Condensed matter systems, that include insulators, semiconductors, metals and superconductors among many others, consist of many particles, of the order of ten to the twenty three. At the microscopic level, the dynamics of individual particles is governed by the well established theoretical framework, quantum mechanics. However, it is extremely difficult to understand and predict macroscopic behaviors of a whole system, starting from the microscopic theory. For the past 30 years, it has become clear that understanding macroscopic properties is not a matter of mere application of the known physical laws. Instead, collective behaviors are governed by novel organizing principles, which are not manifest at all from the microscopic rule of dynamics. Currently, the primary goal of my research is to identify new dynamical principles that are behind novel emergent phenomena in gapless phases of matter such as unconventional metals and systems near quantum critical points, and to apply those principles to understand physical properties observed in experiments. To achieve the goal, I use various theoretical tools, including quantum many-body theory and quantum field theory. I am also interested in the interdisciplinary research areas between condensed matter physics, string theory and gravitational theory. Recently I have been developing the notion of `quantum renormalization group' where coupling constants are promoted to quantum mechanical operators to make a connection between general D-dimensional quantum many-body systems and (D+1)-dimensional quantum theory of gravity via holography.