Hideki Yukawa was the first citizen of Japan to receive a Nobel Prize, an award given to him in 1949 for his theory of the meson, the subatomic particle that binds the nucleus' protons and neutrons. In addition to that honor, Yukawa received the Imperial prize of the Japan Academy in 1940, the Lomonosov Gold Medal of the Soviet Academy of Sciences in 1964, the Order of Merit of the Federal Republic of Germany in 1964, and the Order of the Rising Sun (Japan) in 1977.
Hideki Yukawa was born Hideki Ogawa in Tokyo on January 23, 1907. He was the fifth of seven children born to Takuji and Koyuki Ogawa. His father was employed at the Geological Survey Bureau in Tokyo at the time of Hideki's birth and a year later was appointed professor of geology at Kyoto Imperial University. In his autobiography, Tabibito (The Traveler ), Yukawa describes the experience of growing up in a large household which included, in addition to his parents and siblings, three grandparents. One of these, his maternal grandmother, had once taught at the samurai school at Tokugawa Castle and was a particularly strong influence in Yukawa's life. She taught him to read and write kanji (Chinese pictographs) before he entered elementary school.
Learns Modern Physics
Yukawa attended the Third High School in Kyoto from 1923 to 1926. There he was a classmate of future Nobel Laureate Sin-Itiro Tomonaga who, for his work on quantum electrodynamics, would go on to share the Nobel Prize with Paul Dirac . After graduation, Yukawa entered Kyoto Imperial University, where he majored in physics. Yukawa's interest in the subject had been aroused in high school when he discovered a number of books on quantum mechanics and relativity in the school library. One of the most influential of the books he found, Max Planck 's Introduction to Theoretical Physics, he was able to read only after he had taught himself German.
In 1929, Yukawa received his master's degrees from Kyoto, and then stayed on as a research assistant in the laboratory of Kajuro Tamaki. Then, in 1932, Yukawa accepted an appointment as lecturer in physics at Kyoto. Just prior to accepting his new teaching post, Hideki married Sumi Yukawa, a classical Japanese dancer. He adopted his new wife's family name and went to live with them in Osaka. The Yukawas eventually had two sons, Harumi and Takaai.
In 1933 Yukawa accepted a second position as lecturer in physics, this time at Osaka Imperial University. He continued teaching at Osaka for the next five years, working on his doctorate in physics at the same time. In 1936 he was promoted to associate professor of physics and, two years later, was awarded his Ph.D.
Proposes the Meson Theory
It was during his years at Osaka that Yukawa made the discovery for which he is best known, the meson theory. The early 1930s were a period of some confusion for physicists interested in the atomic nucleus. They had learned from Werner Karl Heisenberg that nuclei consist of only two particles, protons and neutrons. Of these two, only protons have electrical charge, a positive charge. It would appear, then, that nuclei should be inherently unstable: the electrostatic force of repulsion among protons should, according to classical theory, tend to blow the nucleus apart.
Yukawa became interested in this problem in about 1930. It occurred to Yukawa that there must be some force far stronger than the electromagnetic force that could hold nucleons (protons and neutrons) together. As he developed his theory, Yukawa came to the conclusion that such a force must take the form of a particle, carrying a force of attraction back and forth between pairs of nucleons.
In his calculations, Yukawa found that this force-carrying particle would have a mass about 200 times greater than the electron, but only one-ninth that of a proton or neutron. Because of the particle's intermediary mass, it was later given the name meson, from the Greek for "middle." (Scientists actually considered naming the particle the "yukon," in honor of its discoverer, but discarded it to avoid possible geographical confusion.)
Yukawa first announced his theory of the meson at scientific meetings in Osaka and Tokyo in October and November, 1934, and then in the Proceedings of the Physico-Mathematical Society of Japan in February, 1935. For about two years his ideas remained largely ignored; then, in 1937, Carl D. Anderson and Seth Neddermeyer discovered a particle that appeared to have many of the properties predicted by Yukawa. The scientific community's initial enthusiasm for this discovery soon died out, however, as additional studies showed that Anderson's meson--later named the mu-meson, or muon--differed in some fundamental ways from Yukawa's prediction. It was not until 1947 that Yukawa's work was fully confirmed. That year, Cecil Frank Powell found the Yukawa particle--now called the pi-meson--in a cosmic ray shower. Two years later, Yukawa was awarded the 1949 Nobel Prize in physics for his discovery of the meson--an honor that brought substantial pride to the war-torn Japanese scientific community.
In the mean time, Yukawa had returned to Kyoto University where he had been appointed professor of theoretical physics in 1939. The year he was awarded the Nobel Prize, Yukawa came to the United States on a one-year visiting professorship at the Institute for Advanced Studies in Princeton, New Jersey. At the end of that year he accepted an appointment at Columbia University, where he remained for four more years. In 1946 he founded the scientific journal Progress of Theoretical Physics, for which he also served as editor.
In 1953 Columbia awarded Yukawa tenure, but he decided nonetheless to return to Japan. There he assumed his previous post at Kyoto University, as well as the newly-created position as director of the Research Institute for Fundamental Physics, an institute established specifically for him by the Japanese government. Although he retired officially from his academic positions in 1970, Yukawa continued to write, speak, and edit his journal. He was also active in organizations that promoted the peaceful use of science and technology: for example, in 1955 he, along with other scientists, signed the Russell-Einstein paper advocating the settling of political disputes through peaceful means; Yukawa was also in attendance at a number of Pugwash Conferences, in which scientists discussed options for disarmament. He died from pneumonia in Kyoto on September 8, 1981.
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