Influence of N. N. Bogoliubov on the Development of Theoretical Physics in the Soviet Union

(by V. G. Soloviev, in Nikolai Nikolaevich Bogoliubov. Mathematik, mechanik, physik. 1909-1992., Dubna, JINR, 1994, pp. 144 &ndash 155, in Russian)

Great physicists make an everlasting contribution to the treasury of the fundamental sciences, form scientific thinking, and promote understanding of world order. Their talents and scientific generosity lead to the formation of scientific schools. The role of great scientists manifests itself not only in their scientific works, but also in the works and assertions of their disciples and contemporaries. Psychological climate and the prevailing scientific ideology is not fully reflected in the then published papers and reports. The difference in approaches to solving physical problems, disagreements, discussions, and criticism leading to the suppression of alternative lines of research, are not duly reflected in the published scientific papers. Therefore, memoirs of contemporaries give additional information about the scientific atmosphere of that time and the role of individual scientists. Greek scholar Socrates did not write philosophical treatises but they are known two and a half thousand years later thanks to the fact that his ideas were described by Plato and other disciples of Socrates.

Mathematical methods and physical ideas of N. N. Bogoliubov had a great influence on the development of the theory of atomic nucleus. Canonical transformation, the Hartree-Fock-Bogoliubov variational principle, the self-consistent field method, the concept of quasi-averages and others constitute one of the foundations of the microscopic nuclear theory. This is what I wrote in the article published in "Foundation of Physics" (1986, Vol. 16, No. 1, p. 63).

In this article I expounded my subjective vision of the role of N. N. Bogoliubov in the development of theoretical physics in our country after World War II. It is based on my research, my participation in the scientific activity, and on my contacts with N. N. Bogoliubov. Therefore, in this article I cannot do without the description of my scientific activity and difficulties I experienced that period.

1. Theoretical Physics in Dubna and Moscow in 1951-1956

After graduating in December 1950 from Leningrad University I was assigned to work in the Hydraulic Engineering Laboratory (HEL) which was a division of the Laboratory of measuring instruments (now the Kurchatov Institute of Atomic Energy) and was located on the site of the modern city of Dubna. I got into a young team engaged mainly in elementary particle physics. The atmosphere of creative enthusiasm and selfless work dominated there. The undoubted merit of the Director of the Laboratory M. G. Meshcheryakov was the fact that he considered a scientist the main figure. This advantage, unfortunately, was largely lost at JINR. Vice -Directors were M. S. Kozodaev and V. P. Dzhelepov. B. Pontecorvo had also great influence on the scientific atmosphere.

The group of theoretical physics was headed by I. Ya. Pomeranchuk, a scientist with great physical intuition and devotion to science. I. Ya. Pomeranchuk sought to establish here a new branch of the Landau school. He planned to select a group of capable graduates who would become candidates and then doctors of science. Members of the group were required to participate in the Landau seminar in Moscow. Journal articles on different physical issues selected by L. Landau and original papers were discussed at the seminar. The meetings were very lively and discussions were sometimes fierce. It was interesting to watch, whereas a speaker had some difficulties. During my report on my research at this seminar they practically gave me no chance to speak, and after the report I felt like a beaten. Nevertheless, I. Ya. Pomeranchuk said that my report was a success. If the report was approved at the Landau seminar, the publication or thesis were thereby approved. If the line of research was not supported at the seminar, it practically had no chance for development. For example, Landau called the shell nuclear model "a dog's nonsense". Thus, it was hardly possible to publish an article or defend thesis on this subject at that time.

A full participant in the seminar was the one who passed the Landau theoretical minimum consisting of nine examinations. During each exam, conducted by L. D. Landau personally, it was necessary to solve three problems and answer additional questions. Those who did not pass this theoretical minimum considered like second-class theorists. With varying degrees of success I also passed these examinations, having spent on them much time and effort. I passed most of these exams. I told Pomeranchuk that I did not want to spend time on the remaining examinations, especially in mechanics and electrodynamics of continuous media, as I was not a person of encyclopedic learning and this subject was far from my scientific interests. My reluctance to take the remaining exams was influenced by the fact that I learned that some examinees knew in advance the proposed problems. As for me, I did not know in advance any of the tasks that were given to me on the exams and, therefore, I though the conditions for the examinees to be not the same and that there might have a place some hidden selection. As a result, I had to take the remaining exams with A. B. Migdal, as the Landau theoretical minimum was official in the Laboratory. Thus, I automatically dropped out of the Landau school.

In early 1953, Ya. P. Terletsky was appointed the head of our sector, which caused damage to our research. In fact, he suggested me to work against quantum mechanics. I refused to do this work, as even being a student of V. A. Fock's Chair I struggled against those who tried to ban quantum mechanics on the basis of pseudo-philosophic conception. Later, I had to hold debates in defense of quantum mechanics with A. Datsev in Sofia and L. Janossy in Budapest. At that time, it was argued that perturbation theory could not be used in quantum field theory. I did not know how to conduct investigations without perturbation theory, so there came a time of stress and hopelessness for me.

In 1953, M. G. Meshcheryakov introduced me to N. N. Bogoliubov and I started to work under his supervision. From I. Ya. Pomeranchuk I perceived physical thinking. N. N. Bogoliubov charmed me with the elegance of his mathematical apparatus. The work progressed with great difficulty. Once, I remember, I made some calculations and reached a deadlock. I came to N. N. Bogoliubov's home and told him about it. He got his calculations and showed that his calculations went into the same impasse.

In the autumn of 1953 N. N. Bogoliubov was elected an academician of the Department of Physics and Mathematics Sciences. At that time the domination of the Landau school was complete. Since N. N. Bogoliubov's works carried out outside the Landau school caused great interest, it was overemphasized that he was not a theoretical physicist but rather a mathematician.

Interesting Bogoliubov's seminars were held at the MSU Department of Physics and in the Steklov Mathematical Institute. Chapter by chapter we studied the monograph "Introduction to the Theory of Quantum Fields" by N. N. Bogoliubov and D. V. Shirkov. This monograph played a prominent role in the preparation of theoretical physicists worldwide. Later, I found this monograph in the libraries of all scientific centers I visited. The situation of Bogoliubov's students at that time was such that even a candidate thesis on the field theory was difficult to defend. The monopoly of the Landau school continued to exist. Due to the problem of so-called zero charge quantum field theory was declared to be like an outlaw. I defended my dissertation "Construction of Approximate Green Functions in the Pseudoscalar Meson Theory" at Moscow State University in 1956. The opponents were O. S. Parasyuk and A. A. Logunov. O. S. Parasyuk's brilliant review of my dissertation instilled confidence in myself and inspiration for new works.

A major success of N. N. Bogoliubov was the proof of dispersion relations for an arbitrary scattering angle. His talk at the conference in Seattle in 1956 made a strong impression. The works on the dispersion relations strengthened the credibility of Bogoliubov as an outstanding theoretical physicist and strengthened the position of his school. These works stimulated research of D. V. Shirkov, A. A. Logunov, V. S. Vladimirov, B. V. Medvedev, M. K. Polivanov, A. N. Tavkhelidze and his other disciples. At that time I was busy with my thesis and did not participate in this cycle of works.

2. Foundation of the Laboratory of Theoretical Physics and the First Years of Its Existence

In March 1956, the Joint Institute for Nuclear Research (JINR) was established on the basis of the Institute of Nuclear Problems (former HEL) and Electrophysical Laboratory. D. I. Blokhintsev was elected its Director. In May 1956, D. I. Blokhitsev had a conversation with theorists of both divisions in the absence of their leaders M. A. Markov and Ya. P. Terletsky. During this conversation he said he was going to organize a Laboratory of Theoretical Physics (LTP). He regretted that a number of physicists, including V. I. Veksler, I. E. Tamm and M. A. Markov, did not support the establishment of LTP and, therefore, he had not yet managed to find a scientist capable of leading the Laboratory. After the conversation I came up to D. I. Blokhintsev and asked him what he thought of inviting N. N. Bogoliubov to this post. He liked the idea and asked me to find out with N. N. Bogoliubov if he was interested in it. N. N. Bogoliubov expressed interest. The establishment of LTP and the discussion of the candidature for the post of Director were difficult. Several scientists pretended to leadership positions. Some thought that LTP should be led not by mathematician N. N. Bogoliubov but a physicist involved in experimental research. As a result, a compromise was found: N. N. Bogoliubov was elected Director, M. A. Markov and Ya. A. Smorodinsky were elected Heads of the Sectors, and Ya. P. Terletsky was not invited to work at LTP.

D. V. Shirkov, B. V. Medvedev, M. K. Polivanov, V. G. Soloviev and V. Z Blank, soon lost in the Caucasus Mountains, began working in Bogoliubov's Sector. D. V. Shirkov did not want to burden himself with administrative duties and the post of Deputy Director was taken up by A. A. Logunov. In 1957 the following scientists started to work at LTP: I. Zlatev, A. N. Tavkhelidze, Zhou Guangzhao, V. S. Barashenkov, P. S. Isaev, F. Kashlun, V. Tsollner, I. Ulegla, I. Todorov and others. Staggering scientific activity began. Meetings of the general Laboratory seminar with participation of D. I. Blokhintsev and V. Votruba were very interesting. Ya. A. Smorodinsky and other representatives of the Landau school tried to prove that outside the Landau school there were no significant works, save Bogoliubov's works. So during my talk on the hypothesis of conservation of only combined parity in strong, electromagnetic and weak interactions Ya. A. Smorodinsky tried to present my results as absurd. Talks on the theory of dispersion relations made by theorists of the Bogoliubov Sector were also under attack. Artificial obstacles to our publications were created by the Editorial Board of JETP and of the Soviet Editorial Board of "Nuclear Physics".

For the sake of fairness I would like to note that later Ya. A. Smorodinsky organically entered into the collective of the Laboratory of Theoretical Physics. His erudition and encyclopedic knowledge had a positive impact on the studies carried out at LTP and scientific atmosphere.

Strengthening of the scientific credibility of LTP was facilitated by N. N. Bogoliubov's works on the theory of superconductivity. In the Autumn of 1957 N. N. Bogoliubov showed that the method developed by him in constructing a microscopic theory of superfluidity of Bose systems can be generalized to consistent development of the theory of superconductivity based on the Frohlich model. Significant progress in constructing a theory of superconductivity was independently achieved by J. Bardeen, L. Cooper and J. Schrieffer. The series of works by N. N. Bogoliubov followed, in which he used the Bardeen Hamiltonian, a canonical transformation and the principle of compensation of dangerous diagrams, as well as works by S. V. Tyablikov, D. V. Shirkov, D. N. Zubarev, Yu. A. Tserkovnikov and others. The monograph "A New Method in the Theory of Superconductivity" by N. N. Bogoliubov, V. V. Tolmachev and D. V. Shirkov published in 1958 played an important role.

N. N. Bogoliubov's crucial contribution to the construction of the microscopic theory of superconductivity was not recognized immediately. L. Landau and V. Ginzburg were considered recognized authorities in the field of superconductivity. However, the microscopic theory of conductivity was not constructed by them. This led to jealousy that was especially strong to their compatriot N. N. Bogoliubov. I attended P. L. Kapitsa's seminar at which N. N. Bogoliubov reported on his work on the theory of superconductivity in late 1957 or early 1958. Before the seminar he did not feel well and was very excited. Bogoliubov's report aroused considerable interest and was followed by a lively discussion. A. F. Ioffe asked many questions at the seminar. Landau claimed that there should not be any divergences in the theory and, therefore, there was nothing to compensate. The general opinion of participants of the seminar was that N. N. Bogoliubov made an important contribution to the construction of the microscopic theory of superconductivity. Nevertheless, in the review by A. A. Abrikosov and I. M. Khalatnikov published in the Uspekhi Fizicheskikh Nauk in 1958 the creation of the theory of superconductivity was entirely attributed to Bardeen, Cooper, and Schrieffer.

3. Superfluidity of Nuclear Matter and Atomic Nuclei

N. N. Bogoliubov made a statement that superfluidity is a general property of Fermi-systems, including nuclear matter. In the paper published in the Doklady of the USSR Academy of Sciences in 1958 he derived the equation that can be applied to study the problem of superfluidity of nuclear matter.

N. N. Bogoliubov formulated a new variational principle that is the generalization of the Hartree-Fock variational principle. Generalization of the Bogoliubov principle was performed by S. V. Tyablikov, and also by I. A. Kvasnikov and V. V. Tolmachev. My only one paper with N. N. Bogoliubov is devoted to the formulation of the a new variational principle in the many-body problem. The main results of this work belong to Bogoliubov. Here he revealed his exceptional scientific generosity. I believe that he demonstrated the same generosity in many joint papers with his young colleagues.

At the same time, when N. N. Bogoliubov posed the problem of superfluidity of nuclear matter, A. Bohr, B. Mottelson and D. Pines paid attention to the similarity of excited states of nuclei with the spectra of superconducting states of metals and the expediency of using the methods developed in the theory of superconductivity in describing the characteristics of finite nuclei.

I was the first to use Bogoliubov's methods in studying superfluidity of medium and heavy nuclei (published in JETP, DAN USSR and Nuclear Physics in 1958). It was shown that the superfluid state of the nucleus is energetically more favorable than the normal one and is separated from the latter, and that there is a gap in the spectra of even-even nuclei and no gap in the spectra of odd mass nuclei. These my papers provoked objections, according to which the sizes of atomic nuclei were too small for pairing correlations of superconducting type to exist in them.

In 1959, the well-known papers on superfluidity and nuclear moments of inertia by S. T. Belyaev, which were based on the Bogoliubov principle of compensation of dangerous diagrams, and by A. B. Migdal, in which he used the Green function method in the treatment of L. P. Gor'kov, were published. A. B. Migdal asserted that only his interpretation of superfluidity in nuclei was correct.

In my works, published in 1960-1962, the general picture of quasiparticle excited states in nuclei was represented, and the energies and wave functions of excited states of deformed nuclei were calculated. Pairing correlations of superconducting type were shown to influence strongly the probability of alpha- and beta- decays. These works were strongly objected by A. B. Migdal and his disciples saying that one cannot pretend to a detailed quantitative description of characteristics of concrete atomic nuclei.

Before the development of the model taking into account pairing correlations of superconducting type, a few experimentally measured characteristics of atomic nuclei were known which found no explanation. These include the following facts: 1) the existence of a gap in the excited states, associated with internal degrees of freedom, in even-even nuclei and the absence of a gap in the spectra of odd mass nuclei, 2) impossibility of explaining experimental values for moments of inertia of deformed nuclei and a strong excess of moments of inertia of odd mass nuclei as compared to even-even nuclei, 3) observation of beta-decays strictly forbidden in the model of independent particles, 4) 104 - 105 times enhancement of probabilities of favorable alpha-decays as compared to the calculated results and others. Moreover, great difficulties were encountered in describing the shape of atomic nuclei. These and other contradictions were brilliantly removed in the model taking into account pairing correlations of superconducting type.

N. N. Bogoliubov's paper "On the principle of compensation and the method of self-consistent field" published in 1959 had a great effect on the theory of atomic nucleus. The method presented in that paper and the random phase approximation played an important role in the microscopic description of vibrational states in atomic nuclei. Thus, it became possible to describe in a unified way quasiparticle and collective states due to internal degrees of freedom. We managed for the first time to calculate quadrupole and octupole vibraional states of deformed nuclei in the rare-earth and actinide regions. Essential progress was achieved in the description of the equilibrium shape of atomic nuclei. Thus, in 1969 we predicted the existence of a new region of deformed nuclei near the mass equal to 100. In 1970, American physicists found these nuclei experimentally. The description of the equilibrium shape of nuclei was greatly influenced by the method of shell corrections proposed by V. M. Strutinsky.

In the sixties the Bogoliubov school took a strong position in theoretical physics in our country and abroad. Many disciples of N. N. Bogoliubov defended doctor of science dissertations. The results obtained by N. N. Bogoliubov as well as S. V. Tyablikov, D. N. Zubarev, D. V. Shirkov, V. S. Vladimirov, A. A. Logunov, B. V. Medvedev, M. K. Polivanov, A. N. Tavkhelidze, Yu. A. Tserkovnikov, L. D. Soloviev and others in the fields of quantum field theory and statistical physics were generally recognized and have never been disputed. My works in the field of atomic nucleus theory were subjected in our country to bitter and often unfair criticism by A. B. Migdal and his co-workers. At first, those were attacks on Bogoliubov directed through me. Significantly, the papers by S. T. Belyaev based on Bogoliubov's methods did not come under criticism. In contrast with the field theory and statistical physics, where there was a powerful group, I alone had to confront the fierce criticism. My colleagues either evaded the fight or were still very young. Only one of my talks was not criticized by A. B. Migdal. This is my talk in 1964 at the International Congress on Nuclear Physics in Paris that received approval from several well-known scientists. It should be noted that the results in the field of atomic nucleus theory obtained by our group were recognized by the world nuclear physics community.

In 1965, A. B. Migdal formulated the theory of finite Fermi systems as applied to atomic nuclei. This paper is a major step in the development of the theory of atomic nucleus. In his talks at conferences and seminars he advocated that he constructed a "true" nuclear theory and, therefore, nuclear models that came before it lost their importance. In his talks (but not published articles) he insisted on discarding the outdated, in his opinion, models and performing calculations of nuclear properties in the formalism of the theory of finite Fermi systems.

With the advent of the theory of finite Fermi systems I asserted that one more nuclear model was proposed. It can be used along with other models to describe nuclear characteristics. This position was totally rejected by A. B. Migdal and his co-workers. Moreover, the results of our calculations of vibrational states of deformed nuclei suffered false accusation. Only in 1970 R. V. Jolos and I showed that the Bogoliubov method of self-consistent field was the most general. Under certain assumptions the equations of this method result in the equations of the Migdal theory of finite Fermi systems. Thus, it was established that the Migdal theory of finite Fermi systems was one of several microscopic nuclear models. It took several years of withstanding attacks on our quasiparticle-phonon nuclear model until everything fell into place.

The Bogoliubov equations of the method of self-consistent field formed the basis for the development of the time-dependent Hartree-Fock-Bogoliubov approximation. Within the framework of this method many authors studied fast vibrations and slow collective processes with large amplitude. This method is widely used to describe nucleus-nucleus interactions. Great success in solving the nuclear many-body problem is related to the approximate rather than exact allowance for the conservation laws. Mathematically correct formulation of the problem with approximate allowance for the conservation laws became possible after the appearance of Bogoliubov's fundamental work on quasiaverages.

4. General Comments on the Development of Theoretical Physics

The development of theoretical physics in the Soviet Union until 1960 was influences by the exclusive position of the Landau school. The latter united actively and effectively working theoretical physicists. It played an important role in the success of some fields of theoretical and experimental physics. However, any monopoly is bad and dangerous. It inhibits the development of innovative research directions and limits freedom of scientific creativity. A number of scientists from MSU and other institutes opposed the monopoly of the Landau school. They used administrative methods not acceptable in science and, therefore, without success.

N. N. Bogoliubov obtained brilliant scientific results recognized worldwide by the scientific community. These results were obtained outside the Landau school and automatically led to the destruction of the monopoly of this school on theoretical physics. I think that N. N. Bogoliubov played a prominent role in the development of theoretical physics in the Soviet Union, promoting the transition from monopoly to a normal scientific community. I would like to emphasize this role of N. N. Bogoliubov particularly, as it was taken as a matter of course and was soon forgotten. After 1965 the Landau and Bogoliubov schools, as well as other fields of theoretical physics, peacefully coexisted. A new generation of theoretical physicists is working under the conditions largely free from the shackles of the subjective limitations. However, in some fields, such as nuclear theory, the recurrence of the monopoly made itself felt for years.

There were three giants in the field of theoretical physics in the Soviet Union: V. A. Fock, L. D. Landau, and N. N. Bogoliubov. They determined the high level of research in physics in our country.

Such outstanding schools as the Copenhagen, Bogoliubov, Landau and other ones contributed to the rapid development of physics. It is quite possible that the heated discussions and confrontation of different schools helped this development. It is legitimate to pose the question whether such schools exist now. I do not know this kind of schools. What is it: the absence of a brilliant scientist able to create a school, or a new stage of physics associated with the deepening of specialization?

In theoretical nuclear physics not only schools are absent but the heated debates are rare. The alternative nuclear models are developed simultaneously and are seldom compared in discussions. All my attempts to compare the model of interacting bosons with the microscopic models did not meet any response. At present, in Europe the confrontation of competing models is revealed in closed or anonymous reviews of articles submitted for journals. This form of scientific debate seems to me very abnormal.

In conclusion, I consider it my duty to point out the following. Without the memories of contemporary scholars one cannot get a real picture of the state of science at that time. The development of science depends to a great extent on certain purposes and prevailing opinions. They are not as a rule published. I think that it is the duty of a scientist to describe his vision of the departed time and thus the scientific atmosphere, conflict of opinions and the role of individual scientists.

Translated from Russian by G. Sandukovskaya