Hideki Yukawa

1.1. Early Life and Education

Hideki Yukawa was born as Hideki Ogawa on January 23, 1907, in Ichibei-cho, Azabu-ku, Tokyo, which is now part of Roppongi, Minato-ku, Tokyo. His father, Takuji Ogawa, was a distinguished geologist and a professor at Kyoto Imperial University, while his mother was Koyuki Ogawa. He was the third son in a family with two older brothers, two older sisters, and two younger brothers. When he was one year old, in 1908, his family relocated to Kyoto following his father's appointment at Kyoto Imperial University. Although born in Tokyo, Yukawa considered Kyoto his hometown, noting in his autobiography, "My memories begin after moving to Kyoto. Perhaps Kyoto is my true hometown."

His maternal grandfather, Komakitsu Ogawa, a former samurai of the Kishū Domain with deep knowledge of Chinese classics, played a significant role in his early education, teaching him to read classical Chinese texts from the age of five or six. Yukawa later reflected that this early exposure, even without full comprehension, greatly facilitated his later reading by familiarizing him with kanji. Komakitsu was also a man of modern learning, having studied Western sciences and subscribing to London Times until his later years.

During his time at Kyoto Prefectural Kyoto First Middle School (now Kyoto Prefectural Rakuhoku High School), Yukawa was a quiet and unassuming student, often called "Gonbee" (権兵衛^Japanese) or "Iwan-chan" (イワンちゃん^Japanese) due to his reticence. His father, Takuji, initially underestimated his abilities, considering sending him to a technical college rather than a university. However, the middle school principal recognized Yukawa's "high potential" in mathematics and offered to adopt him to ensure he pursued a scholarly career, leading his father to relent.

Yukawa's academic path was marked by deliberate choices. In high school, he decided against a career in mathematics after a teacher marked his correct proof of a theorem as incorrect because it deviated from the expected method. Later, in college, he abandoned experimental physics due to his clumsiness in glassblowing, a required skill for experiments in spectroscopy. These experiences solidified his focus on theoretical physics, particularly the theory of elementary particles.

1.2. Marriage and Family

In 1932, Hideki Ogawa married Sumi Yukawa (born Sumiko), the second daughter of Genyo Yukawa, a doctor who ran the Osaka Gastrointestinal Hospital (later renamed Yukawa Gastrointestinal Hospital). In accordance with Japanese custom, as Sumi's family had no sons, Hideki was adopted into the Yukawa family as a mukoyoshi (adopted son-in-law) and changed his surname from Ogawa to Yukawa. The couple had two sons, Harumi and Takaaki. Harumi Yukawa (born 1933) worked for Heibonsha before becoming a researcher of early modern Japanese theater. Takaaki Yukawa (born 1934) worked for Kodansha before his untimely death from a heart attack in 1971. Solo violinist Diana Yukawa is a distant relative of Hideki Yukawa.
Harumi Yukawa's memoir includes The Child and Mother Who Lived in the Yukawa Family (edited, Dorimusha, 2008).

1.3. Early Career

After graduating from the Department of Physics at Kyoto Imperial University in 1929, Yukawa remained at the university as a lecturer for four years, working in the laboratory of Professor Kajuro Tamaki. In 1933, he became a lecturer at Osaka Imperial University (now Osaka University) at the age of 26. He also held a concurrent lecturer position at Tohoku Imperial University (now Tohoku University) after meeting Hidetsugu Yagi at a meeting of the Physico-Mathematical Society of Japan.

During his early career, Yukawa was known among his students for his soft voice and challenging lectures. He faced pressure from Professor Yagi, who reportedly told him, "I was originally planning to invite Shin'ichirō Tomonaga, but since your brother requested it, I reluctantly hired you. So, you must study hard so as not to lose to Tomonaga."

2.1. Theory of Mesons and Nuclear Force

In the early 1930s, physicists struggled to explain the strong force that binds protons and neutrons together within the atomic nucleus, despite the electrostatic repulsion between protons. This interaction needed to be strong but limited in range. In 1934, Yukawa conceived his meson theory, which he formally published in 1935 in his seminal paper "On the Interaction of Elementary Particles. I." In this paper, he theoretically predicted the existence of a new particle, which he called the meson (now known as the pion or pi meson), as the mediator of the nuclear force (strong interaction). He estimated its mass to be approximately 200 times that of an electron.

Initially, many Western scientists, including Niels Bohr and Werner Heisenberg, were skeptical of Yukawa's proposal of an unobserved new particle. Bohr reportedly questioned Yukawa during his 1937 visit to Japan, asking, "Do you want to create new particles so much?" However, Yukawa's theory gained international attention when Carl David Anderson and Seth Neddermeyer discovered a particle with an intermediate mass (about 207 times the electron mass) in cosmic rays in 1936. This particle, initially called the "mu-meson" (now known as the muon), was first thought to be Yukawa's predicted meson. However, it was later found to interact only weakly with the nucleus, unlike the strong interaction predicted by Yukawa.

The mystery was resolved in 1947 when Cecil Frank Powell, Giuseppe Occhialini, and César Lattes discovered the pi meson (pion) in cosmic rays, which behaved exactly as Yukawa had predicted, mediating the strong nuclear force. This particle rapidly decays into muons, explaining why muons were detected first. Yukawa also contributed to the theory of K-capture, a process where a low-energy electron is absorbed by an atomic nucleus.

2.2. Nobel Prize in Physics

The experimental confirmation of the pi meson in 1947 validated Yukawa's 1935 theory, leading to his recognition with the Nobel Prize. On November 3, 1949, Hideki Yukawa was awarded the Nobel Prize in Physics "for his prediction of the existence of mesons on the basis of theoretical work on nuclear forces." He became the first Japanese person to receive a Nobel Prize and the third Asian laureate, following Rabindranath Tagore and C. V. Raman. This news provided a significant boost to the morale of the Japanese people, who were still recovering from the devastation and loss of confidence following World War II and the Allied Occupation. Physicist Katsuhiko Sato recalled Yukawa as a hero for achieving such a feat with only "paper, pencil, and his own brain" in a poor country like Japan, in contrast to Western scientists working in privileged environments. An analysis of Nobel Prize selection documents in 2000 by Takuji Okamoto noted that the connection between Yukawa's research and the award was "rarely as clear" in the history of the Nobel Prize, with most nominations coming from foreign scientists.

2.3. Later Theoretical Work

After receiving the Nobel Prize, Yukawa continued his research, focusing on problems within quantum field theory. He proposed theories such as the non-local field theory and the elementary domain theory, aiming to address fundamental issues in physics. He also dedicated significant effort to the problem of causality breaking in Minkowski space, a challenge he referred to as "Yukawa's circle" (湯川の丸○_{Yukawa no maru}^Japanese). This problem, also raised by Paul Dirac, concerned the definition of probability amplitude on closed surfaces in Minkowski space, which seemed to violate causality. While Shin'ichirō Tomonaga's super-time theory offered a way to circumvent this by spatially limiting the causality problem, Yukawa's own attempts to address it through non-local fields were not ultimately successful.

Yukawa was also notably critical of Murray Gell-Mann's quark theory, stating that "there is no way that such an ambiguous thing as a charge of 1/3 or 2/3 can exist." Hironari Miyazawa, who first proposed supersymmetry, argued that modern physics has largely avoided Yukawa's fundamental problem of causality, instead focusing on phenomenological approaches.

3.1. Professorships

Yukawa's academic journey began as a lecturer at Kyoto Imperial University from 1929 to 1933. He then moved to Osaka Imperial University, where he served as a lecturer from 1933 and was promoted to assistant professor in 1936. In 1939, he returned to Kyoto Imperial University as a professor. He also held a professorship at Tokyo Imperial University in 1942. After his Nobel Prize, he became a visiting professor at the Institute for Advanced Study in Princeton in 1948 and then a visiting professor at Columbia University in July 1949, becoming a full professor there in 1950. He retired from Kyoto University in 1970 as a Professor Emeritus.

3.2. Yukawa Institute for Theoretical Physics

In 1953, Hideki Yukawa became the first chairman of the Yukawa Institute for Theoretical Physics (YITP) at Kyoto University. The institute was established to foster theoretical physics research in Japan and became a major center for the field, reflecting Yukawa's vision for advancing fundamental physics. He also served as the chairman for the International Conference on Theoretical Physics in Tokyo and Kyoto in the same year.

5.1. Philosophy of Science

Yukawa often reflected on the nature of scientific inquiry, emphasizing the importance of creativity and intuition. In the foreword to his autobiography, Tabibito (The Traveler), he expressed his desire "to be a traveler in a strange land and a colonist in a new country," highlighting his continuous quest for new knowledge. He believed that even early, seemingly unproductive learning, such as his childhood studies of Chinese classics with his grandfather, could yield significant intellectual benefits by fostering familiarity with complex concepts and language. He also explored the relationship between Eastern and Western thought in the context of scientific understanding. His writings, such as Creativity and Intuition: A Physicist Looks at East and West, delve into these themes. He frequently signed his calligraphy with "Chigyōraku" (知魚楽_Chigyōraku^Japanese), meaning "to know the joy of fish," a phrase from Zhuangzi's "Autumn Floods" chapter, reflecting his appreciation for the natural world and a deeper understanding beyond superficial observation.

5.2. Causality and Fundamental Physics

Yukawa maintained a lifelong engagement with fundamental problems in physics, particularly the concept of causality. He raised the issue of "Yukawa's circle" (湯川の丸○_{Yukawa no maru}^Japanese), which posited that defining probability amplitude on a closed surface in Minkowski space could lead to a violation of causality. This problem was also independently raised by Paul Dirac. While Shin'ichirō Tomonaga's super-time theory offered a solution by limiting the causality problem to spatial dimensions, Yukawa continued to pursue a more fundamental resolution through his work on non-local field theories, though these did not ultimately succeed in solving the issue. Physicist Hironari Miyazawa, who first proposed supersymmetry, has argued that the problem of causality breaking, as highlighted by Yukawa, remains unresolved and that much of modern physics has veered towards phenomenology, avoiding this fundamental question.

6.1. Anti-Nuclear Activism

Yukawa's experience during and after World War II profoundly shaped his views on nuclear weapons. In June 1945, towards the end of the Pacific War, he was invited to a meeting for Japan's atomic bomb development project (known as F-research), led by Bunsaku Arakatsu at Kyoto University. However, Japan surrendered before the project could be fully initiated. Following the atomic bombings of Hiroshima and Nagasaki, Yukawa declined requests from newspapers to provide commentary. After the war, he was questioned by the occupying American forces regarding his involvement.

In 1955, Yukawa joined ten other leading scientists and intellectuals, including Bertrand Russell and Albert Einstein, in signing the Russell-Einstein Manifesto, a powerful call for nuclear disarmament. He became a vocal advocate against nuclear weapons, emphasizing the catastrophic consequences of their use. In 1956, he was appointed to Japan's Atomic Energy Commission at the request of its chairman, Matsutarō Shōriki. However, Yukawa strongly opposed Shōriki's push for rapid construction of nuclear power plants through foreign purchases, arguing that neglecting fundamental research would lead to future problems. This conflict led to his resignation from the commission in March 1957, after only one year and three months in the role, citing health reasons.

6.2. Efforts Towards World Peace

Yukawa was actively involved in various initiatives aimed at promoting world peace and global governance. He was a signatory to the agreement to convene a convention for drafting a world constitution, which subsequently led to the World Constituent Assembly drafting and adopting a Constitution for the Federation of Earth.

He presided over the First Kyoto Conference of Scientists, held at Tenryū-ji temple in Kyoto on May 7, 1962. This conference, inspired by the Pugwash Conferences on Science and World Affairs, appealed for the necessity of concluding a Treaty on the Prohibition of Nuclear Weapons. His commitment to peace was further recognized when he was nominated for the Nobel Peace Prize in 1966, as revealed by the Nobel Foundation's public release of nominee lists.

His dedication to peace is also immortalized at the Hiroshima Peace Memorial Park, where his tanka (a form of Japanese poetry) is inscribed on the pedestal of the "Wakaba" (Young Leaves) statue: "May the calamity's fire never come here again; this place is only for those who pray for peace." (まがつびよ ふたたびここにくるなかれ 平和をいのる人のみぞここは_{Magatsubiyo futatabi koko ni kurunakare Heiwa o inoru hito no mizo koko wa}^Japanese). The term "magatsubi" refers to "calamity's fire," signifying the atomic bomb. Even in his final years, despite declining health, he continued his advocacy. In June 1981, he attended the Fourth Kyoto Conference of Scientists, held after a 15-year hiatus, appearing in a wheelchair to passionately appeal for nuclear abolition.

9.1. Physics and Science Books

• Theory of β-Ray Radioactivity (1936)
• Introduction to Quantum Mechanics (1947)
• Introduction to the Theory of Elementary Particles, Vol. 1 (1948)
• Physics Lectures (1975, edited by Osamu Hara)
• Speaking of Theoretical Physics (1997, edited by Hiroshi Ezawa)
• Reading "Hideki Yukawa's Physics Lectures" (2007, supervised by Michiji Konuma)
• Theory of Non-Local Fields (1952, translated and annotated by Ken Inoue)
• Atomic Nucleus and Artificial Transformation of Elements, Vol. 1 & 2 (1938-1940, co-authored with Seishi Kikuchi)
• Nuclear Theory (1949, co-edited with Minoru Kobayashi)
• Direction of Physics (1949, co-authored with Ken Inoue and others)
• Continued Theoretical Physics Lectures (1949, co-authored with Hiroshi Suzuki)
• Elementary Particle Theory (1951, co-edited with Minoru Kobayashi)
• Cosmic Rays and Meson Theory (1955, co-edited with Minoru Kobayashi and Ken Inoue)
• General Physics, Vol. 1, 2, 3 (1955, 1961, 1962, co-authored with Matsuhei Tamura)
• Elementary Particles (1961, co-authored with Yasuhisa Katayama and Hideo Fukutome)
• The World of Physics (1964, co-authored with Yasuhisa Katayama and Eiji Yamada)
• Statistical Physics (1978, supervised by Hideki Yukawa, authored by Morikazu Toda, edited by Ryogo Kubo)

9.2. Philosophy, Essays, and Autobiography

• Recent Views on Matter (1939)
• The Principle of Existence (1943)
• The Invisible (1946)
• Lectures on Theoretical Physics (1946)
• Nature and Reason (1947)
• Thought and Observation (1948, 1949)
• Views on Matter and Worldview (1948)
• The World of the Extremely Small (1950)
• Creative Human (1966)
• Leap to Creation (1968)
• My Theory of Creation: Identification and Combination (1981)
• Aspiring to Physics (1944) - Autobiography
• Tabibito (The Traveler): Recollections of a Physicist (1958) - Autobiography
• Hideki Yukawa's Diary, 1934: The Path to Meson Theory (2007, edited by Michiji Konuma)
• Hideki Yukawa's Diary, 1945: Wartime and Postwar Notes from Kyoto (2020, edited by Michiji Konuma)
• Self-Discovery (1972)
• The World of Genius (1973, 1975, 1979, interviews with Kikuyasu Ichikawa)
• The Scientist's Heart (1977)
• The Outer World and the Inner World (1976)
• Atomic and Human Beings (1948)
• Science and Humanity (1948)
• A Brief Happiness (1954)
• Modern Science and Humanity (1961)
• The World Inside Books (1963)
• Rich in Heart: Collected Essays (1969)
• Academia and Life (1971)
• Universe and Human Beings: Seven Mysteries (1974, illustrated by Josaku Maeda)
• Born on This Earth (1975)
• Living Science: Hideki Yukawa's Essays (2015, edited by Satoru Ikeuchi)
• Hideki Yukawa's Collection of Poems and Writings (2016, selected by Mitsuhiro Hosokawa)
• Hideki Yukawa: Poetry and Science (2017)
• The Creativity of Scientists - From the Magazine 'Nature' (2021)
• Dialogue: On Human Progress (1948, co-authored with Hideo Kobayashi)
• Science and Literature: Dialogue (1949, co-authored with Taizo Ebara)
• Standing in the Field of Truth (1951, co-authored with Shoichi Sakata and Mitsuo Taketani)
• Kyoto: My Childhood Days... (1960, co-authored with Tsutomu Iijima and Yanosuke Kawabata)
• To Create a Peaceful Era: Scientists Appeal (1963, co-edited with Shin'ichirō Tomonaga and Shoichi Sakata)
• People Who Changed Human Life: New World Biographies 7 (1965, supervised by Hideki Yukawa)
• Primitive and Human Beings (1965, edited by Kiyonobu Itakura and others)
• Modern Dialogue (1966, co-authored with Takeshi Umehara, Takeo Kuwabara, Hiroshi Suekawa)
• What is Science for Humanity? (1967, co-authored with Tadao Umesao)
• Creation of Purpose in Life: Dialogue on Creation (1967, co-authored with Kikuyasu Ichikawa)
• Relationships: Essays Commemorating Dr. Yukawa's 60th Birthday (1968, edited by Yasutaka Tanikawa and others)
• Creation of Japanese Culture: What is a Japanese? (1968, co-authored with Masaaki Ueda)
• Beyond the Nuclear Age: Aiming for the Creation of Peace (1968, co-edited with Shin'ichirō Tomonaga and Shoichi Sakata)
• Dialogue: The World of the Universe and Mind (1969, co-authored with Tetsuzo Tanikawa)
• The World of Academia: Collected Dialogues (1970, edited by Hideki Yukawa)
• Modern Academic Theory (1970, co-authored with Shoichi Sakata and Mitsuo Taketani)
• The World of Physics, The World of Mathematics: Dialogue (1971, dialogue with Toshio Kitagawa)
• Half-Day Chats: Collected Dialogues (1971, 1980)
• Rediscovery of Humanity (1971, dialogue with Kikuyasu Ichikawa and Takeshi Umehara)
• Physicist Group Portrait: Dialogue (1972, dialogue with Shin'ichirō Tomonaga)
• The Philosophy of Peace (1973, edited by Hideki Yukawa, general editor Hiroshi Suekawa)
• The Future of Science and Humanity: Continued Half-Day Chats (1973, 1981, edited by Hideki Yukawa)
• Schrödinger's Selected Works, Vol. 1 & 2 (1974, supervised by Hideki Yukawa)
• Discovery of Humanity: Hideki Yukawa's Collected Dialogues (1976, 1981)
• New Conception for Nuclear Disarmament (1977, co-edited with Shin'ichirō Tomonaga and Toshiyuki Toyoda)
• Impressions of the Countryside and Other Works, Egg of Risshun and Other Works, A Certain Voyage and Other Works (1984, co-authored with Torahiko Terada and Ukichiro Nakaya)