In fact, not all great physicists are proficient in quantum field theory, but mastering quantum field theory is increasingly becoming the need of mainstream research in physics. Quantum field theory is a combination of quantum mechanics theory and relativity, which is used in systems (microscopic high-energy particle systems, etc.). Both theories need to be considered at the same time. It was originally a frontier theory developed in modern times. At the same time, the research of physics is divided into many directions, and various systems need to be explored. The mainstream research of condensed matter physics and particle physics need to use quantum field theory, so mastering quantum field theory has become the requirement and necessity for great physicists in today's era.

If you say that great physicists once mastered quantum field theory.

Well, after all, it was founded by a group of physicists like Paulidirac Feynman, who are undoubtedly proficient.