Well, he was awarded a Nobel prize jointly with Sin-Itiro Tomonaga and Julian Schwinger. Here is the speech given on awarding the prize that summarizes what it

Well, he was awarded a Nobel prize jointly with Sin-Itiro Tomonaga and Julian Schwinger. Here is the speech given on awarding the prize that summarizes what it was for: https://www.nobelprize.org/prizes/physics/1965/ceremony-speech/ https://www.nobelprize.org/prizes/physics/1965/ceremony-spee...