Yukawa Mind

Historical materials for a better understanding of Hideki Yukawa

Modern Physics and common beliefs – Described by Yukawa

Yukawa wrote this manuscript for the lecture, “Modern Physics and Common Beliefs,” presented at Suiyokai on December 18, 1935. The first sheet is a blank piece of manuscript paper, followed by 20 sheets of 400-character vertical writing manuscript paper. In the lecture, Yukawa explained to the general audience from a broad perspective how the world of physics had changed since quantum mechanics was established. This manuscript is one of the invaluable historical materials that gives a glimpse into the origin of Yukawa’s speculations.


The first noteworthy point is that Yukawa wrote the manuscript on vertical writing manuscript paper. During his years at Osaka Imperial University, he devoted himself to physics research, so he rarely wrote a lecture manuscript on vertical writing paper. When Yukawa wrote Japanese vertically, his cursive handwriting was fluent and beautiful as he learned Japanese calligraphy in his young age. Some of the kanji characters he wrote are simplified elegantly in his original style.


Yukawa started the lecture by mentioning that the world of physics (modern physics) in the 20th century, especially after the birth of quantum mechanics, was significantly different from that in the 19th century. Ordinary people might have had an impression that modern physics was departing from common beliefs. The laws of physics break down phenomena—any phenomena in the natural world—into simple and fundamental phenomena in order to seek the general rules of nature. From that standpoint, the research on cosmic rays, for example, dealt with high-energy phenomena involving extraordinary and immense energy, and it was a very important field of study for the future of physics. Yukawa talked what modern particle physics was about by discussing how protons, neutrons, electrons and photons moved and changed.


Yukawa then described about the basic principle and general idea of the quantum theory. He stated that light behaved much like particles, described the experimental evidence to support it, and emphasized that quantum mechanics dealt with phenomena in a probabilistic way. It is interesting that Yukawa gave an analogy using a business activity in daily life as an example and referred to an episode of an election to make his explanations easier to understand. According to Yukawa, physics is a branch of science that assumes even a phenomenon that seems to have occurred by accident according to our common sense beliefs is the inevitable result of some cause. Simple, common sense thinking is surprisingly similar to the way of theorizing in modern physics, according to Yukawa. He also stated that further advancement of natural science would enable the application of the way of thinking in physics to spiritual matters in the future. Such an era is indeed approaching.

Written by Yutaka Hosotani (Osaka University)

Historical material OU1935-B3
Lecture: “Modern Physics and Common Beliefs”