Julian Schwinger


Julian Schwinger

Infobox_Scientist
name =Julian Schwinger


imagesize = 180px
caption =
birth_date = birth date|1918|2|12|mf=y
birth_place = New York City, New York, USA
death_date = death date and age|1994|7|16|1918|2|12
death_place = Los Angeles, California, USA
nationality = United States
field = Physics
work_institution = University of California, Berkeley
Purdue University
Massachusetts Institute of Technology
Harvard University
University of California, Los Angeles
alma_mater = City College of New York
Columbia University
doctoral_advisor = Isidor Isaac Rabi
doctoral_students = Roy Glauber
Ben R. Mottelson
Sheldon Lee Glashow

Walter Kohn
Bryce DeWitt
Daniel Kleitman
known_for = Quantum electrodynamics
prizes = nowrap|Nobel Prize in Physics (1965)
religion =
footnotes =

Julian Seymour Schwinger (February 12, 1918 – July 16, 1994) was an American theoretical physicist. He is best known for his work on the theory of quantum electrodynamics, in particular for developing a relativistically invariant perturbation theory, and for renormalizing QED to one loop order.

Schwinger is recognized as one of the greatest physicists of the twentieth century, responsible for much of modern quantum field theory, including a differential form of path integration, and the equations of motion for quantum fields. He developed the first electroweak model, and the first example of confinement in 1+1 dimensions. He is responsible for the theory of multiple neutrinos, Schwinger terms, and the theory of the spin 3/2 field.

Biography

Schwinger was born in New York City where he 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 his Ph.D. (overseen by I.I. Rabi) in 1939. He worked at the University of California, Berkeley (under J. Robert Oppenheimer) and was later appointed to a position at Purdue University.

Career

During World War II Schwinger worked at the Radiation Laboratory at MIT, providing theoretical support for the development of radar. After the war, Schwinger left Purdue for Harvard University, where he taught from 1945 to 1974.

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 unambiguously calculate the first corrections to the electron magnetic moment in quantum electrodynamics. Earlier noncovariant 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.

In the same era, he introduced nonperturbative methods into quantum field theory, by calculating the rate at which electron-positron pairs are created by tunneling in an electric field, a process now known as the Schwinger effect. This effect could not be seen in any finite order in perturbation theory.

Schwinger's foundational work on quantum field theory constructed the modern framework of field correlation functions and their equations of motion. He expressed the Feynman path integral in differential form, a formalism which allowed bosons and fermions to be treated equally for the first time, 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 lead to anomalous Schwinger terms in various classical identities, because of short distance singularities. This 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 is known to be exactly three neutrinos, the third is the partner 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, an SU(2) gauge group spontaneously broken to electromagnetic U(1) at long distances. This 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.

Having supervised more than seventy 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).

Schwinger had a mixed relationship with his colleagues, largely because of his source theory. Schwinger considered source theory as a substitute for field theory, although it is only a different point of view, a version of effective field theory. It treats quantum fields as long-distance phenomena, and does not require a well defined continuum limit. Source theory was considered overly formal and lacking in distinctness from quantum field theory, and the criticisms by his Harvard colleagues led Schwinger to leave the faculty in 1972 for UCLA. His work there was further from the mainstream, but he continued to find source theory reformulations of quantum field theoretic results for the rest of his career.

After 1989 Schwinger took a keen interest in the non-mainstream research of low-energy nuclear fusion reactions (AKA 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. He wrote: "The pressure for conformity is enormous. I have experienced it in editors’ rejection of submitted papers, based on venomous criticism of anonymous referees. The replacement of impartial reviewing by censorship will be the death of science."

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. Standard explanations, now supported by experiments, focus on superheated gas atoms inside the bubble as the source of the light, but Schwinger's methods tie back to his old quantum electrodynamic papers.

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.

Schwinger and Feynman

As a famous physicist, Schwinger was often compared to another legendary physicist of his generation, Richard Feynman. Schwinger was more formally inclined and favored symbolic manipulations in Quantum Field Theory. He worked with local field operators, and found relations between them, and he felt that physicists should understand the algebra of local fields, no matter how paradoxical.

By contrast, Feynman was more intuitive, believing that the physics could be extracted entirely from the Feynman diagrams, which gave a particle picture. Schwinger commented on Feynman diagrams in the following way,

Schwinger disliked Feynman diagrams, because he felt that they made the student focus on the particles and forget about local fields, which in his view inhibited understanding. He went so far as to ban them altogether from his class, although he understood them perfectly well and was observed to use them in private.

Despite sharing the Nobel Prize, Schwinger and Feynman had a different approach to quantum electrodynamics and to quantum field theory in general. Feynman used a regulator, while Schwinger was able to formally renormalize to one loop without an explicit regulator. Schwinger believed in the formalism of local fields, while Feynman had faith in the particle paths. They followed each other's work closely, and each respected the other. On Feynman's death, Schwinger described him as

Personal life

Schwinger is buried at Mount Auburn Cemetery; frac{alpha}{2pi} is engraved above his name on his tombstone.These symbols refer to his calculation of the correction ("anomalous") to the magnetic moment of the electron.

ee also

*Schwinger model
*Schwinger-Dyson equations
*Schwinger's variational principle
*Rarita-Schwinger action
*Lippmann-Schwinger equation
*Schwinger parametrization

References

Publications

*Feshbach, H., Schwinger, J. & J. A. Harr. [http://www.osti.gov/cgi-bin/rd_accomplishments/display_biblio.cgi?id=ACC0109&numPages=57&fp=N "Effect of Tensor Range in Nuclear Two-Body Problems"] , Computation Laboratory of Harvard University, United States Department of Energy (through predecessor agency the Atomic Energy Commission), (November 1949).
*Schwinger, J. [http://www.osti.gov/cgi-bin/rd_accomplishments/display_biblio.cgi?id=ACC0111&numPages=92&fp=N "On Angular Momentum"] , Harvard University, Nuclear Development Associates, Inc., United States Department of Energy (through predecessor agency the Atomic Energy Commission), (January 26, 1952).
*Schwinger, J. [http://www.osti.gov/cgi-bin/rd_accomplishments/display_biblio.cgi?id=ACC0110&numPages=51&fp=N "The Theory of Quantized Fields. II"] , Harvard University, United States Department of Energy (through predecessor agency the Atomic Energy Commission), (1951).
*Schwinger, J. [http://www.osti.gov/cgi-bin/rd_accomplishments/display_biblio.cgi?id=ACC0112&numPages=35&fp=N "The Theory of Quantizied Fields. III"] , Harvard University, United States Department of Energy (through predecessor agency the Atomic Energy Commission), (May 1953).

Further reading

*Mehra, Jagdish and Milton, Kimball A. "Climbing the Mountain: the scientific biography of Julian Schwinger", Oxford University Press, 2000.
*cite web |last=Milton |first=Kimball |title=Julian Schwinger: Nuclear Physics, the Radiation Laboratory, Renormalized QED, Source Theory, and Beyond |date=2006-10-09 |publisher=arXiv |url=http://arxiv.org/abs/physics/0610054; revised version published as "Julian Schwinger: From Nuclear Physics and Quantum Electrodynamics to Source Theory and Beyond," Physics in Perspective, 9, 70-114 (2007)
*Schweber, Sylvan S., "QED and the men who made it : Dyson, Feynman, Schwinger, and Tomonaga". Princeton Univ. Press, 1994.
*Ng, Y. Jack, Ed., "Julian Schwinger; the Physicist, the Teacher, and the Man" ,World Scientific, Singapore , 1996 ISBN 9810225318

External links

* [http://www.osti.gov/accomplishments/schwinger.html Photograph, Biography and Bibliographic Resources] , from the Office of Scientific and Technical Information, United States Department of Energy
* [http://www.nobel-winners.com/Physics/julian_seymour_schwinger.html Julian Schwinger]
* [http://www.nobel.se/physics/laureates/1965/schwinger-bio.html Nobel Museum Biography]
* [http://holiker.narod.ru/five/schwinger-press.html fundamental work in quantum electrodynamics, with deep-ploughing consequences for the physics of elementary particles]

Persondata
NAME= Schwinger, Julian
ALTERNATIVE NAMES=
SHORT DESCRIPTION= Physicist
DATE OF BIRTH= February 12, 1918
PLACE OF BIRTH= New York City, U.S.
DATE OF DEATH=July 16, 1994
PLACE OF DEATH= Los Angeles, U.S.


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