I was in Columbia in 1938 and Schwinger was in trouble; he couldn't get his Ph.D. because he didn't go to lectures of the mathematicians and he didn't have enough credits. So Rabi had told Schwinger that he had to go to my lectures at Columbia; of course, he didn't because it was early in the morning, and I asked Rabi, 'What shall I do?'. I was of course perfectly willing to give him an 'A' on the course because he needed the credits. ... He clearly knew as much as I did - we talked as complete equals. ... Rabi said, 'No, you shouldn't do that, you should give him an exam and make it a tough one.' So I did. We made an appointment, and of course he knew everything. He somehow had got the notes.After the award of his doctorate Schwinger worked at the University of California, Berkeley from 1939 to 1941. During the first year he was a National Research Council Fellow and then he became J Robert Oppenheimer's assistant. In 1941 he was appointed as an instructor in physics at Purdue University and the following year he was promoted to Assistant Professor.
During World War II, beginning in 1943, Schwinger was given leave of absence from Purdue and was sent to the Radiation Laboratory in the Massachusetts Institute of Technology. Later he was sent to the Metallurgical Laboratory (atom-bomb project) of the University of Chicago where Wigner was working. Schwinger did not like the work on atomic bombs so he got in his car and drove to Boston where Uhlenbeck was working on radar at the Radiation Laboratory. Schwinger asked Uhlenbeck if he could work there and it was agreed. Uhlenbeck said :-
And that he liked; he was in my group and he did all these mathematical problems on wave guides which was very good, of course. ... He was a real computer, really remarkable. He was mathematically and technically really remarkably good.Schwinger worked at night beginning :-
... in the evening at about 4 o'clock. I finally got him to give a seminar at 4.30. Julian was always out of breath when he came in, but then I had a certain influence on him so he did it and very conscientiously.In 1961 Schwinger was awarded an honorary doctorate from Purdue. In nominating him for this degree Hubert M James wrote on 6 December 1960 about Schwinger's contributions. Speaking of his time in the Radiation Laboratory James wrote:-
While at the Radiation Laboratory Schwinger invented important methods in electromagnetic field theory, which were extensively employed in the development of the theory of wave guides. He developed variational techniques that produced major advances in several fields of mathematical physics. Still more important were his contributions to the development of the modern form of quantum electrodynamics, through introduction of the "renormalization" technique. For this work he received the Nature of Light Award of the National Academy of Sciences, and shared with Kurt Gödel the first award of the $15 000 Albert Einstein Prize for achievement in Natural Science.Despite being on leave of absence at Purdue he was promoted to Research Professor in Theoretical Physics there. However, when he had finished his war work he resigned his position at Purdue to take up a post at Harvard. He worked at Harvard University from 1945 to 1972, first as an Associate Professor but being promoted to full Professor in 1947. The year he became a full professor he married Clarice Carrol of Boston.
Schwinger was one of the inventors in the 1940s of the theory of renormalization, mentioned above. This theory allows individual particles to be considered from a distant viewpoint. Virtual particle pairs are not considered individually but rather surrounding virtual particles influence the appearance of the original particle. In 1951 he proposed, what is today called the Schwinger effect in quantum electrodynamics, where electron-positron pairs are sucked out of a vacuum by an electric field. This has not yet been confirmed by experiment.
In 1957 his theoretical work led him to conclude that there were two different neutrinos one associated with the electron and one with the muon. Later experimental work has verified these theoretical conclusions. He invented source theory, which deals uniformly with strongly interacting particles, photons, and gravitons. His development of these ideas provided a general framework for all physical phenomena.
Schwinger was joint winner of the Nobel Prize for Physics (1965) for his work in formulating quantum electrodynamics and thus reconciling quantum mechanics with Einstein's special theory of relativity. This topic, originating with the work of Dirac, was independently studied by Feynman who was a joint winner of the prize.
The Presentation Speech given by Ivar Waller on the occasion when Schwinger received the Nobel Prize put his work in context as follows:-
The electrons of an atom move according to the laws of quantum mechanics established in 1925 and the next following years. For the hydrogen atom, which has only one electron and consequently is the simplest atom to investigate theoretically, the calculation of the motion of the electron in the electric field of the nucleus led to results of such accuracy that 20 years elapsed until any error of the theory could be found experimentally. This occurred, however, in 1947 when Lamb and his collaborator Retherford discovered that some energy levels of hydrogen which should coincide theoretically were in fact somewhat shifted relative to each other. One important result of the work of this year's Nobel Prize winners ... was the explanation of the Lamb-shift. ...Almost simultaneously with the discovery of the Lamb-shift another peculiarity was found by Kusch and his collaborator Foley, which made it clear that the magnetic moment of the electron is somewhat larger than had been assumed before. Using the method of renormalization which he also developed Schwinger was able to prove that a small anomalous contribution should be added to the value of the magnetic moment accepted until then. His calculation agreed with the experiments. Schwinger's calculation was indeed earlier than and very important for the proper interpretation of these measurements.
Schwinger had developed the formalism of the new quantum electrodynamics in several fundamental papers .... He has also made this formalism more useful for practical calculations.From 1972 until his death in 1994 Schwinger worked at the University of California, Los Angeles. He was enormously respected, was a highly gifted lecturer, and supervised a string of impressive graduate students. Over his career he supervised over 70 doctoral students, 3 of whom have received Nobel prizes.
Schwinger gave his students much more than guidance on their research. He gave them a depth of understanding and a mastery of the field which permitted each to become not a Schwinger disciple, but an independent scientist.Despite this remarkable record of achievements, he tended to become more and more solitary in his work as he grew older. This meant that he did not have as much impact on the later developments as one would have expected.
The cover notes of  give this summary of his contributions:-
Schwinger was one of the most important and influential scientists of the twentieth century. The list of his contributions is staggering, from his early work leading to the Schwinger action principle, Euclidean quantum field theory, and the genesis of the standard model, to later valuable work on magnetic charge and the Casimir effect.In  he is described as follows:-
Julian Schwinger's legacy goes far beyond his published work. His lectures were elegant, lucid and original (he never did anything the same way twice), works of art and physics both.The Nobel Prize for Physics was certainly not the only honour Schwinger received. On the contrary he received many honours, some of which we have already mentioned above, including the first Einstein Prize (1951), the National Medal of Science (1964), honorary doctorates from Purdue University (1961) and Harvard University (1962), and the Nature of Light Award of the National Academy of Sciences of the United States (1949).
Article by: J J O'Connor and E F Robertson