Erwin Schrödinger in 1922, three years before working on his equation, connected Weyl's group concept to electron charge. Schrödinger showed that the group U ( 1 ) produced a phase shift e i θ θ in electromagnetic fields that matched the conservation of electric charge. As the theory of quantum electrodynamics developed in the 1930's and 1940's the U ( 1 ) group transformations played a central role. Many physicists thought there must be an analog for the dynamics of nucleons. Chen Ning Yang in particular was obsessed with this possibility.