During a series of conferences in New York from 1947 through 1949, physicists transitioned from war work back to theoretical issues. Under Oppenheimer's direction, physicists tackled the greatest outstanding problem of the pre-war years: infinite, divergent, and seemingly nonsensical expressions in the quantum electrodynamics of elementary particles. Julian Schwinger, Richard Feynman and Shin'ichiro Tomonaga tackled the problem of regularization, and developed techniques that became known as renormalization. Freeman Dyson was able to prove that their procedures gave similar results. The problem of meson absorption and Hideki Yukawa 's theory of mesons as the carrier particles of the strong nuclear force were also tackled. Probing questions from Oppenheimer prompted Robert Marshak 's innovative two-meson hypothesis : that there are actually two types of mesons, pions and muons. This led to Cecil Frank Powell 's breakthrough and subsequent Nobel Prize for the discovery of the pion.