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Understand the reason of things (things)

Dr. Yuto Ashida (Institute for Physics of Intelligence / Associate Professor, Department of Physics)

Theoretical physics is the study of clarifying the laws that describe natural phenomena - the laws of physics - and making predictions that can be verified by experiment. This is a common explanation, but few people know how theoretical physics research is actually conducted. The other day, a friend of mine who is an experimentalist said to me, "Theorists are good, just drinking coffee. Well, that may be true from the outside (laughs), but inside my head, I am agonizing over and over again by trial and error every day. In any case, I guess it is difficult to imagine how the research is done.

Theoretical physics may give you an image that you are doing research alone. However, in my (not many) experiences, I have mostly been able to get new ideas and advance my research by interacting with people who have various interests. This may be because I am indifferent to the field and like to combine various theories and methods. However, this style of research requires caution. If I am not aware of it, my interest will diverge unnecessarily and I will end up on the road. You need to look for opportunities without preconceived notions, and see where you can enter, while keeping your own field as the axis of your research.

Since an abstract story does not give you a clear image, I would like to introduce a recent example of research that is fresh in my mind. In condensed matter and statistical physics, there is a problem of quantum dissipative phase transition of Josephson junctions. It is a well-known topic that can be found in textbooks, but as an uninitiated student, I happened to learn about it from an experienced professor in my third year of doctoral course. I was intrigued by it, but at that time, I had many other topics to work on, so I did not start it. The turning point came three years later when I met a Faculty student who was interested in this topic. Together, we found that the phase transition predicted by the theory had not yet been observed, and there was a bit of a controversy.

Overview of the author's research

In general, theoretical physics attempts to understand phenomena by abstracting them from their essence. The way of abstraction requires a good sense. If the model is too difficult, it cannot be solved; if it is oversimplified, it is too far removed from the actual phenomena; and if it is too simple, it is uninteresting. In this study, too, we went back to the basics and carefully examined simplifications of existing models one by one. We found that some ambiguous assumptions were implicitly made. We treated this point precisely and performed a numerical renormalization group analysis. The result was astonishing: a phase diagram completely different from the one in the textbook was obtained. Since it was hard to believe, we also verified the prediction by the method of nuclear theory (functional renormalization group), but the result was still the same (see Phys. Rev. Lett. 129, 087001 (2022)). The fact that the same prediction can be derived independently from different approaches is one of the most exciting and powerful moments in theoretical physics.

Here is another example. Critical and topological phenomena have been a central topic of research in condensed matter and statistical physics for many years. On the other hand, quantum open systems are also important topics that have been steadily developed in quantum optics and atom molecular optical (AMO) physics. By combining ideas from these different fields, we aimed to explore the research frontier of non-Hermitian physics (cf. Adv. Phys. 69, 3 (2020)). Finding hidden connections between seemingly unrelated fields often leads to innovative progress.

Theoretical physics is the study of how to clarify the nature of things, and its scope is infinitely wide. The aforementioned ideas of renormalization groups and non-Hermitian physics have recently been spreading to machine learning, active matter, and so on. Research Students and other students who are interested in science, why don't you go on to RIKEN RIKEN RIMS and try to do research freely without being bound by the boundaries of your field of study?

Published in the November 2022 issue of Faculty of Science News

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