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“He was the prototype of the superior mathematical mind; but while in others this had coexisted with a multitude of interests, in Dirac’s case everything went into the performance of his great historical mission, the establishment of the new science, quantum mechanics, to which he probably contributed as much as any other man.” This was how the German physicist Walter Elsasser characterized Dirac. Elsasser, who was a student of Born and later in life, would turn to a distinguished career in the earth sciences, had first met Dirac in Göttingen in 1927, and later, on several other occasions. Recall that in 1927, Raymond Birge had observed that “Dirac thinks of absolutely nothing but physics.” While it is true that Dirac’s life was very much focused on physics, these two statements exaggerate his singular mindset. Dirac did have other interests. One of them was chess, a game he played very well and as often as he could, both in Cambridge and abroad. He served for many years as president of the chess club of St. John’s College.

More important than his interest in chess was Dirac’s insatiable appetite for travel. At a time when the concept of global tourism was not yet developed, Dirac traveled all over the world. Many of his trips were connected with conferences and lectures while others were just vacations. His idea of a holiday, however, was not relaxing at a resort hotel near a sunny beach. More typically, it involved long and strenuous hikes on difficult terrains.

Dirac’s idea of a pleasant vacation may be illustrated by the first of his many visits to the Soviet Union. While staying in Göttingen in 1928, he was invited to an international physics conference that took part in Moscow and continued aboard a steamboat along the Volga River. After the conference had ended, Dirac traveled alone through the Caucasus to the Black Sea coast, using the occasion to climb a mountain to an altitude of about 3,000 meters (10,000 feet). Then, as he reported to Tamm upon his return to Cambridge: “I spent three days in Tiflis [Tbilisi, the present capital of Georgia], mostly resting and making up for lost sleep, and then went to Batoum [Batumi, a seaside city in Georgia] to try to get a boat for Constantinople.” After a couple of days and much trouble, he succeeded in getting a Turkish visa. “From Constantinople I took a ship to Marseilles, visiting Athens and Naples on the way, and then I came home across France and ended a most pleasant holiday.” During another visit to the Soviet Union eight years later, Dirac, Tamm, and a group of other Russians climbed the 5,640-meter (18,510-feet) Mount Elbrus in the Caucasus Mountains. The leader of the expedition recalled how he tried in vain to get Dirac to accept the “USSR Alpine Climber” badge for having reached the top of Europe’s highest mountain.

Dirac was also a great walker who had the stamina and physical energy that surprised those who knew him only from conferences and dinner parties. During the Christmas of 1935, he went to visit Margit Wigner in Austria, whom he would marry two years later. She recalled: “He used to go off for long walks; he knew no fatigue, meals were unimportant to him, but not to me. … I often accompanied Paul, but usually regretted it. His enduring capacity would have been too much for most mortals.” Lack of personal comfort never bothered Dirac, who for a long time lived an almost ascetic life. He never touched alcoholic drinks and never smoked. “Dirac is rather like one’s idea of Gandhi,” wrote a Cambridge physicist in a letter in 1931. “He is quite indifferent to cold, discomfort, food, etc. We had him to … a nice little supper here, but I am sure he would not have minded if we had only given him porridge.”

In the spring of 1929, Dirac paid his first visit to the United States to take up a position as a visiting professor at the University of Wisconsin in Madison. In a humorous interview in the local Wisconsin State Journal, the strange creature from Europe was introduced as “a mathematical physicist, or something, they call him—who is pushing Sir Isaac Newton, Einstein and all the others off the front page.” So the interviewer paid a visit to the new professor and reported: “His name is Dirac and he is an Englishman. He has been given lectures for the intelligentsia of the math and physics department—and a few other guys who got in by mistake. … I knock at the door of Dr. Dirac’s office in Sterling Hall and a pleasant voice says ‘Come in.’ And I want to say here and now that this sentence ‘come in’ was about the longest one emitted by the doctor during our interview. He sure is for efficiency in conversation. … The thing that hit me in the eye about him was that he did not seem to be at all busy. … He seems to have all the time there is in the world and his heaviest work is looking out of the window. If he is a typical Englishman it’s me for England on my next vacation!”

Having completed his lectures in Wisconsin, Dirac traveled extensively through the United States, visiting, among other places, Chicago, Los Angeles, the Grand Canyon, and the Yosemite National Park. The summer of 1929 found him in Berkeley, where he gave lectures on quantum mechanics. Heisenberg was also lecturing in Berkeley at the time, and since they had both been invited to Japan, the two quantum pioneers decided to go together and return to Europe westward. Dirac and Heisenberg left San Francisco on a steamer that brought them to Hawaii and from there on to Japan. After having completed their lectures in Tokyo, the two young physicists separated. While Heisenberg returned to Europe by boat, Dirac insisted on taking the harder way to Siberia. He had originally planned to go through Manchuria, but troubles at the Chinese-Soviet border prevented it. “I shall leave Vladivostok in the early morning,” he wrote from Kyoto to Tamm. “I cannot remember the exact time of my arrival in Moscow and have left my time-table in Tokyo, but as there is now only one train a week from Vladivostok I expect you will be able to find it out without difficulty.” From the Russian capital, Dirac went to Leningrad by train and from there to Berlin by airplane—probably his first experience in the air. Train and ship finally brought him back to Cambridge.

After his return to Cambridge, Dirac received a letter from Heisenberg, who told him about his own journey from Japan via Shanghai and Hong Kong to Germany. Heisenberg shared Dirac’s interest in walking and hiking. “The best part was the trip to the biggest and best mountains,” he wrote. “In India itself it was very hot and rather rainy. Once our train went off the rails in the middle of the Jungle and people were very afraid of tigers; the tigers probably were pretty afraid too.” Dirac did not stay long in his room in St. John’s College. Apart from several shorter visits to Europe, in the summer of 1931, he returned to the United States, this time, to give lectures at Princeton University.

Before taking up his academic duties, he spent time with his friend Van Vleck in Madison. Dirac was not used to dining in American hotels where the ice water was very cold and the soup very hot. As Van Vleck recalled, Dirac took hand of the situation “with his characteristic directness by transferring an ice cube from the water to the soup.” Together with his companion, Dirac hiked and camped in the Rocky Mountains. “I enjoyed myself very much in the Rockies,” he wrote to Tamm, “although I did not do any difficult mountaineering but mostly kept to the trails.” Three years later, again in Van Vleck’s company, he did some pretty difficult mountaineering in the Rockies, which included an ascent of the 4,360-meter (14,308-feet) Uncompagrhe Peak, the sixth highest summit of the Rocky Mountains. He subsequently went to Princeton’s famous Institute for Advanced Study—where Einstein had settled the year before—and where Dirac spent two terms. However, instead of going directly back to England after his appointment at Princeton ended, he went westwards for another tour around the world. The route of this journey was largely the same as that taken five years earlier, although this time he entered the Trans-Siberian Railway at Irkutsk after having traveled through China.

At the Institute for Advanced Study in Princeton, Dirac’s room was close to Einstein’s but apparently, the two geniuses were not in contact. This is a bit strange, especially because a year earlier Einstein had recommended Dirac for a new chair at the Institute, rating him above Pauli. On the other hand, Dirac came to know the Hungarian theoretical physicist Eugene Paul Wigner closely. Three months younger than Dirac, Wigner was originally trained in chemical engineering. More attracted by mathematical subjects, he switched to atomic and quantum physics; in these areas, he became an important part of the German physics community. However, as a Jew he had no hope of a career in Germany after 1933, so he settled in the United States and became a naturalized citizen four years later. Wigner and Dirac had first met in 1928 when Dirac gave a lecture in Göttingen. Although impressed by the content of the lecture, Wigner was surprised at Dirac’s style of presentation, which he described as “detached, almost like a recitation of a technical text.” Dirac spoke about the exciting new quantum field theory “without giving any sign of enjoying his own lecture.”

In the fall of 1934, Wigner’s younger sister Margit visited him in Princeton, where Dirac met her for the first time. Margit Wigner Balasz was divorced and had two children, Gabriel and Judith. Until that time, Dirac’s relationship with women had been peripheral and platonic. He was generally thought to be an inveterate bachelor, perhaps gay and repressed. How could a woman possibly occupy a place in his mind (not to mention his heart), filled, as it was, with equations? Yet, the “genius who fears all women” (as Sunday Dispatch described him in 1933) eventually fell in love. Margit went back to Budapest while Dirac continued with his physics and the second world tour. However, he had not forgotten the Hungarian divorcée, whom he visited in August of 1935 on his way home from Moscow. When Dirac returned to Cambridge, he wrote Margit a letter to tell her that he missed her very much. “I do not understand why this should be, as I do not usually miss people when I leave them.” Unlike the British quantum genius, Margit was talkative, passionate, and extremely social. But Dirac was warming up to her, proving that opposites sometimes really do attract. It took time for Dirac to commit, but finally, on January 2, 1937, the two married.

Physicist Ernest Rutherford mistakenly believed that Margit was a widow. In a letter of early 1937, he told a friend about the surprising wedding news, commenting, “I think it will require the ability of an experienced widow to look after him adequately!” However, despite their personality differences, the marriage was happy and lasting. Paul and Margit (known as “Manci”) had two daughters of their own, Mary Elizabeth and Florence Monica. Paul’s stepson Gabriel studied mathematics at Cambridge University, graduating with a Ph.D. degree in 1951. He later went to work in the Mathematics Department at Aarhus University in Denmark. When he died at 59 in July of 1984, his stepfather attended the interment in Aarhus. It was Dirac’s next-to-last trip. He passed away three months later in Tallahassee, Florida.

Under Margit’s influence, Paul became more social. A year after their marriage, he wrote his wife a moving letter in which he gratefully admitted the change in his life that she had incited: “You have made me human. I shall be able to live happily with you even if I have no more success in my work.” Although Dirac became more “human,” he was not exactly ordinary. Gamow told the story of how one of Dirac’s old friends, a physicist, came to see him shortly after the wedding. Unaware of the marriage, the friend was surprised to find a woman in Dirac’s house. When Dirac noticed his curiosity, he said, “Oh, I’m sorry. I forgot to introduce you. This is … this is Wigner’s sister.” 

Under Stalin’s rule, the Soviet Union attracted only a few foreign scientists and almost no tourists. However, Dirac found the country most interesting and visited it no less than seven times between 1928 and 1937, the last time in the company of his wife. This was the year of what is known as the “great terror ”—Stalin’s bloody eradication of his enemies, real or perceived. Many of the victims were scientists, and a few of them, foreigners. In a letter he penned in 1937, Kapitza mentioned that many of the theoretical physicists at Leningrad University had been arrested, charged with being enemies of the state. “In fact,” Kapitza wrote, “so many were arrested that in the university faculty of mathematics and physics no one could be found to lecture to students.” When Dirac wanted to visit the country yet another time in 1938, he was refused a visa.

During the 1930s, Dirac had very close relations with Soviet physicists and a genuine interest in what he, as well as many other Western scientists and intellectuals, saw as a promising social and economic experiment. Of Dirac’s almost 200 publications, only four were collaborative papers and, of these, two were written with Russian co-authors. In 1931, he was elected a corresponding member of the USSR Academy of Science, a rare honor for a 29-year-old physicist and one with political overtones. On the advice of Kapitza, six years later, Dirac contributed a paper to a special issue of the Bulletin of the USSR Academy of Science commemorating the 20th anniversary of the October Revolution. The paper could be seen as a political manifestation, but this was hardly Dirac’s intention. It was of a purely scientific nature.

Although Dirac was highly regarded in the Soviet Union, his view of quantum theory did not please the new communist society’s ideological guardians. To them, it smelled a little too much of bourgeois idealism. The 1932 Russian translation of Principles of Quantum Mechanics was supplied with a preface by the publisher warning naïve readers that the book “contains many views and statements completely at variance with dialectical materialism.” Likewise, in the second edition of 1937, the publisher pointed out that Dirac “makes some philosophical and methodological generalizations that contradict the only truly scientific method of cognition—dialectical materialism.”

The autumn of 1933 was busy for Dirac. First, he attended a conference in Copenhagen, and from there he proceeded to another one in Leningrad. He wanted Bohr to join him. “You may be sure of a warm welcome from the Russian physicists and I think you will find it interesting to see something of the modern Russia. (The economic situation there is completely different from everywhere else).” Dirac was impressed by the socialist experiment and took great interest in the improvements of industry, living standards, and availability of consumer goods. It did not occur to him that what he saw was only what the Soviet authorities allowed him to see.

After the Leningrad conference, Dirac participated in his third Solvay Congress in Brussels, the subject of which was the physics of atomic nuclei. In December, he was in Stockholm to receive the Nobel Prize. Following the etiquette, he gave a brief speech at the traditional Nobel banquet. It was quite unusual. Most laureates use the speech to summarize their scientific work or put it into some larger perspective, but Dirac instead dealt with the “great similarity between the problems provided by the mysterious behaviour of the atom and those provided by the present economic paradoxes confronting the world.” The effects of the great economic depression were still all too visible. Undoubtedly to the surprise of the audience, Dirac argued that the cause of these troubles was “an economic system which tries to maintain an equality of value between two things, which it would be better to recognise from the beginning as of unequal value.” He was referring to regular incomes versus single payments. To him, “a regular income is worth incomparably more, in fact infinitely more, in the mathematical sense, than any single payment.” It is not quite clear what Dirac meant by his speech, but it might have been an allusion to the Marxist view of economy. At least, that is what Erwin Schrödinger’s wife Annemarie thought. She described the speech as “a tirade of communist propaganda.”

If most of the audience in Stockholm found Dirac’s speculation about a similarity between quantum physics and economy to be slightly odd, his co-laureate Schrödinger did not. He gave Dirac’s speech much thought and responded to it at length in a letter he wrote on Christmas Eve of 1933. In it, Schrödinger commented on a mathematical paper Dirac had recently published and which Schrödinger found hard to understand because of its brevity and condensed style. “My dear Dirac,” he wrote, “one has the impression, that you are frequently afraid of using up too much paper and print. Are you not aware of the fact, that pages and pages are used up for the reproduction of thoughts, which are considerably less important than yours.”

One of the physicists Dirac visited during his travels to Russia was his old friend Kapitza, who in 1934 had been prevented by the Soviet authorities from returning to Cambridge. In letters from the summer of 1935, Kapitza told about Dirac’s stay with him in Bolshevo, outside Moscow. He emphasized the human traits of the great theorist: “Dirac treats me so simply and so well that I can feel what a good and loyal friend he is.” In another letter: “Dirac and I get on very pleasantly together, chatting and discussing only when we feel like it. … As always, Dirac is somewhat eccentric but I find him easy to get along with. I tried to get Dirac into a flirtation with a good looking girl at 18, who is a language student and speaks English—but I had no success.”

Although Dirac’s attitude to the Soviet Union and its economic system was clearly positive, he was probably not a Marxist and at no time was he a member of the communist party, or any other political party. Nevertheless, for a period, he was friendly to the economic and social system of the Soviet regime, which he tacitly endorsed and valued from a scientific and logical point of view. Dirac traveled with British socialists and communists, but in a detached and passive way only. His many travels to the Soviet Union and general sympathy for the country later got him into trouble. In 1954, for the fourth time after the war, Dirac wanted once again to visit the United States but, this time, he was denied a visa. No reason was given, but this was at the height of the Cold War, and there is little doubt that his many contacts with Soviet physicists made him suspicious in the eyes of McCarthyist circles. After vehement protests from the American scientific community, the decision was reversed.

When Britain declared war on Germany in 1939, Dirac could not completely withstand the pressure to become engaged in matters outside pure physics. British scientists investigated at an early date the possibility of constructing an atomic bomb, and Dirac became involved in the work, if only peripherally and on a consultancy basis. He contributed to theoretical models of a uranium bomb and the separation of isotopes through centrifuges. This work was relatively important and highly classified. When the British bomb project was taken over by the huge Manhattan Project in the United States, the American-British group of physicists at Los Alamos wanted Dirac to join them. He refused. Dirac spent the rest of the war years in Cambridge and Dublin, where he attempted to reformulate quantum electrodynamics, a line of pure research with no connection whatsoever to military applications.

Dirac’s rather sporadic involvement in war-related physics did not spring from a patriotic desire to contribute to the war efforts. What primarily motivated him to do research on uranium physics and centrifuge technology was not the military context, but the scientific relevance. Neither should his decision not to participate in the Manhattan Project be seen as a conscious opposition to using physics for military purposes. This was not a question that preoccupied Dirac. He also did not seem to have cared much about the scientists’ ethical responsibility and the problems raised by the new nuclear weapons. These topics were widely discussed in the immediate post-war period, but not by Dirac. On the whole, he was the arch-typical ivory-tower scientist.