On the Thirtieth Anniversary of Soviet Physics

Paths of Development of Theoretical Physics in the USSR

D. I. Blokhintsev

“In science there is no broad high road, and only he can attain its shining summits who, unafraid of fatigue, climbs along its stony paths.”

If these words of K. Marx are applicable to any scientific achievement, under any conditions, then in the conditions of old Russia the path along these trails was immeasurably burdened by the oppressive social conditions and technical backwardness, which were reflected especially unfavorably in the development of experimental, and at the same time theoretical, physics.

Russian thought was always rich in ideas, but these ideas by no means always received proper development and worthy recognition. In the “prison of nations” both people and ideas withered and perished. Only a few isolated individuals managed to make their way along this stony path.

Among such people was, for example, N. A. Umov, a brilliant thinker and public figure, whom we must regard as one of the first theoretical physicists in Russia, who created a remarkable doctrine—revolutionary for its time—of the motion of energy. Besides N. A. Umov, there were at that time other physicists as well who pursued theoretical physics as their specialty.

Among them was the talented S. A. Boguslavskii, who died early and who examined the classical dynamics of the electron with exhaustive completeness. In his time N. P. Kasterin successfully worked in theoretical acoustics. Nevertheless, no theoretical schools were created in the pre-revolutionary period.

The richest possibilities hidden in the peoples of our country could not reveal themselves in all their fullness and strength.

Trips by scientists “abroad” were a common thing, a forced step that no physicist wishing to ... could avoid.

to study modern physics in order later to develop it in their own country. Countless are the names of those who sacrificed their scientific interests to their social principles.

Because of these difficult conditions, the powerful development of theoretical physics in the first two decades of our century, expressed in the creation of the theory of relativity and quantum theory, took place without the participation of Russian science.

The country was late for the time when the foundation of modern theoretical physics was being laid. Our scientists entered the field later, from the time when the October Revolution liberated the forces of the people.

Theoretical physics as an independent discipline in our country begins to develop only under Soviet power.

The twenties, marked by the discovery of quantum mechanics, were years of remarkable achievements for theoretical physics. The application of the ideas of quantum mechanics and the theory of relativity to the world of atomic phenomena, like a magic key, opened now one door, now another in the secret chambers of the microworld.

It was precisely in this period that Soviet scientists, with all their passion, first entered the field of theoretical physics.

At the Leningrad Physico-Technical Institute, headed by A. F. Ioffe, a group of theorists was organized, most of them at that time young, beginning scientists.

Somewhat later a group arose at Moscow State University, where the theorists were concentrated around L. I. Mandelstam, who was not only an outstanding and profound scientist, but also a true teacher of many theorists.

One of the most remarkable physicists of our time—P. S. Ehrenfest, who had previously worked in Petersburg and belonged to the number of people who had found no place for themselves in tsarist Russia—exerted a great and fruitful influence on the development of theoretical physics, especially in Leningrad. P. S. Ehrenfest also visited Moscow University, and I remember his words addressed to the students of MSU: “I have never anywhere met young people who strove toward science with such enthusiasm as I see here, in Soviet Russia.”

In this same period, lively discussions on the foundations of the scientific world outlook took place among theoretical physicists. At that time the influence of bourgeois ideology was still great among our scientists.

These discussions were the beginning of those battles which were waged by the philosophy of dialectical materialism—liberated from the fetters of censorship—against reactionary, antiscientific idealism and against mechanistic materialism, which discredited materialism.

Especially sharp disputes took place around the fundamental concepts and representations of modern theoretical physics.

The concepts of force and energy, action at a distance and action at close range, the origin of statistical regularity, the law of conservation of energy, philosophical problems connected with the interpretation of quantum mechanics and the theory of relativity, and many other questions relating to the very foundations of modern theory were the subject of discussions and polemical debates.

Soviet theorists more than once appeared in the Soviet philosophical press with a critical analysis of idealist currents in contemporary bourgeois physics and philosophy, which juggle the concepts of modern science. This ideological work by physicists, in which S. I. Vavilov, A. F. Ioffe, M. A. Markov and many others took part, contributed to strengthening the positions of dialectical materialism.

In this atmosphere of struggle against backward, outmoded views, Soviet theoretical thought grew and became stronger.

At the same time, theoretical schools of our own arose and developed.

Among our theoretical physicists, besides L. I. Mandelstam, who exerted so profound an influence on the entire Moscow school that it is difficult to overestimate his importance, especially much was done in the training of theorists by L. D. Landau, I. E. Tamm, V. A. Fock and Ya. I. Frenkel.

Directly from the scientific schools created by these physicists, and also indirectly, under the influence of the general rise in the level of theoretical physics in our country, over the thirty years of development numerous indigenous cadres of Soviet theorists have grown up.

The scientific activity of Soviet theorists now encompasses the whole extensive field of theoretical physics: field theory, the theory of relativity, thermodynamics and statistical physics, quantum mechanics, and the most diverse applications of these theories to various areas of physics.

In the field of field theory, a significant contribution has been made by the work of V. A. Fock, M. A. Leontovich, E. L. Feinberg and others on the propagation of electromagnetic waves. To this same circle of works one may assign S. M. Rytov’s elegant work on the skin effect. Many of the most difficult problems of field theory were successfully solved by G. A. Grinberg.

V. A. Fock is responsible for a first-rate work in which an approximate solution is given to the many-body problem in the general theory of relativity. Besides this fundamental work by V. A. Fock, in the field of the theory of relativity there are the interesting works of G. A. Mandel on five-dimensional theory, and the works of A. A. Friedmann and E. M. Lifshitz on cosmological solutions of the equations of the gravitational field.

Soviet theorists also possess a number of works on the connection between quantum theory and the general theory of relativity.

In the field of thermodynamics, first of all, one should note the brilliant works of L. D. Landau on the theory of phase transitions of the second kind.

M. A. Leontovich is responsible for an important investigation of the thermodynamics of nonequilibrium processes and many other subtle studies on the foundations of statistical physics, whose characteristic feature is the precise formulation of the question and a rigorous solution. Many works by Soviet scientists are devoted to the kinetics of nonequilibrium processes (B. I. Davydov, L. E. Gurevich, and others).

Recently N. N. Bogoliubov, in close connection with the work of A. A. Vlasov on taking long-range forces into account, has made a fundamental contribution to statistical physics, giving a consistent method for solving the basic equations of statistical physics for stationary and nonstationary processes.

In the development of nonrelativistic quantum mechanics, Soviet theoreticians also achieved significant results. V. A. Fock gave a method for the approximate solution of the many-body problem with allowance for exchange phenomena (the so-called “self-consistent field method”). This work may rightly be considered classical. A very significant contribution to the interpretation of quantum mechanics was made by L. I. Mandelstam in his teaching on “direct” and “indirect” measurements. Of great interest is the interpretation of the relation $\Delta E \cdot \Delta t > h$, given by L. I. Mandelstam and I. E. Tamm; interesting, too, is the work of V. A. Fock and N. S. Krylov connected with this work. Among works belonging to this field, one may note the elegant work of L. D. Landau on the “quasiclassical” approximation. In another aspect, the connection between quantum and classical mechanics was studied by D. I. Blokhintsev, P. E. Nemirovsky, and Ya. P. Terletsky.

Soviet theoreticians have worked, and are still working, extensively and interestingly on the basic problems of modern theoretical physics—relativistic quantum mechanics, the theory of elementary particles, and the theory of nuclear forces.

Ya. I. Frenkel was the first to develop the theory of the spinning electron. He also wrote the generalized Maxwell equations for particles with mass not equal to zero, which in our country, because of the bad habit of “not recognizing prophets in one’s own fatherland,” are called Proca equations (Proca wrote them much later).

I. E. Tamm is responsible for a fundamental work on the scattering of hard radiation by free electrons, in which, consistently and on the basis of quantum field theory, a formula was derived for the scattering of radiation in the Compton effect. V. A. Fock developed a remarkable apparatus of “functionals” as applied to problems of quantum electrodynamics. He also (jointly with P. Dirac and B. Podolsky) is responsible for work on the “many-time formalism,” in which a consistent, relativistically invariant formulation of quantum electrodynamics is given. This work—

was included in all monographs relating to this field. M. A. Markov applied this same scheme to the classical theory and gave a criterion for the relativistic invariance of the Hamiltonian method. A number of works by K. V. Nikol’skii belong to this same circle of problems.

As is known, at the present time the theory of elementary particles is encountering fundamental difficulties connected with the fact that, according to modern theory, the self-energy of particles turns out to be infinite. This problem occupies the minds of theorists throughout the world. Our Soviet theorists have advanced a number of ideas, along the development of which one may expect a way out of the indicated contradiction. Thus, V. A. Ambartsumian and D. D. Ivanenko put forward the idea of the quantization of space, which has now received an interesting development in Snyder’s works.

L. D. Landau, in a joint work with R. Peierls, pointed to the fundamental possibility of “individual errors” in measurements of particle coordinates.

M. A. Markov is responsible for the remarkable idea of the “four-dimensionally extended” electron. From another point of view, this same thought is now being developed by the author of the article.

The elegant idea of particles with excited spin states is being successfully developed by V. L. Ginzburg. In connection with the probable existence of various kinds of mesons, this idea acquires special interest. V. L. Ginzburg also has a number of works on quantum electrodynamics.

Quite recently I. Ya. Pomeranchuk gave an excellent development and critical analysis of the so-called λ-process—one of the methods for the “elimination of infinities.”

Within this same circle of fundamental problems lie the works of Soviet theorists on taking into account the role of damping in the processes of scattering of particles (D. D. Ivanenko and A. A. Sokolov, P. E. Nemirovskii, Ya. A. Smorodinskii, and others).

In the theory of nuclear forces one should first of all note the basic idea of D. D. Ivanenko concerning the structure of the nucleus from protons and neutrons, which was later developed by Heisenberg.

I. E. Tamm and D. D. Ivanenko are also responsible for the idea of the origin of nuclear forces. According to this idea, elaborated in detail by I. E. Tamm, nuclear forces are due to the exchange of electrons and neutrinos (“pair forces”). This work of I. E. Tamm had a substantial influence on the entire development of the doctrine of nuclear forces. Recently I. E. Tamm formulated relativistic equations for the interaction of nuclear particles.

L. I. Mandelstam and M. A. Leontovich were the first to give a theory of passage through potential barriers, later laid at the foundation of the theory of radioactive decay and other phenomena. Soviet theorists have also worked extensively on the theory of particle collisions and its applications. Especially significant

a contribution to the theory of cosmic rays was made by the works of L. D. Landau, I. E. Tamm, S. Z. Belenky, as well as A. B. Migdal and others, who substantially developed and improved the cascade theory of cosmic rays. Ya. P. Terletsky pointed out the curious possibility of explaining cosmic rays by the acceleration of particles in the electromagnetic fields of stars.

Among works relating to the passage of particles through matter, of considerable interest are the works of A. I. Akhiezer and I. Ya. Pomeranchuk on the scattering of slow neutrons. In the theory of collisions, a substantial contribution was also made by the works of L. D. Landau, E. M. Lifshitz, Ya. A. Smorodinsky, P. E. Nemirovsky, and others, who studied various effects in collisions of elementary particles.

The achievements of Soviet theorists are especially manifold in applications of quantum theory and statistical physics to the theory of electrical conductivity, to magnetism, to optical and other properties of matter in various physical states. Ya. I. Frenkel was one of the first successfully to develop the theory of the electrical conductivity of metals on the basis of wave mechanics.

I. E. Tamm gave for the first time the correct theory of the photoelectric effect.

Substantial results on the thermal conductivity of solids, including metals, were obtained by I. Ya. Pomeranchuk. Many other physicists also worked successfully on questions of the theory of metals and various effects in them. Especially outstanding results belong to Soviet theorists in the theory of magnetism.

L. D. Landau proved the quantum nature of the diamagnetism of metals—a result that at the time attracted the attention of all theorists.

Ya. I. Frenkel, much earlier than Heisenberg, pointed to exchange forces as the cause of spontaneous magnetization.

Fundamental work on ferromagnetism was done by N. S. Akulov, who developed the phenomenological theory of anisotropy and magnetostriction. In the field of the theory of ferromagnetism many other substantial results were obtained by A. I. Akhiezer, S. V. Vonsovsky, L. D. Landau, E. M. Lifshitz, E. I. Kondorsky, I. Ya. Pomeranchuk, and others.

In the theory of semiconductors and dielectrics, Soviet theorists are responsible for a number of first-rate ideas. Thus, in an elegant work I. E. Tamm proved the existence of local surface levels in an ideal crystal.

L. D. Landau put forward the fruitful idea of the “self-trapping” of the electron in a crystal lattice, which received further development and application in many works, among which the work of S. I. Pekar should especially be noted. Ya. I. Frenkel introduced

introduced into the theory of crystalline semiconductors and dielectrics two very important ideas—the idea of “hole” conduction and the idea of so-called “excitons”—quanta of excitation propagating in a crystal in the form of waves. D. I. Blokhintsev, B. I. Davydov, and S. I. Pekar developed the theory of rectification at a metal–semiconductor contact. In an original work, V. L. Ginzburg succeeded in giving an interpretation of the phenomenon, discovered by B. M. Vul, of the anomalously large dielectric constant of certain crystals.

Substantial and elegant results on the theory of imperfect crystals, which semiconductors in fact are, were obtained by E. M. Lifshitz.

Among the works relating to the theory of semiconductors should also be included D. I. Blokhintsev’s work on the theory of phosphorescence, in which this phenomenon was for the first time interpreted on the basis of the quantum theory of semiconductors and qualitative agreement with experimental facts was indicated. Substantial results on the law of decay of phosphors were obtained by V. V. Antonov-Romanovskii and Z. I. Adirovich and V. L. Levshin.

In the theory of liquids, a significant result was achieved by L. D. Landau, who succeeded in constructing a phenomenological theory of quantum liquid and thereby explaining the striking and, at first glance, enigmatic phenomena observed in liquid helium II. This work of Landau’s was recently supplemented by the microscopic theory of N. N. Bogoliubov.

Many valuable contributions to the development of the doctrine of liquids were made by the ideas of Ya. I. Frenkel, who regards a liquid as something closer to a crystal than to a gas.

Soviet theorists also worked on applications of kinetic theory to plasma—a medium consisting of electrons and ions and occupying an intermediate position between a gas and a liquid. In this connection, the works of L. D. Landau and A. A. Vlasov should be noted; Vlasov was the first to write the kinetic equations for a medium in which long-range forces are essential. Closely related to this circle of problems are V. L. Ginzburg’s works on the ionosphere.

In the field of optics, the works of I. E. Tamm and M. A. Leontovich on the theory of light scattering are of fundamental importance. These works are directly associated with the name of the late L. I. Mandelstam, who was their inspirer.

In theoretical optics, the works of S. I. Vavilov and his pupils (V. L. Levshin, I. M. Frank, and others) on the theory of luminescence in solutions and solids must also be noted. The theory of the width of spectral lines in gases was given in the work of A. A. Vlasov and V. S. Fursov.

One of the most elegant works in optics belongs to I. M. Frank and I. E. Tamm, who gave the theory of the radiation of an electron,

of a particle moving in a medium with superluminal velocity—a phenomenon discovered under the guidance of S. I. Vavilov by his student P. A. Cherenkov.

In conclusion, let us note that our theoreticians have also successfully penetrated into adjacent fields: hydrodynamics, acoustics, and others. Here one may note the works of L. D. Landau (on the theory of detonation and turbulence), S. Z. Belen’kii (shock waves), the significant works of Ya. B. Zel’dovich (on the theory of combustion, detonation, and the propagation of shock waves), and the numerous works of N. N. Andreev and his school (D. I. Blokhintsev, L. M. Brekhovskikh, and others) on various questions of acoustics.

Far from all the works can be covered in this concise list. But Soviet physicists may be satisfied by the fact that the bounds of a journal article cannot accommodate even a list of their works.

Theoretical physics has become firmly established in university curricula. Courses are now being taught in the theory of the electromagnetic field, statistical physics and thermodynamics, quantum theory, and others.

Over the past thirty years an enormous Soviet literature on questions of theoretical physics has been created.

This is, first and foremost, our physics journals, to a large extent filled with original and review articles on questions of theoretical physics, whose pages are studied closely by our own and foreign scholars. It is the numerous original and translated monographs on individual branches of theoretical physics. It is, finally, the diverse textbooks, on which theoreticians have worked a great deal and which have won the deserved approval of our teachers, graduate students, and students. Among these textbooks on theoretical physics one may mention Fundamentals of the Theory of Electricity by I. E. Tamm, Principles of Quantum Mechanics by V. A. Fock, the courses Electrodynamics and Wave Mechanics by Ya. I. Frenkel, Introduction to Quantum Mechanics by D. I. Blokhintsev, the many-volume course of theoretical physics by L. D. Landau and E. M. Lifshitz, Statistical Physics by M. A. Leontovich, and others.

This brief survey of the activity of Soviet theoreticians shows that Soviet theoretical physics has fully taken an independent path of development and stands at the level of the foremost world science.

It successfully competes with foreign science in solving the manifold problems of contemporary physical theory. In this competition we have been given the task of overtaking bourgeois science. This task is considerably more difficult, for the fundamental problems that now face theoretical physics are deep and complex. To solve these problems, even talented but artisanal work is insufficient. What is necessary is collective creative work, work of deep thinking, capable of

toward broad generalizations and possessing the advanced philosophy of revolutionary materialism.

Our theoreticians have had to struggle considerably for the propagation of modern science, for the “purity” of understanding its spirit.

But it would be dangerous to absolutize the principles of modern theory.

The noble task of our theoreticians consists in supporting innovative currents in our science, carefully nurturing the sprouts of new ideas in an atmosphere of strict but benevolent criticism, defending the common interests and honor of Soviet theoretical thought.