Yukawa Hideki: Nobel Laureate in Physics in the Cause of Peace

A Scientist Schooled in the Classics

The Japanese physicist Yukawa Hideki was born Ogawa Hideki in 1907 in Tokyo, and moved to Kyoto in the following year when his father Ogawa Takuji, a geographer and geologist, took a professorial position at Kyoto University. (He took the surname Yukawa upon his marriage.) A precocious child, Hideki was learning the Analects of Confucius and other Confucian classics under his grandfather’s tutelage before he entered elementary school. The style of study was to read the texts aloud in Japanese way repeatedly, with no explanation of their meaning, in order to make the kanbun classical Chinese writing familiar. This does not mean that the young Hideki read nothing for his own enjoyment; he was free to read from his father’s collection, his mother bought magazines for him and his siblings, and his friends lent him popular books for children, which he enjoyed as well.

His father, a great reader, ensured that Hideki grew up in a home filled with books. The Taoist text Zhuangzi, which he came across as a junior high school boy, was one that would captivate his imagination for the rest of his life. He was also deeply interested in Western histories, and he read translated foreign novels as well; but it was geometry that truly captivated him. As a young learner he was drawn toward logical thought even more than areas requiring observation or memorization skills.

Upon entering high school, he narrowed the focus of his studies to the sciences. Even so, early in these years, he spent time in the school library engrossed in philosophical texts. As time passed, though, he gravitated toward books like the philosopher Tanabe Hajime’s 1915 Saikin no shizen kagaku (Recent Developments in the Natural Sciences) and 1918 Kagaku gairon (A General Overview of the Sciences), which honed his desire to study physics himself.

Clearing Up Subatomic Mysteries with Meson Theory

The world of physics had seen great change around the turn of the century. In 1900, the German scientist Max Planck published his discovery that the energy contained in emitted light attains only certain discrete values, which he labeled quanta. In 1905 Albert Einstein discovered that light has the properties of both waves and particles and created the Special Theory of Relativity. Previously accepted knowledge showed the limits of its validity and the physics community entered an age of discovery and upheaval.

Yukawa, right, visits Albert Einstein at the Institute for Advanced Study, Princeton, New Jersey, in June 1953. (© Mainichi Newspapers/Aflo)

This upheaval continued on through the mid- to late 1920s, when the field of quantum mechanics came into its own. In 1924, when he was still a teenager, Yukawa bought a copy of The Quantum Theory, by the German physicist Fritz Reiche, in an imported bookstore. “This was more exciting to me than any novel I had read before,” he later recalled. Two years later, when he entered the physics course at Kyoto Imperial University, he plunged directly into the world of theoretical physics, purchasing a steady stream of the latest physics texts from abroad and educating himself on quantum theory.

Upon his graduation in 1929, Yukawa had determined to pursue a research career, and was considering two major themes to explore in his work. The first was a new theory to be built on the frameworks of quantum mechanics and relativity. The second was an attempt to use quantum mechanics to shed light on the mysteries of the atomic nucleus. Both of these areas were difficult problems to scientists, who were working on them all over the world. Yukawa would end up pursuing work in both of them for the rest of his career.

After graduating, he spent five years in difficult research with no mentor to guide him. In 1934, though, he arrived at an achievement: his “meson theory,” which stated that the protons and neutrons in the atomic nucleus were bound together by subatomic particles later called mesons, via the nuclear force. He presented his theory at the Physico-Mathematical Society of Japan and submitted a paper in English; the following year, this was published in the Proceedings of the society.

In 1937, cosmic-ray experiments carried out in the United States and Japan confirmed the existence of particles with the mass Yukawa had predicted, and he gained worldwide fame. Work began to expand his theory, with teams vying fiercely for further progress in Britain and other major scientific powers abroad, as well as in Japan.

It eventually became clear, though, that there were inconsistencies between the mesons proposed by Yukawa and the particles detected in cosmic-ray research. In 1942, two scientists from Yukawa’s research group, Tanikawa Yasutaka and Sakata Shoichi, predicted that the particles detected in cosmic rays were formed through the decay of a Yukawa particle. The British physicist Cecil Powell in 1947 successfully confirmed that Yukawa’s particle —which he labeled the pi meson, or pion—was present in cosmic rays, where it decays to form the particles found in 1937. The following year, accelerator experiments in the United States created artificial Yukawa particles. The Japanese scientist’s mesons were confirmed by both natural observation and laboratory work. His first published work earned him the 1949 Nobel Prize in physics, making him the first Japanese Nobel prize winner. In 1950, Powell also received the Nobel prize for his meson work.

The Power of the Atom Becomes a Threat

After Japan went to war against the United States, Britain, and the other Allied powers in 1941, Yukawa—then a professor at Kyoto Imperial University—found himself wondering on several occasions whether he should proceed with his research in the fundamental sciences. Each time, though, he returned to the same conclusions: that it was important for him to contribute to the field for which he was best equipped, and that basic research was just as necessary as applied technologies. In 1943, however, the Japanese government proclaimed the achievement of the country’s war goals to be the sole, supreme purpose of scientific research, mobilizing all Japanese researchers toward this end. Yukawa felt that he too had to take part in military research, but had difficulty identifying research themes where he could contribute. In the end, he and several of his colleagues went to work under the physicist Arakatsu Bunsaku on a project commissioned by the Imperial Japanese Navy, seeking applications for uranium-based nuclear energy. Uranium could not be mined in Japan, though, and the research was called off before Japan’s defeat. In the end, Yukawa made no direct contributions to the Japanese war effort.

Two days before that final defeat, on August 13, 1945, Yukawa listened to a report from Arakatsu, who had been to Hiroshima to carry out a detailed survey of the destruction, especially the radiation damage, from the atomic bomb that had been dropped on the city on August 6. He began a lengthy period of reflection and contemplation, during which he made no public statements and turned down all requests for articles or other writing. At last, in October that year, he published the essay “Shizuka ni omou” (Quiet Thoughts) in a weekly magazine. This marked a decision he had made on the course he would chart in the postwar era, based on a self-examination of his previous life. In it he wrote: “For a nation’s goals and the means it uses to achieve them to be justified, they must be consistent with the advancement of the welfare of humanity as a whole.” Based on this conviction, he concluded: “Out of the ordering of individual, family, society, nation, and world, it was a grave error to take the nation alone and give it supreme authority.”

The United States remained the sole nuclear power for some years after World War II, carrying out repeated bomb tests. In 1949, when the Soviet Union also built its first atomic weapon, Washington embarked on the development of the much more powerful hydrogen bomb. The Soviets quickly followed suit, producing an escalating development race. America’s March 1, 1954, Bravo hydrogen bomb test in Operation Castle at Bikini Atoll in the South Pacific dealt severe damage to many fishing vessels and island residents located outside the designated hazard zone. The radiation sickness that afflicted all crew members of the Daigo Fukuryu Maru, a Japanese vessel—one of whom succumbed to the radiation in September—and the vast quantities of irradiated tuna that had to be discarded at markets drove home the threat of these weapons all around the world, not just in Japan.

Following the Bikini test, Yukawa contributed an essay to the March 30 Mainichi Shimbun titled “Genshiryoku to jinrui no tenki” (Atomic Energy and the Turning Point for Mankind). In it he argued forcefully: “Protecting ourselves from the threat of the power of the atom is a goal that must be placed above all others.”

Worldwide Efforts for Peace

At the time of the Bravo test, Yukawa, then 47, was director of the Research Institute for Fundamental Physics at Kyoto University. In addition to his research and teaching duties, he had a number of other tasks, but now to these he added fervent writing and speechmaking aimed at the abolition of nuclear weapons. The man of thought became a man of action.

The British philosopher and mathematician Bertrand Russell, who was also shocked by the Bikini test, conferred with Albert Einstein and organized the Russell-Einstein Manifesto, released on July 9, 1955, with 11 scientists in total, including Yukawa, as signatories. The manifesto resolved: “In view of the fact that in any future world war nuclear weapons will certainly be employed, and that such weapons threaten the continued existence of mankind, we urge the governments of the world to realize, and to acknowledge publicly, that their purpose cannot be furthered by a world war, and we urge them, consequently, to find peaceful means for the settlement of all matters of dispute between them.” The document urged that “scientists should assemble in conference” to deliberate problems like these, extending its call beyond the scientific community to all people around the world.

The first of the Pugwash Conferences on Science and World Affairs, the outcome of this manifesto, was held in Pugwash, a small fishing village in Canada’s Nova Scotia, in July 1957. Despite the overarching Cold War structure pitting East against West, celebrated scientists from around the world, including US and Soviet researchers, came in their individual capacity and managed to reach unanimous agreement on almost all the issues they discussed. These conferences continue to be held today. Yukawa, who attended the first Pugwash Conference, continued speaking out to ensure their robust development. In 1962, along with fellow Japanese physicist Tomonaga Sin-itiro, he founded the Kyoto Conference of Scientists, a Pugwash affiliate group for supporters of the Russell-Einstein Manifesto and its goals.

Alongside these efforts, in 1955 Yukawa responded to a call from Shimonaka Yasaburo, member of the World Federalists, to form a nonpartisan group of experts, the Committee of Seven to Appeal for World Peace. This group’s first action was an appeal that the United Nations, which had just celebrated the tenth anniversary of its foundation, be fundamentally reformed and strengthened. Yukawa was inspired by the World Federalists’ idea that the planet needed a single government capable of tackling problems on a global scale, while allowing nations to maintain their independence, and he was elected as president of the World Association of World Federalists at the group’s 1961 World Congress, a role he would fill with great energy for the next four years.

Yukawa, seated in a wheelchair, greets foreign scholars at the 1975 Pugwash Symposium, the first held in Japan, at the Kyoto International Conference Center. (© Jiji)

In 1975, the twenty-fifth Pugwash Symposium was held in Kyoto, with the theme “A New Design Toward Complete Nuclear Disarmament.” Although he at the time recovering from serious illness, Yukawa was in attendance with Tomonaga, presenting “Beyond Nuclear Deterrence: The Yukawa-Tomonaga Declaration,” an argument that the nuclear deterrence doctrine would not bring about peace. His activities continued until 1981, when he hosted and attended from his sickbed the Kyoto Conference of Scientists, calling for a new global order and the abolition of nuclear weapons.

Ten days after the Kyoto Conference gathering, he penned “Heiwa e no negai” (A Wish for Peace), which was his final writing. Three months later, on September 8, 1981—eight years before the end of the Cold War—Nobel laureate Yukawa Hideki breathed his last.

(Originally written in Japanese. In line with the author’s preference, macrons have been removed from romanized Japanese terms in this article. Banner photo: Yukawa Hideki in 1965. © Jiji.)