# Julian Seymour Schwinger

# Julian Seymour Schwinger

**1918-1994**

**American Physicist**

Julian Schwinger was one of a small number of physicists to be involved in the creation of quantum electrodynamics, the physical theory of electrons, positrons, and photons. The theory is unique in providing a complete description of a well-defined set of physical phenomena that is in complete agreement with experiment. As such, quantum electrodynamics has strongly influenced the search for comprehensive theories of the less-well understood nuclear interactions. It also represents a new level of mathematical sophistication to be required of theoretical physicists in the future.

Schwinger was a child prodigy, the son of a businessman and his wife, who taught himself physics by reading library books and the *Encyclopedia Britannica,* and, skipping several grades, graduated from high school at the age of 14. While attending the City College of New York, he published a scientific paper in the *Physical Review* that attracted the attention of I. I. Rabi, who then arranged a scholarship for Schwinger at Columbia University. He received his Ph.D. degree from Columbia in 1939. Like many American physicists, Schwinger was drawn into military research, working on both the atomic bomb and radar. After the war, he joined the faculty at Harvard University, becoming a full professor at the age of 29.

The quantum theory developed by Erwin Schroödinger and Werner Heisenberg in the 1920s provided a description of the wavelike properties of electrons moving at low velocities. A form of this theory consistent with Einstein's theory of relativity was published by Paul Dirac soon after. While the new quantum mechanics made it possible to explain the absorption and emission of light by atoms and many aspects of the chemical bonds formed by atoms, the theory was unsatisfactory in that it did not provide a treatment of the particle-like properties of light in a manner consistent with its treatment of electrons.

The need for a more comprehensive theory was highlighted at one of the first postwar conferences on theoretical physics, a meeting held at Shelter Island, New York, in 1947. Schwinger and another former child prodigy, Richard Feynmann, attended. Much of the discussion at the Shelter Island conference focused on two new experimental results: a precise measurement of the difference between two energy levels of the hydrogen atom, the simplest atom; and new precise measurements of the magnetic moments of the electron. Both of these results disagreed slightly but indisputably with what was expected from the existing quantum theory. A consensus thus developed that a proper treatment of the electron interaction with the electromagnetic field would explain the new experimental results.

Because the electron has an electrical charge, it interacts with the electromagnetic field. Attempts to compute the energy of this interaction were beset with serious mathematical difficulties. Chief among them was the fact that traditional approaches gave results that involved adding together an infinite number of small quantities, and, depending on the order in which the terms were added, partial sums of the terms could be infinitely large. An effective way to deal with these mathematical problems, using measured quantities to replace some of the mathematics, was found by three individuals, the Americans Schwinger and Feynmann and the Japanese Sin-Itero Tomonoga, who would share the Nobel Prize for physics in 1965 for their contributions. Schwinger's contribution, which was characterized by logical clarity and mathematical elegance, was published in a series of papers in the *Physical Review* beginning in 1948. Feynman's approach was more intuitive, based on a graphical representation that has come to be known as the Feynman diagram. Tomonaga had actually anticipated some of the Americans' results but was prohibited from publication by wartime conditions in Japan.

Schwinger remained active in the study of particle physics until the time of his death, influencing research on the weak force and attempts to unify the fundamental forces. He is remembered as a brilliant lecturer who often spoke without notes and as a reserved individual with strong opinions but an open mind.

**DONALD R. FRANCESCHETTI**

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**Julian Seymour Schwinger**