On February 12, 1918, US-american theoretical physicist and Nobel Laureate Julian Seymour Schwinger was born. Schwinger is best known for his work on the theory of quantum electrodynamics (QED), in particular for developing a relativistically invariant perturbation theory, and for renormalizing QED to one loop order.
“Is the purpose of theoretical physics to be no more than a cataloging of all the things that can happen when particles interact with each other and separate? Or is it to be an understanding at a deeper level in which there are things that are not directly observable (as the underlying quantized fields are) but in terms of which we shall have a more fundamental understanding?”
— Julian Schwinger, Quantum Mechanics – Symbolism of Atomic Measurements (2001) p. 24 f.
Julian Schwinger – Youth and Education
Julian Seymour Schwinger was born in New York City, to Orthodox Jewish parents originally from Poland, Belle and Benjamin Schwinger, a prosperous garment manufacturer. Julian attended Townsend Harris High School and then the City College of New York as an undergraduate before transferring to Columbia University, where he received his B.A. in 1936 and published his first physics paper at the age of sixteen. In 1939 at the age of 21, Schwinger earned his Ph.D. under the supervision of Isidor Isaac Rabi, who led the molecular beam laboratory at Columbia University, with a dissertation “On the Magnetic Scattering of Neutrons“. From 1939 to 1941, Schwinger worked at the University of California, Berkeley under J. Robert Oppenheimer, and in 1941 was appointed to a position at Purdue University.
World War 2 and Los Alamos
While on leave from Purdue during World War II, Schwinger worked at the Radiation Laboratory at MIT instead of at the Los Alamos National Laboratory, where he provided theoretical support for the development of radar. After the war, Schwinger left Purdue for Harvard University, where he taught from 1945 to 1974.
Quantum Electrodynamics and Quantum Chromodynamics
Schwinger developed an affinity for Green’s functions from his radar work, and he used these methods to formulate quantum field theory in terms of local Green’s functions in a relativistically invariant way. This allowed him to calculate unambiguously the first corrections to the electron magnetic moment in quantum electrodynamics. Earlier non-covariant work had arrived at infinite answers, but the extra symmetry in his methods allowed Schwinger to isolate the correct finite corrections. Schwinger developed renormalization, formulating quantum electrodynamics unambiguously to one-loop order. 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.
Quantum Field Theory
Schwinger’s foundational work on quantum field theory constructed the modern framework of field correlation functions and their equations of motion. His approach started with a quantum action and allowed bosons and fermions to be treated equally for the first time, using a differential form of Grassman integration. He gave elegant proofs for the spin-statistics theorem and the CPT theorem, and noted that the field algebra led to anomalous Schwinger terms in various classical identities, because of short distance singularities. These were foundational results in field theory, instrumental for the proper understanding of anomalies.
In other notable early work, Rarita and Schwinger formulated the abstract Pauli and Fierz theory of the spin 3/2 field in a concrete form, as a vector of Dirac spinors. In order for the spin-3/2 field to interact consistently, some form of supersymmetry is required, and Schwinger later regretted that he had not followed up on this work far enough to discover supersymmetry. Schwinger discovered that neutrinos come in multiple varieties, one for the electron and one for the muon. Nowadays there are known to be three light neutrinos; the third is the partner of the tau lepton.
In the 1960s, Schwinger formulated and analyzed what is now known as the Schwinger model, quantum electrodynamics in one space and one time dimension, the first example of a confining theory. He was also the first to suggest an electroweak gauge theory, which was extended by his student Sheldon Glashow into the accepted pattern of electroweak unification. He attempted to formulate a theory of quantum electrodynamics with point magnetic monopoles, a program which met with limited success because monopoles are strongly interacting when the quantum of charge is small.
A Prolific Academic Advisor
Having supervised 73 doctoral dissertations , Schwinger is known as one of the most prolific graduate advisors in physics. Four of his students won Nobel prizes: Roy Glauber, Benjamin Roy Mottelson, Sheldon Glashow and Walter Kohn (in chemistry).
Particles, Sources, and Fields
In later years, Schwinger has followed his own advice about the practical importance of a phenomenological theory of particles. He has invented and systematically developed source theory, which deals uniformly with strongly interacting particles, photons, and gravitons, thus providing a general approach to all physical phenomena. This work has been described in two volumes published under the title “Particles, Sources, and Fields“.
Non Mainstream Physics
From 1972 until his death in 1994 Schwinger worked at the University of California, Los Angeles. Despite this remarkable record of achievements, he tended to become more and more solitary in his work as he grew older. After 1989 Schwinger took a keen interest in the non-mainstream research of cold fusion. He wrote eight theory papers about it. He resigned from the American Physical Society after their refusal to publish his papers. He felt that cold fusion research was being suppressed and academic freedom violated. In his last publications, Schwinger proposed a theory of sonoluminescence as a long distance quantum radiative phenomenon associated not with atoms, but with fast-moving surfaces in the collapsing bubble, where there are discontinuities in the dielectric constant. Mechanism of sonoluminescence now supported by experiments focuses on superheated gas inside the bubble as the source of the light.
The Nobel Prize in Physics
Schwinger was jointly awarded the Nobel Prize in Physics in 1965 for his work on quantum electrodynamics (QED), along with Richard Feynman and Shinichiro Tomonaga. This topic, originating with the work of Dirac, was independently studied by Feynman who was a joint winner of the prize.
Julian Schwinger died of pancreatic cancer in 1994, aged 76.
Porter Williams: Julian Schwinger and the Audacity of Scope, 
References and Further Reading:
-  Julian Schwinger, Biographical, Nobelprize.org
-  O’Connor, John J.; Robertson, Edmund F., “Julian Schwinger“, MacTutor History of Mathematics archive, University of St Andrews.
-  Sin-Itiro Tomonaga and Quantum Electrodynamics, SciHi blog, March 31, 2015.
-  The Case of J. Robert Oppenheimer, SciHi Blog, April 22, 2017.
-  Isidor Isaac Rabi and the Nuclear Magnetic Resonance, SciHi Blog, July 29, 2015.
-  Schwinger, J (1948). “On Quantum-Electrodynamics and the Magnetic Moment of the Electron”. Phys. Rev. 73 (4): 416–417.
-  Schwinger, J. “On Angular Momentum”, Harvard University, Nuclear Development Associates, Inc., United States Department of Energy (through predecessor agency the Atomic Energy Commission) (January 26, 1952).
-  Mehra, Jagdish (2000). Climbing the mountain: the scientific biography of Julian Schwinger. Oxford University Press. pp. 1–5.
-  Porter Williams: Julian Schwinger and the Audacity of Scope, Rotman Institute of Philosophy @ youtube
-  Julian Schwinger at Wikidata