Effects of Chinese Classic Literature on H. Yukawa's Work

The following is the outline of my talk to be delivered at Himeji Kyoyukai meeting on February 18, 2010, in Himeji.

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The Nobel-winning physicist Hideki Yukawa developed the theory of elementary domains for particle physics together with his coworkers in his later years [1, 2]. In the preface of a book [3], he writes that the important factor that led him to the idea of this theory was the words of the Chinese poet Li Po (701­­–762), "Heaven and Earth are hotels for everything, and days and nights are travelers of hundreds of generations." He also writes that scientific research and literature are not separate things for him.

Yukawa also writes in a textbook on particle physics [4] that Li Po's words mentioned above was one of the motivations that brought his unconscious idea about the elementary domains to his consciousness. While being rather lengthy for introducing the concept of the elementary domains in a textbook, the paragraphs including the above words are considered the valuable record of Yukawa's processes of thinking.

In his lecture on experimental nuclear physics, Kiichi Kimura, a classmate of Yukawa's at Kyoto University, told us that some words in the Taoist book Zhuangzi might have been a hint on Yukawa's meson theory that brought him the Nobel Prize. I do not remember the words Kimura quoted from Zhuangzi. In his 1961 essay [5] on Zhuangzi, however, Yukawa writes that he had read Taoist books Zhuangzi and Laozi in his middle school days, but was forgetting philosophical thoughts in them for many years after that. This means: When he was thinking about meson theory, Yukawa was forgetting Zhuangzi. Thus, the words of Zhuangzi Kimura mentioned had not been a direct hint, at least, on the idea of the meson.

It was reported [6] that the meson theory had been motivated by the occasion of the childbirth of Yukawa's wife. Namely, the judo expert Sachio Ashida remembered to have been told in a commuting tramcar by Yukawa, "A baby can exert an attractive force between the parents. There may be a similar existence in the nucleus." I suppose that this was Yukawa's jocular explanation of his idea for the layperson. Yukawa should have gotten the idea of the meson from the study of earlier papers by Werner Heisenberg and Enrico Fermi together with the analogy of the electromagnetic field mediated by the photon. Note here that he used a good analogy.

In his essay mentioned above [5], Yukawa writes that he reminded himself of an allegory in Zhuangzi when he was thinking about something at a deeper level than "elementary particles," which amounted to more than 30 kinds those days. The allegory was about the king who lacked all the seven holes in the face, i.e., ears, eyes, nostrils and a mouth, and was named Chaos ("Konton" in Japanese). The underdeveloped state of the king was appropriate for Yukawa to express his idea of something at a deeper level.

Yukawa also liked the following words from Zhuangzi: "On the basis of the beauty of Heaven and Earth, we arrive at the reason of everything." He made these words in Chinese characters use as a watermark of the invitation letter for "International Meeting on Elementary Particles: The Thirtieth Anniversary of the Meson Theory" [7].

To develop a new theory in the field of particle physics, it is necessary to have good mathematical skill of formulating equations to express a physical model, as well as strong imaginative power to think of a new model. One of the important device with which Yukawa cultivated the latter ability should have been Chinese classic literature, which includes a lot of allegories and helps one to develop thinking by the use of an analogy. Thus his reading of Chinese classic literatures in his young days was useful not only directly for the idea of elementary domains, but, quite possibly, also indirectly for the idea of the meson.

(This article owes much to the discussion we have had at Osaka Science Museum among the members of "Citizens' Study Group on Hideki Yukawa.")

References

1. Y. Katayama and H. Yukawa, "Field Theory of Elementary Domains and Particles. I" Progress of Theoretical Physics Supplement No. 41, p. 1 (1968).
2. Y. Katayama, I. Umemura and H. Yukawa, "Field Theory of Elementary Domains and Particles. II" ibid. p. 22 (1968).
3. H. Yukawa, Auto-selection of Hideki Yukawa's Essays Vol. 5 (Asahi-shimbun, Tokyo, 1971) [in Japanese].
4. H. Yukawa, Unified Theories of Particles, in Iwanami Lectures: Foundations of Modern Physics, Vol. 11, H. Yukawa and Y. Katayama, ed., p. 563 (Iwanami, Tokyo, 1974) [in Japanese].
5. H. Yukawa, Zhuangzi, in Auto-selection of Hideki Yukawa's Essays Vol. 3, p. 363 (Asahi-shimbun, Tokyo, 1971) [in Japanese].
6. Personal connections in Japan: The spirit of Judo (8), Asahi-shimbun, Evening issue (May 25, 2006) [in Japanese].
7. The 30th anniversary of Meson theory (1), Asahi-shimbun (September 20, 1965) [in Japanese].