Interdisciplinary message I get from Yukawa

Interview with the faculty staff in the Graduate School of Science, The University of Osaka

Interdisciplinary message I get from Yukawa

Kenichi Asano (Professor)

Department of Physics, Graduate School of Science, The University of Osaka

Q: What does Hideki Yukawa mean to you?

I first encountered Hideki Yukawa through his autobiography, Tabibito, during my high school days, when I had started thinking about my future. There, I felt an affinity with his knowledge of classical Chinese texts and his interest in the Taikōki about Hideyoshi. Although at that time I was much more interested in biology and natural history, this book deeply inspired me by showing how physics seeks to describe the world in terms of mathematics.

Q: What aspects of Hideki Yukawa’s research attract you?

One of the most celebrated aspects of the meson theory is often referred to as postulating a completely new particle at a time when nothing was known.
However, what impresses me more is his ability to clarify the physics by pinning the problem down to the simplest concept within the landscape of many competing, complex theories.
In my own field of condensed matter physics, we try to understand the phenomena arising from the interplay of multiple degrees of freedom — charges, spins, orbitals, and phonons — which cannot be treated all at once and therefore require physical insight to extract the simplest model or picture that captures the core of the problem.
In this respect, there is a strong parallel with how Yukawa was working on his problem, learning from previous studies, gathering information, and refining ideas step by step by discarding inessential aspects. This spirit can be felt in the materials preserved in the Yukawa Memorial Room.

Q: In what sense do you think Hideki Yukawa has influenced the research of theoretical physics in Japan?

Another outstanding quality of Yukawa, in my view, was not being too mathematical nor merely following experiment. It was precisely this balance that was essential to the construction of the Yukawa theory. Physics employs mathematics and computation as tools, relies on thought experiments, and requires a high degree of abstraction and systematic reasoning. Yet, in the end, empiricism overrides everything. No matter how beautiful a theory is, it can never be relevant unless it withstands experimental verification. As a theorist, one must keep a close eye on the experiment while pursuing what is truly correct as a theory. I sometimes feel that theoretical physics research in Japan — both in particle physics and in condensed matter theory — has entered a phase in which this balance is in danger. I feel that thinking about Yukawa can help us remaid of the importance of such a balance.

Q: Could you also share the relevance to condensed matter physics?

I work on condensed matter theory, a field that takes a different perspective from particle physics, in which phenomena arising from many degrees of freedom cannot be understood simply by probing a single degree of freedom.
Yet even in this field, we encounter “Yukawa potential”. It was originally introduced to describe the nuclear force, while it appears in condensed matter as an effective interaction felt by electrons in systems with many mobile charges in its background, such as metals or plasmas, where the long-range part of the Coulomb interaction is screened. Interestingly, the same functional form emerges in these entirely different contexts.

Moreover, Tomonaga, who was closely associated with Yukawa, proposed the Tomonaga–Luttinger liquid, a prototypical state of one-dimensional quantum many-body systems. The theory of superconductivity also connects to the achievements of Yoichiro Nambu, who had strong ties to the University of Osaka. In recent years, as research on topological materials has rapidly advanced, the concept of gauge fields has likewise gained increasing importance in condensed matter physics.
These facts indicate that the seemingly unrelated research fields can share many important universal concepts.