Eugene Paul "E. P." Wigner (Hungarian Wigner Jenő Pál; November 17, 1902 – January 1, 1995) was a Hungarian American physicist and mathematician.

He received a share of the Nobel Prize in Physics in 1963 "for his contributions to the theory of the atomic nucleus and the elementary particles, particularly through the discovery and application of fundamental symmetry principles"; the other half of the award was shared between Maria Goeppert - Mayer and J. Hans D. Jensen. Wigner is important for having laid the foundation for the theory of symmetries in quantum mechanics as well as for his research into the structure of the atomic nucleus, and for his several mathematical theorems. It was Eugene Wigner who first identified Xe-135 "poisoning" in nuclear reactors, and for this reason it is sometimes referred to as Wigner poisoning.

Wigner was born in Budapest, Austria - Hungary, into a middle class Jewish family. At the age of 11, Wigner contracted what his parents believed to be tuberculosis. They sent him to live for six weeks in a sanatorium in the Austrian mountains. During this period, Wigner developed an interest in mathematical problems. From 1915 through 1919, together with John von Neumann, Wigner studied at the Fasori Evangélikus Gimnázium, where they both benefited from the instruction of the noted mathematics teacher László Rátz. In 1919, to escape the Béla Kun communist regime, the Wigner family briefly moved to Austria, returning to Hungary after Kun's downfall. Partly as a reaction to the prominence of Jews in the Kun regime, the family converted to Lutheranism.

In 1921, Wigner studied chemical engineering at the Technische Hochschule in Berlin (today the Technische Universität Berlin). He also attended the Wednesday afternoon colloquia of the German Physical Society. These colloquia featured such luminaries as Max Planck, Max von Laue, Rudolf Ladenburg, Werner Heisenberg, Walther Nernst, Wolfgang Pauli, and Albert Einstein. Wigner also met the physicist Leo Szilard, who at once became Wigner's closest friend. A third experience in Berlin was formative. Wigner worked at the Kaiser Wilhelm Institute for Physical Chemistry and Elektrochemistry (now the Fritz Haber Institute), and there he met Michael Polanyi, who became, after László Rátz, Wigner's greatest teacher.

In the late 1920s, Wigner deeply explored the field of quantum mechanics. A period at Göttingen as an assistant to the great mathematician David Hilbert proved a disappointment, as Hilbert was no longer active in his works. Wigner nonetheless studied independently. He laid the foundation for the theory of symmetries in quantum mechanics and in 1927 introduced what is now known as the Wigner D-matrix. It is safe to state that he and Hermann Weyl carry the whole responsibility for the introduction of group theory into quantum mechanics (they spread the "Gruppenpest"). In the late 1930s, he extended his research into atomic nuclei. He developed an important general theory of nuclear reactions (for instance the Wigner – Eckart theorem). By 1929, his papers were drawing notice in the world of physics. In 1930, Princeton University recruited Wigner, which was very timely, since the Nazis soon rose to power in Germany. At Princeton in 1934, Wigner introduced his sister Manci to the physicist Paul Dirac, whom she married.

In 1936, Princeton University did not rehire Wigner, hence he searched for new employment. He found this at the University of Wisconsin. There he met his first wife, Amelia Frank, who was a physics student there. However she died unexpectedly in 1937, naturally leaving Wigner distraught.

On January 8, 1937, Wigner became a naturalized citizen of the United States. Princeton University soon invited Wigner back into its employment, and he rejoined its faculty in Fall 1938. Although he was a professed political amateur, in 1939 and 1940 he played a major role in prompting the U.S. Government to establish the Manhattan Project, which developed the first atomic bomb by 1945. However, by his personal beliefs, Wigner was at heart a pacifist. Wigner was present at a converted squash courts at the University of Chicago's abandoned Stagg Field on December 2, 1942, when the world's first atomic reactor, Chicago Pile One (CP-1) achieved a nuclear chain reaction (a critical reaction). He later contributed to civil defense in the U.S. In 1946, Wigner accepted a position as the Director of Research and Development at the Clinton Laboratory (now the Oak Ridge National Laboratory) in Oak Ridge, Tennessee. When his duties there did not work out especially well, Wigner returned to Princeton University.

In 1941, Wigner married his second wife, Mary Annette Wheeler, a professor of physics at Vassar College, who had completed her Ph.D. at Yale University in 1932. They remained married until her death in 1977, and they were the parents of two children.

Wigner was known for his exquisite politeness. It was related that he returned a car to a swindling used car dealer with the words "Go to hell, please".

In 1960, Wigner published a now classic article on the philosophy of mathematics and of physics, which has become his best - known work outside of technical mathematics and physics, " The Unreasonable Effectiveness of Mathematics in the Natural Sciences". He argued that biology and cognition could be the origin of physical concepts, as we humans perceive them, and that the happy coincidence that mathematics and physics were so well matched, seemed to be "unreasonable" and hard to explain. His reasoning was resisted by the Harvard mathematician Andrew M. Gleason.

In 1963, Wigner was awarded the Nobel Prize in Physics. Wigner professed to never have considered the possibility that this might occur, and he added: "I never expected to get my name in the newspapers without doing something wicked." Wigner later won the Enrico Fermi award, and the National Medal of Science. In 1992, at the age of 90, Wigner published a memoir, The Recollections of Eugene P. Wigner with Andrew Szanton. Wigner died three years later in Princeton, New Jersey. One of his significant students was Abner Shimony. Wigner's third wife was Eileen Clare - Patton Hamilton Wigner ("Pat"), the widow of another physicist, Donald Ross Hamilton, who had died in 1972. (He had been the Dean of the Graduate School at Princeton University.)

Near the end of his life, Wigner's thoughts turned more philosophical. In his memoirs, Wigner said: "The full meaning of life, the collective meaning of all human desires, is fundamentally a mystery beyond our grasp. As a young man, I chafed at this state of affairs. But by now I have made peace with it. I even feel a certain honor to be associated with such a mystery." He became interested in the Vedanta philosophy of Hinduism, particularly its ideas of the universe as an all pervading consciousness. In his collection of essays Symmetries and Reflections – Scientific Essays, he commented "It was not possible to formulate the laws (of quantum theory) in a fully consistent way without reference to consciousness."

Wigner also conceived the Wigner's friend thought experiment in physics, which is an extension of the Schrödinger's cat thought experiment. The Wigner's friend experiment asks the question: "At what stage does a 'measurement' take place?" Wigner designed the experiment to highlight how he believed that consciousness is necessary to the quantum - mechanical measurement processes.