Theoretical physics to transcend time and space and travel to far off places

“There were times at graduate school when I oscillated between becoming a physicist or choosing another path,” Yamamoto tells us. His embarkation on a physics path, his choice of competitive dance as his club at university, and his experiences amassed as a post-doc overseas, were all a result of an honest adherence to fleeting sensations of “inspiration.” Yamamoto’s keen sensibilities are now being demonstrated in his education and research activities at Keio University.


Naoki Yamamoto

Department of Physics

Specializes in elementary particles and nuclear theory. 2005, Graduates from the Department of Physics, Faculty of Science, University of Tokyo. 2010, Completes doctoral degree at same university’s Graduate School of Science. Ph.D. in Science. Assistant Professor at the Department of Physics, Faculty of Science and Technology, Keio University since 2014, following completion of a post-doc at the University of Washington’s Institute for Nuclear Theory, Kyoto University’s Yukawa Institute for Theoretical Physics, and the University of Maryland. In his current position since 2017. Concurrent appointment as a KIPAS principal investigator since 2019.

The Research

The star of our current interrogations is Associate Professor Naoki Yamamoto, who is battling the fundamental problem of “the nature of matter” using theoretical physics.

Where are the limits of theoretical physics?

From elementary particles inside atoms to supernova explosions

From elementary particles inside atoms to supernova explosions, all things that exist in this world are compelled by particular “laws of physics,” all of which are subjects for research within “theoretical physics.” Dr. Yamamoto from the Department of Physics is steadily expanding his research scope in the vast field of “theoretical physics,” propelled by the dictates of his curiosity and intuition. This piece will introduce one thread of the research of a young theoretical physicist.

Deduction of a nontrivial phenomenon from a simple theory

Dr. Yamamoto of the Department of Physics at the Faculty of Science and Technology, Keio University is a theoretical physicist engaged in a wide range of logical research, which takes in “nuclear and particle physics,” on the micro level of quarks and neutrinos, through to macro level research, such as that on astrophysics.
Paper and pen remain the fundamental tools at his disposal for such dynamic research. “Well, I may have recently replaced these with an iPad and Apple Pen. It may appear that I am simply staring into space during my research,” says Yamamoto, reflecting on how he may appear when researching. Nevertheless, a huge amount of mental turnover on Yamamoto’s part is required to clarify a non-trivial phenomenon based on a simple theory during these ruminations. The ultimate result may be “a theoretical prediction of a previously overlooked physical phenomenon” or “a new theory to supersede a fundamentally flawed conventional theory.”

My life’s work of “quark confinement”

“Quark confinement”— the original problem which inspired Yamamoto’s incursion into theoretical physics (Fig. 1). This is also one of seven others in mathematics identified in 2000 as Millennium Prize Problems, with a million-dollar prize being offered to the person who can solve it. However, it remains unsolved today and is one vein of Yamamoto’s life’s work. But just why is this problem so dastardly difficult? To learn the answer to this, we must familiarize ourselves with the very constituents of matter.
“All matter is made up of atoms”— this is common knowledge to anyone over junior high school age. While atoms were originally thought to be the smallest components of matter, it is now known that they are made up of a “nucleus” surrounded by “electrons.” Further, these nuclei are made up of nuclear particles such as protons and neutrons, with such nuclear particles in turn comprised of still smaller particles called “quarks.” Although yet smaller components may remain to be discovered in the future, quarks, neutrinos, electrons, and their compatriots are currently the smallest known units of matter, and referred to as "elementary particles." The presence of quarks within nuclei has been confirmed in experiments conducted using large accelerators.
In everyday life, just as the movement of matter conforms to the rules of “Newtonian Mechanics,” the miniscule world of quarks is animated by the rules of “quantum chromodynamics.” This is not to say that quarks have actual “colors.”Rather, the three primary colors of light are paralleled to the three degrees of freedom which quarks have to move.
“According to quantum chromodynamics, it is not possible to extract a single quark from a nucleon. It’s certainly a puzzler,” says Yamamoto. This is the “quark confinement” problem. While it is expounded that “quarks are connected by a ‘powerful force,’” to date no one has been able to offer analytical proof of this based in quantum chromodynamics.

I wish to learn “the nature of matter”

“Unless we solve the problem of ‘quark confinement,’ we will never truly understand the matter in which quarks are contained. There are any number of fundamental things which we have yet to explain about matter, not confined to this particular problem.” Yamamoto’s theoretical research is simultaneously oriented towards clarification of “the nature of matter” and the pursuit of the challenge of “quark confinement.”
So, just how does he go about approaching these objectives?
“Theoretical physics often presupposes extreme states. For example, these quarks which now cannot be isolated from the nucleus earlier existed in a disjointed plasma state in the extremely high temperatures which directly followed the Big Bang, known as ‘quark-gluon plasma.’ What then, conversely happens when this matter is compressed into an ultra-high-density state? It is thought that quarks ultimately assume superconductive or superfluid states. For now, though, our attention is on the nature of the interim states they go through before reaching this point.” This is the way that Yamamoto is seeking to clarify “the nature of matter.”

Can “Chiral Transport Theory” clarify the mysteries of supernova explosions?

Yamamoto’s recent research focus has been on supernova explosions. Elements which are formed in stars are scattered throughout space in the massive explosion which marks the end of a heavy star. Supernova explosions are thus the source of everything — as these elements are the constituents of the matter and biological life which surround us. However, explosions do not readily arise under the conventional theory. We may be able to find the solution to this problem by applying the “Chiral Transport Theory.”
Chiral Transport is a theory presented in a paper by Yamamoto in 2012, offering a theoretical description of the transport phenomenon with its roots in the nature of elementary particles referred to as “chirality.”
One familiar transport phenomenon is Ohm’s Law, which states that a current will flow when an electrical field is applied. However, in this case heat is generated with the current and energy is lost. Meanwhile, the nature of chirality is such that particle transport without energy loss not normally seen in regular matter becomes possible.
Under conventional supernovae theory, there is the issue of the elementary particles known as neutrinos being expelled without having imparted enough energy to the surrounding matter to cause an explosion. However, this overlooks the property that “only left-handed chirality is found in the neutrino, causing the left-right symmetry to be compromised” (Fig. 2). Since Chiral Transport Theory also accounts for the phenomenon of transport without energy loss, due to the properties of neutrinos, it is suggesting and elaborating new directions in research to explore the mysteries of the supernova explosion.

“This world” chock full of things we don’t understand

The diligently research-driven Yamamoto meanwhile recounts that, “There are many things that I wish to unravel, with over 50 problems which I have yet to turn my hand to still in my ‘jottings book.’” Among these, he says, are those which require vast calculations and years to figure out before the full picture is revealed. This includes research into the supernova explosion. Meanwhile, there are also those which would take around a week to turn into a paper if the right ideas were to make themselves known. Whatever the problem, if solved these will result in a “new world” revealing itself. I am getting more and more excited at the prospect of the worlds Dr. Yamamoto has yet to reveal to us.

(Interview and text writer: Akiko Ikeda)


Associate Professor Naoki Yamamoto

Junior and senior high school days marked by an intense absorption in mathematics

Tell me a little about your childhood.

I was born in Shiga Prefecture and raised in Nagoya and Osaka. As the youngest of three brothers, my older brothers definitely helped toughen me up. I remember being made to play soccer, and chasing the ball with all my might.
In junior and senior high school, I was intensely absorbed in mathematics and would solve and enter problem contests I found in magazines as part of my search for challenging problems. The “Homework” column by Péter Frankl in the magazine Daigaku e no Sugaku (“Mathematics to University”) in particular was replete with difficult problems which one does not learn at senior high school. In my second year of senior high school, I participated in a mathematics seminar (training camp) chaired by Péter on the University of Tokyo Komaba Campus where I was permitted to stay overnight and at which I studied alongside the other participants. My arrival in Tokyo was prompted by my wish to study mathematics at the University of Tokyo.

This training camp was a gathering ground for hardcore math heads?

As I went on to study physics at university, I lost touch with the participants of the training camp. However, I recently occasioned to look some of them up and found that one of their number had gone on to become an associate professor in the Department of Mathematics at Kyoto University; another was researching on AI for patient diagnostics and medicine at Riken; and that in short all remain active in their various spheres in the world.
I happened to run into one of the participants of the training camp at the University of Maryland, where I worked as a post-doc for around a year and a half from 2012. This person was there as part of his doctoral studies, but he spoke about entering the Department of Earth and Planetary Physics, occasioned by an interest in fluid dynamics at university. Subsequently, he found work at the Meteorological Office and is researching on enhancing the accuracy of weather forecasts based on numerical simulations deploying fluid dynamics. This certainly made me think “It’s a small world.”

Choosing physics to learn about space

Why did you go the physics route rather than mathematics which you loved so much?

While my image of physics up until senior high school involved doing calculations using various formulae, at university I again encountered quantum mechanics and the theory of relativity. As a result, I learned that things such as space and elementary particles could facilitate an understanding of the origins of the natural world. This is what prompted my switch from mathematics to physics. In fact, as a senior high school student I studied quantum mechanics on my own initiative. Nevertheless, the wave function of complex units is produced by the Schrodinger equation... It struck me as confounding that despite the fact that we are not able to measure the wave function itself, this has a tangible bearing on actual physical quantities. During my university classes, I learned about the historical background that led to quantum mechanics, and was able to achieve insights into its significance which had eluded me during my schooldays.

I see. So, you chose physics over mathematics because you wished to learn more about space and elementary particles?

While I was interested in both space and mathematics as a youngster, I was largely unaware of the best avenues to approach space, and this thus manifested in a vague yearning to become an astronaut. However, when I started university, I learned that physics can be regarded as a means to comprehend the actual phenomena occurring in space through logical thinking. This is not limited to space as it is today; neither are there limits in terms of distances from our own planet. Consequently, physics can allow us to envisage what the early universe looked like or what is happening in a far distant black hole or during a supernova. The access thus provided to space is of a broader nature than would be possible merely by going there. Taking classes in the theory of relativity and quantum mechanics in the spring semester of my first year at university gradually brought me round to the fascinations of physics, and this is what set me on the physics path from my second year.
In physics, in order to understand the vastness of space or conversely the micro world of particles, mathematical formulae are also used and really beneficial.

I believe that you got pretty talented at the dance you started at university?

Hoping to turn my hand to something I had never done before, at university I joined the competitive dance club at the Athletic Foundation of the University of Tokyo. While I checked out a number of martial arts and dance clubs at the welcoming for newcomers, I was most impressed by the competitive dance display by my senior peers. This was an excellent team with the potential to emerge triumphant at the national championships, and I was very much hooked. One of my dance teachers while at graduate school invited me to consider turning pro, a prospect which I initially gave some thought to. However, I ultimately concluded that, if I am to compete on the world stage, theoretical physics would be the best path for me to make my mark.

Going to see the person you wanted to meet

What kind of place was your first lab at the University of Tokyo?

Tetsuo Hatsuda was my supervising faculty member while I was at graduate school at the University of Tokyo. He transferred to Riken in 2012, and has been researching as part of a new project called iTHEMS (creative mathematics program). He seems to be attempting to clarify various phenomena in physics and biology using mathematics, casting his net wide to recruit personnel and find potential academic solutions. Mr. Hatsuda’s interests have been wide-ranging from the get-go, and he is very flexible in incorporating ideas from other fields and applying these to problems in his own field; or conversely, in putting forward his ideas to solve problems in other fields. I think that I am influenced by Mr. Hatsuda in the way in which I think about applying a particular perspective from physics to other fields.

You have also spent time overseas?

After getting my Ph.D. in March 2010, I made my way to the University of Washington under the JSPS Postdoctoral Fellows for Research Abroad program, where Dr. Dam Thanh Son, who I admired most at the time, was stationed. Dr. Son is a Vietnamese researcher who writes truly elegant papers. While it is believed that progressive compressions of matter will eventually bring about a superconductive state in quarks, as a master’s student, I was more concerned with the question of “through what states do quarks progress before they reach this state?” It was around this time that I read a paper written by Son which outlined a theory describing the superconducting state of quarks. This was the first time that I had read a paper and become convinced that I was in the presence of a work of art. It was as if the scales fell from my eyes. My subsequent thoughts were that I would like to speak directly and conduct joint research with Dr. Son in order to learn what allowed him to produce such a paper.

What do you mean when you say that the paper was elegant?

Firstly, if you look at the results alone the conclusions are far from foregone, and in fact quite surprising. Nevertheless, each step along the path to these conclusions is imbued with an exceedingly simple logic which even a master’s student would be capable of following. Then, before you know it, you find yourself at a point extremely remote from the one at which you started. Dr. Son’s research is wide-ranging, and he has written any number of important papers in various fields of physics.
In theoretical physics, where problems are set up and then tackled, the methodology of setting-up problems is especially important. I learned many things form Dr. Son in this regard. Also, the Chiral Transport Theory, which ties in with neutrino transport theory is something I put together in a paper with Dr. Son in 2012.
 In the end, I spent two years and five months at the University of Washington. I then made my way to Thomas Cohen of the University of Maryland to coincide with Dr. Son’s move to the University of Chicago. While Thomas generally has a jocular persona, he is also the type of person who would rather formulate and present a counterargument to an interesting physical phenomenon that has been proposed and which researchers from all over the world were tuned into. While he attacks things from the opposite direction to others, this is not mere contrarianism. His ability to clarify problematic aspects theoretically having achieved a profound understanding of the problem is what I found so amazing. It was interesting to interact with various physicists with different approaches.

Life at Keio as a faculty member and researcher

What kind of place do you find Keio University to be?

I was first appointed to the faculty at Keio in 2014. What surprised me was that the students were extremely friendly, and that they would come to me with questions at the end of the class. They would ask about things that had no relation whatsoever to the class, saying: “Excuse me, but I didn’t understand this from a book I read recently...” I was delighted to be approached in this way.
 The relationships between the faculty members and the research environment are excellent, and in 2015, we launched the “Topological Science” Project (Strategic Research Foundation Grant-aided Project for Private Universities). This is an outcome of our wishing to get something started with the faculty members of the Physics Department at Yagami Campus and those teaching physics at Hiyoshi Campus. We invite researchers from Japan and overseas who are engaged in interesting research to talk as well as holding international symposiums.
 I was selected as a member of the Keio Institute of Pure and Applied Sciences (KiPAS) at the Faculty of Science and Technology from the 2019 academic year, meaning that a research-conducive environment is at my disposal for five years. I am putting more energy than ever before into the problem of supernova explosions. My days are satisfying and productive, both as a faculty member and a researcher.

Some words from Students

Professor Yamamoto is a consistently sympathetic teacher who is also capable of pointed observations on the subject of physics. I took his class when I was an undergraduate student, and the emphasis he placed on the universal aspects of physics as opposed to unravelling a specific problem really resonated with me. I really wanted to debate things with Professor Yamamoto, and so I joined his lab.

His level of intuition when we are discussing and working through calculations and physics principles on the board is truly overwhelming. I believe that it is this thorough knowledge of physics which imparts the insight I need to unravel physics problems.

His “Chiral Transport Theory,” published in 2012 is likely to have repercussions throughout many fields besides on supernova explosions, and it is currently attracting attention throughout the world.
(3rd year doctoral candidate)

(Interview and text writer: Akiko Ikeda)