Hideki Yukawa

Japanese physicist

• Born: January 23, 1907; Tokyo, Japan
• Died: September 8, 1981; Kyoto, Japan

When Hideki Yukawa won the 1949 Nobel Prize in Physics, he became the first Japanese person to win a Nobel Prize. Though he was often regarded as soft-spoken and shy, his ideas about the nature of atoms spoke loudly, forming the foundation for modern particle studies.

Primary field: Physics

Specialties: Theoretical physics; atomic and molecular physics

Early Life

Hideki Yukawa was born Hideki Ogawa on January 23, 1907, in Tokyo, Japan. He was the fifth of seven children in a well-educated family. His grandparents were professors and scholars, and his father, Takuji Ogawa, became a professor of geology at Kyoto Imperial University (now Kyoto University) in 1908. Three of his brothers also became professors.

Though Yukawa mainly studied mathematics and literature as a young student, he quickly became interested in the sciences when he entered Kyoto Imperial University. He eventually decided to study theoretical physics, partly due to his stated inability to work in more experimental fields. He credited two of his professors, Yoshio Nishina and K. Tamaki, with encouraging him to study theoretical physics.

When he graduated from Kyoto Imperial University in 1929, Yukawa stayed on at the school as an unpaid assistant, working on solving the so-called divergence problems of quantum electrodynamics. Yukawa failed to make any headway with these problems, so he began looking into nuclear forces.

The next few years proved both fruitful and rewarding for Yukawa. While continuing to lecture at Kyoto Imperial University, he married Sumiko Yukawa, and was adopted by her family (taking the name Yukawa), in 1932. The couple’s first son, Harumi, was born in 1933, and their second son, Takaki, was born in 1935.

In 1933, Yukawa was awarded an assistant professorship at Osaka Imperial University. In 1934, at the age of twenty-seven, he began the work that would later earn him the Nobel Prize in Physics. He successfully predicted the existence of a particle called a meson—the subatomic particle responsible for the force that holds all atoms together.

Life’s Work

In 1935, Yukawa published his theory of mesons in the paper “On the Interaction of Elementary Particles. I.” Just three years earlier, Werner Karl Heisenberg had updated the standard model of the atomic nucleus. Heisenberg correctly proposed that the nucleus is comprised of protons and neutrons, rather than protons and electrons as had been previously assumed.

This new theory posed a further problem: Since protons are positively charged, and neutrons have no charge, the protons in the nucleus should repel each other; this would break up the nucleus and, by extension, the atom. This mystery of atomic force intrigued Yukawa and eventually led him to his meson theory.

Working with Eugene Paul Wigner, another future Nobel laureate, Yukawa proposed the existence of a force that keeps protons from repelling each other. Existing experimental data gave him two important clues in his investigation. The first clue was that the atomic force must have a very short range in order to not interfere with the electrons orbiting the nucleus. Yukawa used an electromagnetic field as a model for his theoretical force and adapted it to produce short-range forces. The second clue was that any field must be associated with a particle. Therefore, Yukawa determined, there had to be a previously unknown particle in the nucleus of an atom that acted to hold the nucleus together.

Yukawa found there was a relationship between the range of a force and the mass of the particles associated with that force. Based on the assumed range of atomic forces, he estimated the particle’s mass at about two hundred times the mass of an electron. These particles, he theorized, are constantly being exchanged between the protons and the neutrons (collectively known as nucleons) in an atomic nucleus.

Each nucleon, Yukawa deduced, has a cloud of particles orbiting around it, and each nucleon shares its cloud with neighboring nucleons, much in the same way that molecules are formed from the shared electrons in the outer layers of atoms. Yukawa thought these new particles were too unstable to exist outside of an atomic nucleus but that they were likely to appear in cosmic radiation, which comes from outer space.

Though Yukawa’s theory did not receive a lot of attention at the time, it eventually became the basis for modern particle physics. Two years after Yukawa published his theory, scientists Carl David Anderson and Seth Neddermeyer discovered what they thought were mesons. Though the particles (called mu-mesons or muons) had a mass roughly two hundred times that of electrons and appeared in cosmic rays, they did not interact with nucleons in the way that Yukawa had predicted. They turned out to be a type of particle created when mesons decay.

In 1947, a scientist named Cecil Powell discovered actual mesons, which he labeled pi-mesons, or pions. These pions perfectly fit all the specifications of Yukawa’s theory. Interestingly, Powell also found that mesons lose mass during decay and that the lost mass then turns into energy, which illustrates Albert Einstein’s theory of the relationship between mass and energy.

The discovery of the meson, thirteen years after the idea was first introduced, finally confirmed Yukawa’s theory. He was awarded the Nobel Prize in Physics in 1949 for his prediction of such a crucial elementary particle.

Meanwhile, Yukawa had developed another important theory about atoms. His theory put forth that sometimes the nucleus of an atom will “capture” one of the electrons from the innermost shell of electrons, called the K-shell. He predicted that this process—called K-capture—would alter the charge of the atom and make it unstable. As with his theory of mesons, Yukawa’s K-capture theory was also confirmed several years later.

Hideki Yukawa had a special relationship with the city of Kyoto and with Kyoto University, always returning there after periods of teaching at other schools. In 1938, he went back to study at his alma mater to earn his doctorate. The next year, he became a full professor in the school’s theoretical physics department. In 1953, after teaching overseas at the Institute for Advanced Study in Princeton, New Jersey, and at Columbia University in New York City, Yukawa again returned to Kyoto to become the director of an interuniversity research institute for fundamental physics founded in his honor. Progress of Theoretical Physics, the monthly journal he founded in 1946, continues to be published by the research institute, which was renamed the Yukawa Institute for Theoretical Physics.

Later in life, Yukawa was an active protestor of the use of atomic weapons. He was only two hundred miles from Hiroshima when the United States dropped an atom bomb on the city in 1945 at the end of World War II. A few days later, the United States dropped another atom bomb on the Japanese city of Nagasaki. The two events resulted in the deaths of more than 100,000 Japanese citizens.

Impact

Although Yukawa recognized the extraordinary scientific achievement that atomic weapons represented, he denounced their use. He kept his feelings about atomic weaponry quiet for several years, but in 1954, he finally spoke publicly and said that the use of atomic energy for defensive purposes would be harmful to all of humankind. In 1955, Yukawa joined with ten other prominent scientists, including Albert Einstein, to sign a document called the Russell-Einstein Manifesto. The document urged the world to find peaceful means to resolve disputes rather than resorting to nuclear weapons.

Yukawa retired from his position at Kyoto University in 1970. On September 8, 1981, he died of pneumonia in Kyoto at the age of seventy-four. Many of his notes, manuscripts, and letters were discovered after his death. They are preserved in the Yukawa Hall Archival Library at Kyoto University.

Bibliography

Brown, Laurie M. “Hideki Yukawa and the Meson Theory.” Physics Today 39.12 (1986): 55–62. Print. Explains Yukawa’s meson theory, the development of the theory, and its impact on particle physics.

Nambu, Yoichiro. “The Legacies of Yukawa and His Disciples.” Nuclear Physics A 805.1–4 (2008): 90c–97c. Print. Discusses the impact of Yukawa’s work in developing the field of particle physics and examines the influence he had on his students.

Sato, Humitaka. “Biography of Hideki Yukawa.” Nuclear Physics A 805.1–4 (2008): 21c–28c. Print. Presents biographical information on Hideki Yukawa and another Japanese physicist, Sin-Itiro Tomonaga.