Many?Body Problems and Effective Field Theory: From Two?Trillion?Degree Plasmas to Quantum Liquids at Absolute Zero

Exploring broad areas of theoretical physics through effective field theory.

Research (Rikka) July 07, 2025

HONGO Masaru Associate Professor, Faculty of Science

Theoretical physics, which studies the structure and properties of matter, is often pursued within distinct subfields such as atomic physics, nuclear physics, and particle physics, with researchers typically working within those conventional boundaries. Associate Professor Hongo, however, uses the theoretical framework known as “effective field theory” to explore broad areas of theoretical physics without being constrained by traditional divisions.

“The term ‘effective field theory’ is most often used within the subfield called ‘quantum field theory,’ but I believe it provides a unifying perspective that underlies all areas of modern physics. Specifically, by focusing on the separation of phenomena by their characteristic scales of short?distance versus long?distance behavior, it gives a theoretical framework that can be applied to a wide variety of physical phenomena. Based on this way of thinking, I have pursued research that spans from two?trillion?degree plasmas to the behavior of quantum liquids at temperatures approaching absolute zero, unconstrained by disciplinary labels.”

In 2022, in an international collaboration with Research Fellow Keisuke Fujii and Professor Tilman Enss at Heidelberg University in Germany, they showed through theoretical calculations that particles immersed in an ultracold quantum fluid experience a long-range force analogous to the van der Waals interaction known from molecular physics.

“Atomic ensembles cooled by laser techniques to temperatures near absolute zero have recently attracted attention as systems in which quantum states can be controlled with high precision, making them an ideal stage for probing forces in the microscopic world. In our research, we found that long-range forces arising from quantum fluctuations act on impurity particles in atomic superfluids realized at extremely low temperatures. Many intriguing questions remain, such as what characteristic phenomena or structures emerge from this newly identified force.”

In addition, because impurities in a superfluid share similarities with electrons in solids and with heavy particles in high?temperature plasmas, combining the theoretical description of the force discovered in this research with experimental results from ultracold atomic gases may also contribute to a deeper understanding of related physical systems across different fields.

Schematic of the long?range force acting between impurity particles (red) immersed in a superfluid of an ultracold atomic gas (blue). An attractive interaction resembling the molecular van der Waals force arises from the simultaneous exchange of two superfluid phonons (sound waves in the superfluid).

Profile

HONGO Masaru

Associate Professor, Faculty of Science

Ph.D. (Science). Specializes in particle and nuclear theory and non?equilibrium physics. His research aims to bridge fluid dynamics and quantum field theory, a fundamental microscopic framework underlying modern physics. Appointed faculty under Niigata University’s 'Swing-by' Program for Fostering Young Faculty (Term II).

Researcher Directory

Associate Professor Hongo enjoys reading; he often chooses mystery novels.
“I find solving puzzles in mysteries feels close to doing physics, so I keep reading them.”

*Article content and profile information are current as of June 2025.