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BIOGRAPHIES OF GREAT SCIENTISTS
Dirac Paul Adrien Maurice. Biography of a scientist
Directory / Biographies of great scientists
English physicist Paul Adrien Maurice Dirac was born on August 8, 1902 in Bristol, in the family of Charles Adrien Ladislav Dirac, a native of Sweden, a French teacher in a private school, and an Englishwoman, Florence Hannah (Holten) Dirac. At first, Paul studied at a commercial school in Bristol. He then studied electrical engineering at the University of Bristol from 1918 to 1921 and graduated with a Bachelor of Science degree. After that, Paul also took a two-year course in applied mathematics at the same university. “During this mathematical education, Fraser influenced me the most ... he was an excellent teacher, able to inspire his students with a feeling of real admiration for the fundamental ideas of mathematics ... - Dirac recalled. - I learned two things from Fraser. First, rigorous mathematics. Before I used only non-rigorous mathematics, which satisfied the engineers ... They did not care about the exact definition of the limit, how long to add the series, and other such things. Fraser taught that strict logical ideas were sometimes necessary to handle these objects. And further: “The second thing I learned from Fraser was projective geometry. It had a profound effect on me because of its inherent mathematical beauty… Projective geometry always works on flat space… it provides you with methods such as the one-to-one correspondence method, which, like magic, they get results; the theorems of Euclidean geometry, over which you have long tormented, are deduced in the simplest ways, if you use the arguments of projective geometry. Dirac continued to be interested in projective geometry after becoming a graduate student at the University of Cambridge in late 1923, specializing in theoretical physics under Ralph Howard Fowler. In particular, he regularly attended tea parties at Professor Baker's house on Saturday evenings. After each of these tea parties, someone gave a report on a geometric problem. Dirac himself also "worked with projective geometry ... and made one of the reports at such a tea party. It was the first lecture in my life, and, of course, I remember it well. It was about a new method for solving projective problems." Dirac then entered graduate school in mathematics at St. John at Cambridge and in 1926 he defended his doctoral dissertation. The following year, Dirac became a member of the scientific council of the same college. While still at university, Dirac became interested in Albert Einstein's theory of relativity. During Dirac's postgraduate years at Cambridge, Heisenberg and Schrödinger developed their formulations of quantum mechanics by applying quantum theory to describe the behavior of atomic and subatomic systems and the motion of particles such as the electron. Dirac began studying the Heisenberg and Schrödinger equations as soon as they were published in 1925, making several useful remarks in the process. One of the shortcomings of quantum mechanics was that it was developed only for particles with low speed (compared to the speed of light), and this made it possible to neglect the effects considered by Einstein's theory of relativity. The effects of the theory of relativity, such as the increase in the mass of a particle with increasing speed, become significant only when the speeds begin to approach the speed of light. At the Solvay Congress in October 1927, Bohr approached Dirac. Here is how Dirac himself recalls this: “Bohr came up to me and asked: “What are you working on now?” I answered: “I am trying to get a relativistic theory of the electron.” Bohr then said: “But Klein had already solved this problem.” I was somewhat discouraged. I began to explain to him that the solution of the Klein problem based on the Klein-Gordon equation is unsatisfactory, since it cannot be consistent with my general physical interpretation of quantum mechanics. However, I could not explain anything to Bohr, since our the conversation was interrupted by the beginning of the lecture and the question hung in the air." Dirac was not pleased. He sought to obtain equations for a single electron, and not for a system of particles with different charges. He got his way, but the decision surprised him. Two-dimensional Pauli particles, which describe the spin well in the nonrelativistic case, were clearly lacking. The electron in theory had an extra degree of freedom - the freedom, as it turned out, of the transition to a state with negative energy. It looked so wild that it was just right to abandon everything that had been done. In search of a way out, Dirac came up with a strange idea. He suggested that all electrons in the universe occupy levels with negative energy, according to the Pauli principle, forming an unobservable background. Only electrons with positive energy are observable. "Electrons," writes Dirac, "are distributed throughout the world with a high density at each point. Perfect emptiness is the region where all states with negative energy are occupied." "Unoccupied states with negative energy will appear as something with positive energy, because in order for them to disappear, it is necessary to introduce one electron with negative energy into it. We assume that these unoccupied states with negative energy are protons." Dirac's theory was met with skepticism. The hypothetical background of electrons caused distrust, moreover, Dirac's theory, in his words, "was very symmetrical with respect to electrons and protons." But the proton differs from the electron not only in the sign of charge, but also in mass. The discovery of the positron, a particle truly symmetrical to the electron, forced a new appreciation of Dirac's theory, which essentially predicted the existence of the positron and other antiparticles. At the Leningrad conference in 1933, Dirac stated the essence of the positron theory as follows: “Let us assume that in the world that we know, almost all electronic states with negative energy are occupied by electrons. This set of electrons sitting at negative energy levels, due to its homogeneity, cannot be perceived by our senses and measuring instruments, and only the levels not occupied by electrons, being something exceptional, some kind of violation of homogeneity, can be noticed by us in exactly the same way as we notice the occupied states of electrons with positive energies. energy, i.e., a "hole" in the distribution of electrons with negative energy will be perceived by us as a particle with positive energy, because the absence of negative kinetic energy is equivalent to the presence of positive kinetic energy, since minus by minus gives a plus ... It seems reasonable to identify such a "hole" Withpositron, i.e., to assert that the positron is a "hole" in the distribution of electrons with negative energy. "According to Dirac's theory," F. Joliot wrote, "a positive electron in a collision with a free or weakly bound negative electron can disappear, forming two photons emitted in opposite directions." There is also a reverse process - the "materialization" of photons, when "photons with sufficiently high energy can create positive electrons when colliding with heavy nuclei ... A photon, interacting with a nucleus, can create two electrons with opposite charges." Derived by an English scientist and published in 1928, the equation is now called the Dirac equation. It made it possible to reach agreement with the experimental data. In particular, the spin, which was previously a hypothesis, was confirmed by the Dirac equation. This was the triumph of his theory. In addition, the Dirac equation made it possible to predict the magnetic properties of the electron (magnetic moment). Dirac also belongs to the theoretical prediction of the possibility of the birth of an electron-antielectronic pair from a photon of sufficiently high energy. The antielectron predicted by Dirac was discovered in 1932 by Carl D. Andersen and was named the positron. Later, Dirac's assumption about the possibility of the birth of a couple was also confirmed. Subsequently, Dirac hypothesized that other particles, such as the proton, must also have their antimatter counterparts, but a more sophisticated theory would be required to describe such pairs of particles and antiparticles. The existence of the antiproton was confirmed experimentally in 1955 by Owen Chamberlain. Many other antiparticles are now known. The Dirac equation made it possible to clarify the problem of X-ray scattering by matter. X-ray radiation first behaves like a wave, then interacts with an electron as a particle (photon) and after a collision it is again like a wave. Dirac's theory provided a detailed quantitative description of this interaction. Dirac later discovered the statistical distribution of energy in a system of electrons, now known as the Fermi-Dirac statistics. This work was of great importance for the theoretical understanding of the electrical properties of metals and semiconductors. Dirac also predicted the existence of magnetic monopoles - isolated positive or negative magnetic particles, similar to positively or negatively charged electrical particles. Attempts to experimentally detect magnetic monopoles have so far been unsuccessful. All known magnets have two poles - north and south, which are inseparable from each other. Dirac suggested that natural physical constants, such as the gravitational constant, may not be constant in the exact sense of the words, but slowly change with time. The weakening of gravity, if it exists at all, is so slow that it is extremely difficult to detect, and therefore remains hypothetical. Dirac and Schrödinger received the 1933 Nobel Prize in Physics "for their discovery of new productive forms of atomic theory". In his lecture, Dirac pointed out the possibility of the existence of "stars consisting mainly of positrons and antiprotons" arising from the symmetry between positive and negative electric charges. Perhaps one half of the stars belongs to one type, and the other to another. These two types of stars should have have the same spectra, and it would be impossible to distinguish them by the methods of modern astronomy." In 1937, Dirac married Margit Wigner, sister of the physicist Eugen P. Wigner. They had two daughters. It is generally accepted that Dirac is a taciturn and not very sociable person. So it was. He preferred to work alone, and he had few direct students. But along with this, he had the ability for sincere and deep friendship. Dirac found two of his closest friends in the Soviet Union. They were Peter Kapitsa and Igor Tamm. The memoirs of Tamm's daughter Irina about Dirac are curious: “For two years in a row, P. A. M. Dirac, who came to Moscow, stayed with us, with whom dad met and became friends in 28 at Ehrenfest in Leiden. I remember how on my second visit in the evening a radiant Dirac enters and, raising his finger, solemnly declares: “Tamm, you have grandiose changes.” In response to everyone's bewilderment, he explained: “Now the light is on in the toilet.” In the autumn of 1934, Kapitsa was not allowed to return to England, to the laboratory he was in charge of, and he was forced to stay in the USSR at first without the opportunity for scientific work. Dirac wanted to come to the Soviet Union in order to try to help Kapitza. This problem was discussed in detail in the correspondence between him and Kapitsa's wife, Anna Alekseevna, who was then in Cambridge. Dirac was lecturing in the United States that year. To rescue Kapitsa, he even collected signatures under a collective letter from American physicists to the government of the USSR, together with R. Milliken paid a visit to the Soviet embassy. Friends and acquaintances of Paul Dirac were often amazed by his unexpected and sometimes "strange" reaction to the topics that arose in the conversation. True, then it became obvious that his remarks were a natural and logical response, and that it was only the purely automatic and thoughtless associations of everyone else that made one expect something else from him. The same property manifested itself in its physics. The resemblance is so clear that many of the famous stories about the scientist can be directly correlated with some of his articles. Here, for example, is the story of pills in a bottle, told by H. R. Ulm. Ulm apologized for the noise in his pocket, explaining that the bottle was no longer full and therefore making noise. Dirac remarked, "I think it makes the most noise when it's half full." He grasped the fact that the bottle makes no noise, not only when empty, which is obvious, but also when it is completely full. This thought is similar to the idea underlying his "hole theory". In another episode, the conversation over tea turned to the fact that among the children born recently to physicists in Cambridge, there was a surprisingly large proportion of girls. When someone flippantly remarked, "There must be something in the air!" - Dirac added after a pause: "Or maybe in the water." He took the expression "in the air" not in its conventional sense, but literally, seeing a possible application. This trend is reflected in many of his works. Perhaps it first appeared in the way he used Heisenberg's observation that quantum variables do not commute. To Heisenberg himself, this seemed like an ugly feature of formalism. Dirac, on the contrary, showed that this circumstance occupies a very important place in the new theory. Another characteristic feature of Dirac appeared in the story that took place in Copenhagen. Friends have noticed that the famous physicist Pauli is gaining weight too quickly. Then Dirac was asked to see to it that he did not eat too much. Pauli took part in this game and asked Dirac how many lumps of sugar he could put in his coffee. "I think one will be enough for you," Dirac said, adding a little later: "I think one is enough for everyone." After some further thought: "I think the pieces are made in such a way that one is enough for everyone." Such a belief in the orderliness of the world is often reflected in his writings and, above all, in a remark in an article showing that the magnetic monopole does not contradict the known laws of quantum mechanics: "It would be surprising if nature did not use it." When Dirac talked about his work, it seemed to the audience that he did not explain the existing world so much, but, like a creator, creates his own, beautiful, mathematically rigorous. Only at the end does he return to reality. Comparing his world with the real world, Dirac sometimes encountered surprises that others would consider a crushing blow to the theory. But this is exactly what Dirac did not have. The decisive criterion of truth for him was logical isolation. Thus, he could never come to terms with the modern theory of relativistic quantum fields based on the renormalization method. After completing his work on relativistic quantum mechanics, Dirac traveled widely, visiting universities in Japan, the Soviet Union, and the United States. From 1932 until he retired in 1968 he was professor of physics at Cambridge. After Dirac left Cambridge, he was invited to the University of Florida, where he remained a professor until the end of his life. In 1973, Dirac was awarded the British Order of Merit. He was elected a foreign member of the American National Academy of Sciences (1949) and a member of the Pontifical Academy of Sciences (1961). Dirac died in Tallahassee on October 20, 1984. Author: Samin D.K.
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