1. The quiet reorganization occurring in theoretical physics

What comes to mind when you hear the term “theoretical physics?” Probably Einstein, Hawking, and Hideki Yukawa. Or, maybe you think of Makoto Kobayashi, Toshihide Maskawa and Yoichiro Nambu, the trio of Japanese scientists who won the Nobel Prize in Physics in 2008. Try looking at several homepages of physics departments at universities. You are sure to see long lists of laboratories. This large number of laboratories can be roughly classified into laboratories which conduct experiments and laboratories which do not. Broadly speaking, these “laboratories which do not conduct experiments” are laboratories involved in research for theoretical physics. Of course, you are most likely aware that theoretical physics can be subdivided into many different research fields. In this article, I will introduce a “movement for quiet reorganization” which is now occurring within theoretical physics, or which may occur in the near future.

2. Broadly classifying the field of theoretical physics

Theoretical physics is discipline which theoretically explores the laws of physics, or matter. Accordingly, the research subject is “laws of matter.” Such research consists of 3 orientations.

The first orientation is classifying matter existing in this world into as small elements as possible, and then clarifying the fundamental properties of those microscopic structural elements. Perhaps you have heard that matter is composed of atoms, that atoms contain an atomic nucleus and electrons, and that the atomic nucleus contains protons and neutrons. When examining these protons and neutrons even more closely, they include 3 structural elements called quarks. Kobayashi and Maskawa were awarded the 2008 Nobel Prize in Physics in recognition of their correct theory that 6 or more types of quarks should exists. These quarks, electrons and other microscopic structural elements are referred to as elementary particles. An elementary particle called neutrino was detected at experimental facilities in Kamioka, ultimate leading to Dr. Koshiba winning the Nobel Prize. In this way, physics research has the orientation of searching for even smaller structural particles and clarifying the fundamental properties of those particles. In this article, I will refer to such research in a broad sense as “elementary particle physics.”

The second orientation focuses on large-scale physics; specifically, on the universe. How was our universe born? How do galaxies move?—Research focuses on such large-scale questions of distance and time. Gravity is an important factor when researching the universe. This is because electricity is defined in terms of plus and minus, with forces of attraction and repulsion that can mutually negate and weaken each other. However, gravity has only the force of attraction, with no weakening by an opposing force. Consequently, gravity is the principal force acting on a universal scale which contains various forms of matter. Accordingly, universe research is closely related to gravity research. The concept of gravity is skillfully explained through the general relativity discovered by Einstein. Everyone has heard of black holes, astronomical objects absorbing even light. The existence of black holes was also predicted by Einstein’s general relativity.

The third orientation of physics is to research what happens when large numbers of particles gather together. Rather than focusing on the properties of individual structural particles, this orientation is concerned with investigating phenomena as a group. When using human beings as an example, it is akin to focusing on large social movements when thousands or millions of people join together, rather than on the detailed character of individual people. In the case of matter, if there is only 1 water molecule, then researchers will analyze the properties unique to that molecule. However, when an extremely large number of water molecules gather together, separate group properties will appear—for example, properties as fluid. In this way, the orientation of researching the properties of numerous particles as a group is referred to as “statistical physics.” Almost all of the matter that we contact during our daily lives is the gathering of vast numbers of molecules. Accordingly, statistical physics is closely related to our lives.

3. Reunion of the three orientations: A new relationship born from the superstring theory

　Until now, the three orientations of elementary particle theory, gravity theory and statistical physics have developed almost independently, even while holding a mutual relationship. However, a new method has recently appeared which deals with all three orientations together.

The elementary particle theory seeks to classify matter into as small elements as possible, and then to clarify the fundamental properties of those microscopic structural elements. Within that process, the idea has arisen that the fundamental structural elements of matter are not point-like particles, but rather like long strings. This idea is known as the superstring theory. Dr. Yoichiro Nambu, winner of the Nobel Prize in Physics, is one of proponents of this idea. Research on the superstring theory has revealed that one type of gravity theory can be understood as “rewritten version” of a separate elementary particle theory. Accordingly, since statistical physics obey the laws of elementary particle groups, statistical physics can also be “rewritten” as gravity theory. In this respect, the three orientations are now rewritten versions of each other and are no longer unrelated fields.

4. Gauge/gravity correspondence and related challenges

This relationship among the flow of physical theories is referred to using the terminology “gauge/gravity correspondence” or “AdS/CFT correspondence.” Scientists are starting to utilize this correspondence in new methods for approaching physics problems. For example, nonequilibrium statistical physics is an unexplored research field in statistical physics. This research field deals with the nonequilibrium state which appears when there is movement in particle groups or when an external force is exerted on the systems. Currently, many aspects of nonequilibrium statistical physics remain unclear. Still, almost all visible phenomena in our daily lives occur in the nonequilibrium state, so this research is of vital importance. When this nonequilibrium state is rewritten as gauge/gravity correspondence, it sometimes permutes into a gravity problem. Furthermore, when reconsidered as a gravity theory problem, the problem can be easily solved in some cases. This is a new orientation of research in relation to nonequilibrium statistical physics.

5. The magic of Einstein: An unexpected link with theoretical physics

In this way, a movement is quietly being born that will consolidate different existing orientations of physics. However, it can be said that this new movement actually originated from the work of a genius physicist who was active 100 years ago.

Physicists refer 1905 as the “miracle year.” This recognizes how the genius physicist Albert Einstein announced three theories, all of which were worthy of the Nobel Prize. Specifically, those theories were the theory of relativity, photoelectric effect, and Brownian motion. Most people have read a science fiction story which includes the strange phenomenon of time slowing down aboard a rocket flying at high speeds. This is one phenomenon deduced from the theory of relativity (special relativity). The general relativity, which is a gravity theory, was developed from the special relativity. Ultimately, Einstein won the Nobel Prize for his other work on the photoelectric effect. The photoelectric effect is the observation that electrons are emitted when light is shined on matter. This theory was important for constructing the foundation of modern physics, also known as quantum mechanics. Also, we must not forget Einstein’s thesis on Brownian motion. Brownian motion refers to the random motion of particles suspended in a medium such as water. The Brownian motion theory is a predecessor to aforementioned field of nonequilibrium physics. It is utterly amazing how Einstein produced three theories worthy of the Nobel Prize in a single year.

Most impressive is how the Brownian motion theory which is the predecessor of nonequilibrium physics and the general relativity which is the basic theory for gravity are theories which Einstein began 100 years ago. At that time, Einstein probably never imagined that these different theories are mutually related. If I may make a statement without fear of being misunderstood, Einstein’s theories were even more intelligent than the man himself.

Today, attention is gradually being attracted by the movement of using gauge/gravity correspondence to research nonequilibrium physics. My original field of expertise is the superstring theory. However, I am recently often invited to research meetings in the separate field of nonequilibrium physics.

More than 50 years have passed since Einstein passed away. However, we physicists still seem to be conducting research in the palm of his hand.

Particle Theory Group Laboratory
http://www.phys.chuo-u.ac.jp/labs/nakamura/index.html