Schwinger had a mixed relationship with his colleagues, because he always pursued independent research, different from mainstream fashion. In particular, Schwinger developed the source theory, a phenomenological theory for the physics of elementary particles, which is a predecessor of the modern effective field theory. It treats quantum fields as long-distance phenomena and uses auxiliary 'sources' that resemble currents in classical field theories. The source theory is a mathematically consistent field theory with clearly derived phenomenological results. The criticisms by his Harvard colleagues led Schwinger to leave the faculty in 1972 for UCLA. It is a story widely told that Steven Weinberg, who inherited Schwinger's paneled office in Lyman Laboratory, there found a pair of old shoes, with the implied message, "think you can fill these?". At UCLA, and for the rest of his career, Schwinger continued to develop the source theory and its various applications.