Attraction of Each Department

Department of Physics

  • Yasushi Suto (Professor, Department of Physics)

Encouraging the study of physics in order to understand the origins of the universe

Figure 0

The Japanese word for physics literally means a science dedicated to the study of laws that govern things. In other words, physics is the search for order (natural laws) that makes it possible to describe our world (or rather, a part of it) with the utmost simplicity and accuracy. Even though such description uses mathematics as its language, it is uncertain whether or not mathematics is able to fully describe the world of nature as it is in reality. Therefore, a worldview based on physics may be only a representation mapped out in our minds, allowing us to comprehend a part of our real world. If this is true, physics can never offer us anything more than the best approximation of the order that encompasses just a fraction of the real world(*1). In this sense, we may say that the progress of physics (or natural sciences at large) is a never-ending, self-evolving, and self-correcting process.

It seems that I started this article with a theme that may be totally irrelevant or unnecessary; please bear with me. If this is the case, however, Figure 1 may be useful, which schematically illustrates how different worldviews relate to each other.

Figure 1

Figure 1: Worldview of physics
(1) World that can be described by classical physics
(2) World that can (or should) be described by known physics
(3) Our real world
(4) World that can be described by mathematics
(5) World without any self-contradiction

(1) World that can be described by classical physics

The word “classical” implies appreciation when it is applied to music, painting and literature. In the case of classical physics, however, it may have the connotation of “older” physics that does not incorporate quantum mechanics. Due to this connotation, young people tend to have the mistaken belief that classical physics is somehow inferior to quantum physics. They have to be convinced of this error before they can be called full-fledged researchers.

(2) World that can (or should) be described by known physics

As people often describe the 20th century as the century of physics, physics has greatly expanded the domain of its applicability beyond (1). I attribute this expanded domain (2) to “known” physics and draw its boundary with broken lines, with the intention of suggesting that this domain is still expanding as physics continues to progress.

(3) Our real world

Beyond the domain of known physics, there undeniably exists the possibility of new physics. Theorists already have several candidates including supersymmetry and superstring theories that only a tiny fraction of physicists understand(*2). However difficult they may be, the “new” physics is gradually passing from the stage of theoretical study to the stage of experimental demonstration.

(4) World that can be described by mathematics

In reality, the relationship between (3) and (4) is uncertain. The representation in Figure 1 merely shows the one most widely accepted. That is to say, it is somewhat arrogant to believe that our world should be fully described by mathematics; it is not surprising that some mathematically self-consistent theories do not correspond to our real world.

(5) World without any self-contradiction

Then, can there be any model without contradictions that does not correspond to our world and cannot be described by mathematics?(*3) Since the Japanese Constitution guarantees freedom of thought, I have no right to prevent anyone from considering this as a possibility. However, if any student comes to me with such a question, I, as one of professors in physics, would advise him/her never to attempt stepping into a world beyond the area shared by (3) and (4). Nevertheless, we are fully aware that modern society is full of contradictions. Thus we may have to admit that there is a world even beyond the boundary of (5).


Having finished the somewhat strange introduction, which has become unusually long, let me start describing our Department of Physics(*4). Kaisei School was established in the first year of the Meiji Period (1868) and was renamed University Southern School in December of the following year (1869). According to the Rules of University Southern School established in October 1870, the faculty of natural science encompassed the following subjects: physics, botany, zoology, chemistry, geology, mechanics, astronomy, trigonometry, conic sections, mensuration, and calculus. Tokyo Kaisei School and Tokyo Medical School merged to form the University of Tokyo on April 12, 1877. At that time, the university had four faculties (law, science, literature and medicine), and the Faculty of Science had five departments: Department of Mathematics, Physics & Stars, Department of Chemistry, Department of Biology, Department of Engineering, and Department of Geology & Mining Engineering. In these early days, physics was taught by professors from abroad. In July 1879, Kenjiro Yamakawa became the first Japanese professor in the Department of Physics and in 1888 he became the first doctorate-holder in the Department. In 1901, he was appointed as the sixth president of the University of Tokyo. His statue was erected in front of Building 1 of the Faculty of Science in December 2006.

The Department of Physics keeps the tradition of celebrating the Newton Festival, organized mainly by third-year students of the undergraduate school, in December every year. This serves as an opportunity for communication among undergraduates, graduate students, current faculty members and retired faculty members. This festival, commemorating Isaac Newton (born December 25, 1642), was first celebrated in December 1879 based on a proposal made by Aikitsu Tanakadate, then a student in the Department of Physics, and other students. Aikitsu Tanakadate was the first faculty member who graduated from the Department of Physics (lecturer from 1883, associate professor from 1886, professor from 1891 to 1917). In 1881, the Department of Mathematics, Physics & Stars divided into three departments (Department of Mathematics, Department of Physics and Department of Astronomy), creating the basis for what is today the Department of Physics.

Figure 2

Figure 2: A photograph from a book titled 100-years History of the University of Tokyo. The photograph was taken on June 18, 1891, before Mr. C. G. Knott, the last foreign professor of physics, returned to his home country. In the front row, from left to right, are Masakazu Toyama (Principal, School of Liberal Arts), Dairoku Kikuchi (Principal, School of Sciences), X. G. Knott, and Kenjiro Yamakawa. Hantaro Nagaoka is standing at the far left of the second row.

Undergraduate Education

The Department of Physics (part of the Faculty of Science) is responsible for undergraduate education in the Hongo campus. The Graduate School of Physics (part of the Graduate School of Science), on the other hand, refers to a large organization that includes, besides the Department of Physics, many institutions mostly outside the Hongo campus such as the Research Center for the Early Universe, the Center for Nuclear Study, the International Center for Elementary Particle Physics, the Institute for Solid State Physics, the Institute for Cosmic Ray Research and the Japan Aerospace Exploration Agency. Please note that in this article I distinguish between the Department of Physics and the Graduate School of Physics according to this organizational structure (This is often very confusing even to our current faculty). The Department of Physics has a large number of faculty members including 21 professors, 10 associate professors, 3 lecturers and 27 assistant professors (as of January 2007). Each professor and associate professor, with an assistant professor, forms an independent unit for running a group. In this manner, by making as many independent groups as possible with the limited number of faculty members, we are able to cover diverse fields of physics and prevent concentration in a limited number of fields.

A clear distinction between theoretical physics and experimental physics could be regarded as one of the characteristics of present-day research in physics. In the Department of Physics, we ensure that students have opportunities for learning both. It is mandatory for third-year students to participate both in experiments and in theoretical studies. While most students find it difficult to choose between experimental physics and theoretical physics in the graduate school, it is important that the choice they make is based on their real experience. In order to acquire such experience, fourth-year students, usually in pairs, go to their chosen group to participate in special experiments or theoretical studies. As a general rule, students who chose a group for experimental research in the summer semester must choose a group for theoretical research in the winter semester (or vice versa), because here again, we emphasize the need to be experienced in both experimental physics and theoretical physics. Even though such lessons are obligatory, it is not mandatory for students to submit a graduation thesis because the subjects covered are still far from the leading edge of present-day research in physics.

Graduate School and Research Areas

More than 90% of undergraduate students in the Department of Physics decide to advance to the graduate school to undertake further study. Out of the approximately 100 students who enter our master course, about 40 are graduates from other universities. Of those who complete the master course, about two-thirds proceed to the doctoral course, while the rest decide on a career in a broad range of fields in the public or private sector. The faculty members in the Graduate School of Physics comprise 74 professors, 58 associate professors and 4 lecturers (136 members in total, as of January 2007). This is four times larger than the number of academic staff employed in the Department of Physics. With so many fields covered by many laboratories in the Department of Physics, it is impossible to describe them in detail. In this article, I will only provide a general description in terms of “sub-course” divisions (Figure 3), which is the basis of graduate school entrance examinations. For more details, please take a look at the other articles that can be found on our website.

Figure 3

Figure 3: List of Sub-courses Offered by the Department of Physics A0: Theoretical nuclear physics
A1: Theoretical particle physics
A2: Experimental nuclear/particle physics and accelerators
A3: Theoretical condensed matter physics
A4: Experimental condensed matter physics
A5: Theoretical general physics (astrophysics, relativity, fluid dynamics and quantum information)
A6: Experimental general physics (nonlinear physics, fluid dynamics, plasma physics, quantum optics, atomic/molecular physics, etc.)
A7: Biophysics
A8: Experimental astrophysics and astro-particle physics (electric waves, visible and infrared rays, x-rays, gamma rays, cosmic rays, neutrinos, gravity waves, dark matter investigation, etc.)

A0 Every element on earth is formed around an atomic nucleus in which nucleons (protons and neutrons) are bonded by strong interactions. While diverse states and properties of all matter arise from the quantum many-body systems of the atomic nucleus, there are still many fundamental and important mysteries concerning the nature of nuclear forces. A theoretical attempt to understand the atomic nucleus in terms of quark many-body systems as more fundamental constituents is deeply connected with astrophysics through links provided by a branch of particle physics called quantum chromodynamics and the study of neutron stars and phase-transition in the early universe.
A1 Theoretical particle physics studies elementary particles (and their interactions) as the most fundamental elements of the natural world. In the Department of Physics, most of those who are fully occupied in exploring outside domain (2) in Figure 1 belong to this sub-course. Those who concentrate on the fringe slightly external to domain (2) are called “phenomenological theorists.’ Those who are interested in the border between domains (3) and (4) or in what exists outside domain (3) are called “pure theorists.”
A2 Experimental physicists conduct experiments to verify or falsify theoretical predictions made by those who belong to sub-courses A0 or A1. Thus, they have the great responsibility of increasing the extent of domain (2) in relation to domain (3), and of giving accurate feedback to those in sub-course A1 to prevent them from becoming too curious about what exists outside domain (3). Those who belong to this sub-course are more closely associated with the “big sciences’ than anyone else in the Department of Physics, and are naturally often involved in international joint research projects.
A3 and A4 What is the most essential foundation of the natural world? When faced with this question, those who belong to sub-courses A0 through A2 would assume a reductionist point of view and say that the foundation lies in the basic laws that govern the world in its simplest form. Assuming that this is the set of values they share, those who belong to sub-courses A3 or A4 would represent a different set of values by saying that they are most attracted by order that becomes manifest in the natural world of reality only when basic elements are clustered into groups (recently often referred to as emergence)(*5).
A5 This sub-course deals with theoretical physics in the “others” category, so to say. It covers very diverse subjects (various in the extreme, in fact) including fluid dynamics, numerical relativity, astrophysics, cosmology, and quantum information.
A6 Experimental research in this sub-course does not always correspond with theoretical studies in sub-course A5. With experimental research conducted in unique and interesting fields such as plasma physics, laser physics and nonlinear/nonequilibrium systems, it is more appropriate to say that this sub-course is dedicated to blazing a trail into new fields.
A7 Historically, it is not an exaggeration to say that our Department of Physics gave birth to biophysics in Japan. While it is already a cliche to say that the 21st century is the century of biology, we stand apart from fashion and aspire to be pioneers of the new age of biophysics by teaching the next generation of biophysicists who embrace the spirit of physics.
A8 The Department of Physics is characterized by the presence of many groups dedicated to observations and experiments in astrophysics. Supported by close partnerships with major astronomical institutions such as the Institute for Cosmic Ray Research, the Japan Aerospace Exploration Agency and the National Astronomical Observatory of Japan, our researchers are engaged in multi-wavelength astronomical observations at large, investigating not only electromagnetic waves but also particle beams and gravity waves. Furthermore, there are intense research activities that overlap with particle physics such as underground experiments for detecting neutrinos and direct search for dark matter. We are so proud that the first detection of neutrinos from a supernova explosion with KAMIOKANDE led by Professor Masatoshi Koshiba, who was awarded the Nobel Prize in Physics in 2002, is a great achievement that originated from this sub-course(*6).

In conclusion, we may say that most of the research in the Department of Physics is concerned with matters just inside the boundary of domain (2), while matters just outside the boundary of domain (2) is covered by phenomenological particle theories and experimental particle physics. Pure theorists in particle physics cover an area between just inside the outer limits of domain (3) and the borderline of domain (4). The potentially vast realm that may exist beyond this is not covered by the Department of Physics under the Faculty or Graduate School of Science. Exploration into domain (5) beyond the boundary of domain (4) is left to the Department of Philosophy under the Faculty of Literature. Exploration beyond the boundary of domain (5) is left to metaphysical studies.

Students in the Komaba campus who are considering studying further in Hongo, or those who are considering entering the graduate school, typically ask the following two questions: what are the differences between the Department of Applied Physics in the School of Engineering and the condensed matter research laboratories that belong to the School of Science? what are the differences between the astronomical laboratories in the Department of Astronomy and those in the Department of Physics? From the viewpoint of research alone, there are no essential differences between them, and these laboratories are closely related to one another through the joint research they undertake, for example. As to specific details of the research, things depend more on preferences specific to each group or faculty member than on the difference between the School of Engineering and the School of Science, or between the Department of Astronomy and the Department of Physics. Taking astrophysics, in which I specialize, as an example, the profiles of the current faculty members suggest that specialists in X-ray astronomy, relativity and quantum cosmology favor the Department of Physics, while specialists in optical and infrared astronomy, solar physics and nucleosynthesis favor the Department of Astronomy. Nevertheless, this distinction does not constitute a clear-cut dividing line between the departments, and it is quite possible that things may change in the future. If I may be permitted to put in a plug for the Department of Physics, I would like to emphasize that our department, with its many faculty members, provides opportunities to make contacts with many areas of physics including particle/nuclear/astrophysics, condensed matter physics and biophysics, allowing the students to find their true interest from a wide variety of choices. Undergraduate students are usually only exposed to a tiny fraction of the numerous fields of physics, with the exposure to any particular field being rather random. I advise students not to be misled by this temporary limitation but study further in the Department of Physics, develop a broader outlook of physics in general, and then think deeply about their future careers.

Culture of the Department of Physics

The types of studies employed in the realm of physics and researchers' conception of nature are extremely diverse. Probably owing to this, people in the Department of Physics are quite ready to accept the peculiarities of others. My office is located on the 9th floor (Physics theory floor) of the West Wing of Faculty of Science Building 1. Most of the occupants of the southern half of the same floor believe that our space and time actually consist of 9+1 dimensions, that is to say, an unperceivable compact space of six dimensions dangles from the three-dimensional space perceived by us. Surely, if one of them mumbles something like “our space and time consist of 9+1 dimensions” in a packed train in Tokyo, a space around him would mysteriously appear regardless of how crowded the train may be, allowing him to have a seat. Students who enter the Department of Physics have a deep longing for and wish to join the ranks of such weird specimens of humanity who live in the 9+1 dimensional world. On the other hand, another part of the same floor is occupied by researchers who are fully engaged in performing calculations within 2+1 dimensions. Furthermore, the 9th floor of the Central Building is partially occupied by researchers who study materials restricted to only a single dimension in space. You will see that such theoretical researchers are all confined to the 9th floor while other researchers have their laboratories on the 6th or lower floors. This fact seems to suggest that even though such eccentricities are accepted in the Department of Physics, there may be a conspiracy to prevent the further spread of a dangerous set of values originating from the higher floor(*7) of the building.

There are similar diversities among different cultures within the department. Serious-minded graduate students often ask me what they should wear when they make a presentation to an academic society for the first time. Of course, I tell them that they will be laughed at if they wear ties and suits. This mindset, however, seems to be part of the culture of particle/nuclear/astrophysicists. I always dress casually, even when invited to give a talk at an international conference, but I have never been criticized for doing so(*8). However, researchers in condensed matter physics tend to dress well because they often meet with researchers from corporations. The acceptance of such diverse cultures within the group could be a secret for the survival of physics through natural selection.


One of the objective measures that indicates the international regard our Department of Physics is held in is the ranking based on the number of citations to our papers published in the last 11 years, announced by the Thomson Scientific Division of Thomson Corporation. According to statistics as of April 2006, the University of Tokyo was ranked second in the world with 161,747 references made to 14,844 papers that it released in various fields of physics. This achievement cannot be solely claimed by the Graduate School of Physics (Astronomy and Engineering Schools contribute to the statistics). Moreover there is some controversy over how the statistics were compiled(*9). Nevertheless, this still constitutes a sufficient demonstration of the outstanding level of research being conducted in physics at the University of Tokyo.

On January 5, 2007, the Asahi Shimbun published an editorial titled “Young men, aspire to be another Yukawa.” The editor regretted that the younger generation today seemed to have lost interest in science, and emphasized the importance of scientific literacy for those who would be responsible for the future of Japan. The Department of Physics aspires to lead the younger generation in this direction, and serves as a base for blazing a trail into new domains of physics and of science at large.


Many objections can be raised to a statement like this. However, throughout this article, I have chosen to ignore such objections.
Since I am not among the chosen few, I am unable to discuss these theories in detail.
Soon after entering the university, many of you were probably impressed, when you were taught in a general education lecture: “Mathematics is not a natural science. In mathematics, something is true if it is logically true. In physics (or any natural science), something may be logically true but it is not accepted as truth if it contradicts reality (experiment and observation).” I have to confess I was one of these students. However, it is difficult to answer students who go on questioning like this: “Why are such logical truths rejected in our world?” or “How do such models stand in relation to the natural world?” Be careful not to bombard teachers with such questions in the Faculty or Graduate School of Science, or you will be shunned! Those who cannot erase such questions from their mind may find themselves more comfortable in another faculty or department that has teachers of similar temperament.
Most of these descriptions are based on 100-years History of the University of Tokyo.
When comparing the different sets of values between condensed matter physics and particle/nuclear/astrophysics, I always remember a scene from the Astro Boy television series that I saw in my childhood. While his human friends enjoyed the beauty of big fireworks in the sky, Astro Boy's eyes were seeing symbols of the elements that constituted the fireworks. If I remember correctly, Astro Boy then mumbles: “How do they find beauty in this?” However, while these two different viewpoints may appear to be contradictory, they are indeed complementary, contributing to the attraction of physics at large.
Speaking more exactly, sub-course divisions at that time were slightly different from now. Today's sub-course A8 was formed from what used to be a part of sub-course A6 and sub-course A2.
Not necessarily the higher level.
It is quite possible that I just did not notice any criticism.
For example, the University of Tokyo used to top the world until 2004 in terms of the number of references made to papers it released in various fields of physics. From FY2005, however, about 80 institutions under the Max-Planck Institute of Germany began to be treated as part of the Max-Planck Institute, propelling the institute into top position.