 87.

A. V. Ioffe, P. S. Tartakovskii, and others). An interesting photomagnetic effect was discovered by I. K. Kikoin and M. M. Noskov: when a cuprous-oxide crystal placed in a transverse magnetic field is illuminated, an electromotive force of the order of 1 V arises.

After the discovery of electron diffraction, Soviet physicists carried out a number of pioneering works (P. S. Tartakovskii, B. E. Lashkarev, A. I. Alikhanian). In the field of electron optics, important theoretical works belong to G. A. Grinberg, who developed (1942) a general theory of the focusing of electrons in electric and magnetic fields.

L. A. Artsimovich considered the electron-optical properties of so-called emission systems, containing objects that are sources of slow electrons. The result of work in the field of experimental electron optics was the creation by A. A. Lebedev and his collaborators of a domestic model of the electron microscope.

Finally, the problem of electrical conductivity in gases has in recent years acquired outstanding interest from both the theoretical and the applied points of view. On the one hand, major successes in the study of the structure of matter and the development of the most refined experimental methods made it possible to pose the question of the elementary processes in a gas discharge; on the other hand, the role of the gas discharge in various electrovacuum devices (amplifiers, thyratrons, gasotrons, etc.) and the intensive development of questions connected with the design of new economical light sources drew attention to the study of the macroscopic characteristics of the gas discharge. Among the works of Soviet physicists in this field we shall note the works of N. A. Kaptsov (ignition of a gas discharge, corona), G. V. Spivak (the role of metastable atoms in a gas discharge, probe theory, accommodation coefficients), N. D. Morgulis (cathode sputtering), and V. L. Granovskii (plasma of a gas discharge). Among works connected with the problem of constructing economical gas-discharge lamps, it is necessary to note the works of V. A. Fabrikant (radiation of a discharge in metal vapors), B. N. Klyarfeld, and A. M. Shemaev.

Magnetism

In the field of the doctrine of magnetism we shall note first of all three fundamentally important works relating to the magnetism of elementary particles. In 1920, a year before the publication of the well-known work of Gerlach and Stern (1921), P. L. Kapitsa and N. N. Semenov developed a project for a method of determining the magnetic moment of the atom by measuring the deflection of an atomic beam in an inhomogeneous magnetic field, with a complete calculation of the apparatus. This is precisely the method by means of which Gerlach and Stern, independently of the Soviet scientists, experimentally proved the existence of spatial quantization*).

Another fundamentally important work relating to the magnetism of elementary particles belongs to L. D. Landau, who showed that a gas of free electrons must exhibit a diamagnetic effect,

) In view of the fact that this work of Kapitsa and Semenov, owing to accidental historical circumstances, remained little known, we give here the exact reference with the title of the work. The work was published in English in 1922, but was dated as early as December 1920: “On the possibility of an experimental determination of the magnetic moment of an atom,” Journal of the Russian Physico-Chemical Society, Physical Section*, vol. 50, nos. 4–6, 1922.

No less important a work was carried out and published in 1937 by B. G. Lazarev and L. V. Shubnikov (Kharkov). As is well known, the spin paramagnetism of the electron, due to its magnetic dipole moment, is a weak phenomenon. But nuclear paramagnetism is still weaker, since it is determined by the intrinsic magnetic moments of the proton and neutron, which in order of magnitude are about 1000 times smaller than the magnetic moment of the electron. Therefore, when Lazarev and Shubnikov succeeded in detecting and measuring by direct experiment (with the aid of a magnetic balance) the nuclear susceptibility of solid hydrogen cooled to 2°K, this result could be placed on a par with the best records of experimental technique. It is not without reason that one of the most prominent contemporary specialists in the field of nuclear magnetism, the American physicist Purcell, called the experiment of Lazarev and Shubnikov a “triumph of physical experiment”*).

Let us note that the conclusion, seemingly paradoxical at first glance, about the existence of a magnetic moment in the neutron was first made (1934) by I. E. Tamm and S. A. Al’tshuler, who correctly estimated both the sign and the value of this moment.

One of the most brilliant achievements in physics during the last 20 years was E. K. Zavoisky’s discovery in 1945 of the so-called “paramagnetic resonance” (at present it is more often called electron spin resonance, to distinguish it from nuclear magnetic resonance). The phenomenon consists in the fact that atoms or molecules possessing “unpaired” electron spins and oriented, according to the rules of spatial quantization, by a strong constant magnetic field, reorient their magnetic moments under the action of the electromagnetic wave’s weak alternating magnetic field perpendicular to the constant field. In this case, when there is resonance between the frequency of the electromagnetic field and the proper frequency of the atoms or molecules in the solid paramagnet, a sharply pronounced selective absorption of the electromagnetic wave arises. Calculation shows that, for a constant field strength of 10,000 oersteds, resonance should be observed at frequencies of electromagnetic waves corresponding to the centimeter range, i.e., the range of waves used in radiolocation technique.

This discovery, awarded in the USSR with the V. I. Lenin Prize in 1956, attracted extraordinarily great attention, and in recent years it has begun to acquire ever broader applications as an analytical method of unusual sensitivity. In the USSR, intensive development of the method of paramagnetic resonance and of its applications is being carried out in the Kazan Branch of the Academy of Sciences of the USSR under the direction of B. M. Kozyrev and S. A. Al’tshuler. Analogous phenomena of electron resonance should be observed not only in paramagnets, but also in ferromagnets. Experimentally, ferromagnetic resonance was also discovered by E. K. Zavoisky (and independently by Griffith in the USA). However, the theory of ferromagnetic resonance had already been developed in 1935 by L. D. Landau and E. M. Lifshitz, while the very idea of magnetic resonance was advanced and substantiated in 1923 by Ya. G. Dorfman, who pointed to the significance of the selective absorption of electromagnetic waves in iron wires discovered by V. K. Arkad’ev as early as 1913 and then studied in the 1920s, which Arkad’ev called “magnetic spectra.”

The most important problem in the theory of magnetism is the explanation of the strong susceptibility of iron and other ferromagnets. Already at the be—

) Purcell, Nuclear Magnetism, Amer. Journ. of Physics, 22*, 1, 1954.

by the beginning of the twentieth century it had become clear that the cause of this strong magnetization must be a special molecular field; but the physical nature of this field could not be satisfactorily explained within the framework of classical physics. On the one hand, experiments with gyromagnetic phenomena (the Einstein–de Haas experiment, the Barnett experiment) showed that the carriers of the elementary magnetic moment (“elementary magnets”) in a ferromagnet are electron spins; on the other hand, the magnetic interactions between electrons are too weak for them to explain the nature of the molecular field. This riddle was qualitatively resolved by Ya. I. Frenkel’s brilliant idea, which pointed out that, owing to the quantum-mechanical exchange effect, electrons must experience interactions far stronger than purely magnetic ones, of the order of electrostatic interaction. In this case the energetically most favorable state may be one in which the electron spins are arranged in parallel. As is well known, the same idea underlies the quantum theory of ferromagnetism developed later by Heisenberg.

A number of important works on the theory of ferromagnetism were carried out by S. V. Vonsovsky, N. S. Akulov, E. I. Kondorsky, and their collaborators. Magnetic methods of quality control of metals (magnetic flaw detection) have found wide application in industry. The basic works in this direction belong to V. K. Arkad’ev and R. I. Yanus.

Acoustics

The most diverse branches of acoustics—from the general theory of the acoustics of a moving medium to problems of architectural acoustics and practical methods of precise measurement of acoustic quantities—are the subject of the works of N. N. Andreev. He created a large school of Soviet acousticians, who are developing theoretical questions of sound propagation in inhomogeneous and stratified media with boundaries (L. M. Brekhovskikh); questions of so-called sound optics, pertaining to the refraction and focusing of sound waves (L. D. Rozenberg); problems of the sounding of open spaces and rooms (Yu. M. Sukharevsky and B. D. Tartakovsky); and questions of noise acoustics (V. S. Grigor’ev). Extensive studies were carried out in 1930–1940 in the field of musical acoustics by a group of collaborators of the Scientific Research Institute of the Musical Industry (A. V. Rimsky-Korsakov, A. A. Kharkevich, V. P. Konstantinov, N. A. D’yakonov, A. I. Belov, I. G. Rusakov, P. A. Matveev, and others). These works dealt with the physics of musical instruments, their materials, construction, and acoustic properties. In architectural acoustics a series of works was performed in connection with the design of the Palace of Soviets. S. N. Rzhevkin and G. D. Maluzhinets carried out a number of theoretical and experimental investigations on special resonance absorbers, which made it possible successfully to solve the problem of sound absorption in very large halls. In the field of ultrasonics one should note the works of P. A. Bazulin on the absorption of ultrasonic waves in liquids and the works of S. Ya. Sokolov, who created an original method of ultrasonic flaw detection.

III. ORGANIZATIONAL QUESTIONS. SCIENTIFIC LITERATURE

The cadres of physicists have grown to an enormous extent. Before the revolution, the number of physicists in Russia engaged in scientific work did not exceed 100 persons; now we have thousands of physicists, among them several hundred doctors. In place of a small number of very modestly equipped

laboratories attached to university departments, we have the largest scientific-research institutes and many hundreds of specialized technical institutes, in each of which physical work is being carried out to one degree or another. The wealth of equipment in our institutes has repeatedly been noted in recent years by foreign scientists visiting the USSR to take part in scientific conferences. As a concrete example one may mention that the value of the equipment transferred free of charge by the Soviet Government to the Joint Institute for Nuclear Research was estimated at about half a billion rubles. Although, in connection with the organization of a large number of powerful research institutes, the center of gravity of all the leading work is naturally concentrated in these institutes, active research work is also being carried out in the departments of universities and other higher educational institutions. The decree of the Council of Ministers of the USSR of April 12, 1956, “On Measures for Improving Scientific-Research Work in Higher Educational Institutions,” opens up new possibilities for the laboratories attached to departments.

Congresses and conferences play a major role in the scientific life of the country. Before the revolution in Russia, the “Congresses of Russian Naturalists and Physicians” met more or less periodically, with physics assigned a single section. In 1908, at such a congress, Lebedev reported with triumph on his work on the pressure of light on gases. After the revolution, beginning with the first congress of 1918 mentioned at the start of the article, congresses of physicists with several sections began to be convened in the twenties and early thirties. The number of participants in them grew steadily; almost all congresses were attended also by major foreign scientists (M. Born, J. Franck, C. Darwin, P. Debye, R. Pohl, and others), who gave papers and took part in discussions. Finally, in the postwar period the volume of work and, correspondingly, the number of participants and papers increased to such an extent that the convening of general congresses of physicists became impractical. The congresses were replaced by thematic conferences: on semiconductors, spectroscopy, luminescence, low temperatures, various nuclear problems, etc. The number of participants in these conferences is also growing continuously, and at some conferences it has become necessary to organize two, and sometimes even more, sections. To give an idea of the scale, let us cite one figure: the spectroscopy conference held in Lvov in the summer of 1956 brought together about 1,500 members; no smaller a number of members was gathered by the conferences on semiconductors and on the physics of high-energy particles. In recent years the participation of foreign physicists in the work of these conferences has become a usual phenomenon.

Russian scientific literature on physics before the revolution was extraordinarily poor. A few excellent textbooks, such as Stoletov’s Introduction to Acoustics and Optics, Eichenwald’s Electricity, the university lectures on physics by N. A. Umov, and also a number of lithographed lectures by professors, made it possible for junior students to study physics. But for a deeper study of this science, in particular for the study of theoretical physics, it was absolutely necessary to resort to monographs and serious manuals in foreign languages. As an exception one may name only O. D. Khvolson’s five-volume Course of Physics, which played a major role in raising the level of culture in the field of physics in Russia and was translated into German and French, but which, nevertheless, had rather the character of an encyclopedia, with the shortcomings inherent in publications of that kind.

From the very beginning of the Revolution, the publication of scientific, educational, and popular-science literature was placed on an entirely different, immeasurably broader scale. As a result, over forty years the literature on physics has grown so much that not only for special university courses, but also for candidate examinations, and likewise for an in-depth study of various problems, serious monographs and manuals in Russian can be indicated on almost all questions. Educational and popular-science literature in the languages of the peoples of the USSR is also gradually developing. A number of Soviet manuals and monographs have been translated into foreign languages.

A survey of Soviet physical literature could form the subject of a special article. Here we shall confine ourselves to a few examples. The unique multivolume course of theoretical physics by L. D. Landau and E. M. Lifshitz occupies an entirely special place in the world literature on physics. In no language is there a higher-level course of theoretical physics that combines such completeness of coverage of all physics with complete originality: the entire course is written from a single point of view and is the complete opposite of the eclectic courses usual on such a scale. Several volumes of Landau and Lifshitz’s Theoretical Physics have been translated into English.

The numerous manuals on almost all sections of theoretical physics and monographs on special questions written by Ya. I. Frenkel were also distinguished by originality, although some of them were controversial. Almost all of them appeared in translations into German and English. V. A. Fock is the author of a manual on quantum mechanics and an original monograph on the general theory of relativity. The courses Foundations of the Theory of Electricity by I. E. Tamm, Statistical Physics and Introduction to Thermodynamics by M. A. Leontovich played a major role in creating a high level in the teaching of theoretical physics. D. I. Blokhintsev’s Foundations of Quantum Mechanics combines pedagogical merits with originality in the treatment of the philosophical problems of quantum mechanics; the book appeared in German translation. Let me also mention my two-volume Atomic Physics, which gained wide circulation in the USSR and far beyond its borders (it has been translated into seven foreign languages).

The important problem of creating fully modern courses in general physics was also successfully solved by Soviet authors. Among the large number of textbooks published over the past forty years we shall mention the widely circulated and foreign-language-translated three-volume university course by S. E. Frish and A. V. Timoreva, which is the result of many years of teaching by the authors at Leningrad University. The experience of teaching physics at Moscow University was used in the multivolume General Course of Physics, each part of which has independent interest and significance. The volumes that have appeared—Mechanics by S. E. Khaikin), as well as Mechanics by S. P. Strelkov, Optics by G. S. Landsberg, and Electricity* by S. G. Kalashnikov—are distinguished by great freshness, originality, and careful didactic thought.

A number of valuable monographs of great scientific significance were issued during the past period. The monograph by A. A. Andronov and S. E. Khaikin on nonlinear oscillations has already been mentioned. Let us mention Physics of Crystals and Physics of Semiconductors by A. F. Ioffe, *The Modern Doctrine

*) Certain parts of this book at one time drew criticism from a philosophical point of view; this, however, does not diminish the pedagogical value of this original book as a whole.

On Magnetism” by S. V. Vonsovskii, the two-volume monograph Molecular Vibrations by M. A. Elyashevich, M. V. Vol’kenshtein, and B. I. Stepanov. D. V. Skobel’tsyn’s Cosmic Rays gave a complete picture of the state of the question up to the time of the book’s publication (1936); a new monograph on the same subject has been written by N. A. Dobrotin. We shall confine ourselves to these examples, although the list could, of course, be continued.

Translations of the most important foreign textbooks and monographs also enriched Soviet literature. Abraham—Becker’s Theory of Electricity, Drude’s Optics, Wood’s Physical Optics, Courant—Hilbert’s Methods of Mathematical Physics, Mie—Franck’s Mathematical Physics, and many other books in good translations are now easily accessible to the Soviet student and research worker. A large number of fundamental scientific monographs by foreign authors have been issued during the last ten years by the Foreign Literature Publishing House, whose activity (the heads of the physics editorial office are A. A. Sokolov and A. A. Gusev) deserves great gratitude. Let us mention the series of monographs on spectroscopy by Condon and Shortley, Herzberg, Harrison, Luftburrow and Lord; fundamental monographs on nuclear physics (the two-volume Experimental Nuclear Physics edited by E. Segrè, Nuclear Physics by E. Fermi, and others), monographs on semiconductors, ultrasound, piezoelectricity, and many other current problems of physics.

Finally, the publication of the classics of science, issued with great affection by the State Technical-Theoretical Publishing House and the Publishing House of the Academy of Sciences, played a major role in raising culture in the field of physics. Over the past forty years, the works of Archimedes, Stevin, Galileo, Newton, Huygens, Lagrange, Gilbert, Leonardo da Vinci, Faraday, Helmholtz, Mayer, Lomonosov, Lebedev, Stoletov, and a number of other classics of our science have been issued in excellent translations and handsome editions. In this important work of illuminating the history of physics, especially great are the merits of the president of the Academy of Sciences, S. I. Vavilov, who died too early; he knew and loved the history of science profoundly and enriched our literature with translations and commentaries of all of Newton’s optical works and with works on the optical writings of Galileo and Lomonosov.

Finally, the collected works of outstanding Russian scientists undertaken by the Academy of Sciences are also very valuable (Mandelstam, Papaleksi, Vavilov, Lazarev).

Periodical publications intended for the publication of original works and reviews also grew significantly. Before the Revolution, there existed only one journal for the publication of original physical papers: the Journal of the Russian Physico-Chemical Society. Physical Section. Its small size was sufficient for publishing 30–40 papers a year, which practically exhausted at that time the original scientific output in physics. The second part of this journal, also issued separately under the title Problems of Physics, published review articles and was of very modest scope.

After the Revolution, the periodicals designated for the publication of original papers were the Journal of Experimental and Theoretical Physics and the Journal of Technical Physics. The Bulletin of the Academy of Sciences. Physical Series had as its task chiefly the publication of the proceedings of scientific conferences. Brief communications on the most important works were published in the Reports of the Academy of Sciences. In recent years, the output of Soviet institutes and laboratories has grown so much that the publications listed proved wholly insufficient for

“timely publication of works. In view of this, the publication of a whole series of new specialized journals was undertaken. These are: Optics and Spectroscopy, Crystallography, Electronics, Atomic Energy, Physics of Metals, Methods and Techniques of Physical Experiment, Acoustical Journal, and others. As a matter of self-criticism, it should be noted that even this greatly increased number of journals cannot cope with the flow of papers arriving at the editorial offices, and their publication is delayed for long periods.

The means of scientific information about new works and about the current state of topical problems in physics have also grown to an enormous extent. In 1918 a new review journal, Uspekhi Fizicheskikh Nauk (Advances in the Physical Sciences), began to appear; since then it has been published continuously and did not cease publication even in the difficult days of the Civil War and the Great Patriotic War. This journal enjoys wide renown in the USSR and abroad. The review articles by Soviet physicists published in it are widely cited in the world scientific literature, and many of them are translated into foreign languages. In 1953 the Academy of Sciences created a powerful Institute of Scientific Information, which publishes abstract journals in various branches of knowledge, including, of course, physics. The abstract journal Fizika (Physics), in the completeness of its coverage of the world literature on physics, far surpasses the analogous foreign abstract journals Physikalische Berichte and Physics Abstracts. Whereas these foreign journals cover 15–17 thousand publications per year, the journal Fizika in 1956 provided more than 36 thousand abstracts and bibliographic descriptions. Moreover, it should also be borne in mind that, for example, works on mechanics are abstracted in Physikalische Berichte, while in the USSR a special journal, Mekhanika (Mechanics), is published for abstracting these works. The same applies to astrophysical works, which are abstracted in the journal Astronomiya (Astronomy).

IV. CONCLUSION

We can now draw the conclusion and return to what was said at the beginning of the article. The preceding, far from complete, outline of the development of physics in the USSR over 40 years speaks for itself: the “contributions to the domain of knowledge” (to repeat the words of N. A. Umov cited at the beginning) by Soviet physicists have more than repaid our country’s debt to the history of science. Indeed, the works of our physicists cover all areas of this science, and there is no branch to which these works have not contributed much that is substantially new. Important discoveries made in our country have opened new paths in science, created new directions that are being developed to a considerable extent by Soviet scientists and, of course, also by scientists of foreign countries. Today, in almost every conscientious review, Soviet theoretical and experimental works play a noticeable, and in some cases a leading, role.

But this is not all. Whereas in pre-revolutionary Russia physics was practically wholly concentrated in university departments, the number of which, for a vast country, was very small, and the subject matter of research took shape quite spontaneously, after the Revolution the center of gravity of research work was transferred to hundreds of large and small research institutes, where a scientific worker could and was obliged to devote all his time to research work, being in every respect provided for by the state; naturally, there arose a need for the planned development of science. This idea of planning in the subject matter of scientific research was initially met with distrust—

L. I. MANDELSTAM.

S. I. VAVILOV.

steadily; it seemed to contradict the creative character of scientific research; the most expedient forms of planning were not found at once. Yet these difficulties made themselves felt only at the very beginning, and the planned development of science soon justified itself. Over the past years it has been an advantage of Soviet science. Moreover, the importance of the systematic development of science, given the colossal role that science now plays in the life of the state, has been understood abroad as well, though not immediately.

Another advantage of the organization of science in the USSR is the close connection between science and the demands of life, with the needs of production. Already the organizers of the first physical research institutes in the USSR—D. S. Rozhdestvenskii, A. F. Ioffe, and P. P. Lazarev—well understood the importance of linking physics with the demands of life, since this connection not only meets the needs of the state but also nourishes science itself with new problems and ideas. In our day, after the Twentieth Congress of the CPSU, which set before scientists the task of steadily raising the role of science in solving the practical tasks of communist construction, this connection between the development of physics and the needs of industry, agriculture, public health, and Soviet culture is increasing sharply.

And, finally, Soviet science in general, and physics in particular, is strong in its connection with the people. Science has ceased to be the affair of a small group of people; an enormous number of talented people have “entered science” from among the workers and peasants. And, only slightly paraphrasing the words of D. I. Mendeleev, taken as the epigraph to this article, we may say with full justification: “Truly, the scientific sowing has sprung up for the people’s harvest!”