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Leo Szilard

Hungarian–American physicist (1898–1964)

Szilard, the son of an architect, studied engineering in his native city of Budapest before moving to the University of Berlin where he began the study of physics and obtained his doctorate in 1922. He remained there until 1933 when, after spending a few years in England working at the Clarendon Laboratory, in Oxford, and at St. Bartholomew's Hospital, London, he emigrated to America in 1938. After the war Szilard moved into biology and in 1946 was appointed to the chair of biophysics at the University of Chicago, where he remained until his death. He became a naturalized American in 1943.

Szilard was one of the first men in the world to see the significance of nuclear fission and the first to bring it to the attention of Roosevelt. In 1934, after hearing of the dismissal of the possibility of atomic energy by Ernest Rutherford, he worked out that an element that is split by neutrons and that would emit two neutrons when it absorbed one neutron could, if assembled in sufficiently large mass, sustain a nuclear chain reaction. Szilard applied for a patent, which he assigned to the British Admiralty to preserve secrecy.

When in 1938–39 he heard of the work of Otto Hahn and Lise Meitner on the fission of uranium he was well prepared. After quickly confirming that the necessary neutrons would be present Szilard, fearing the consequences that would ensue from Hitler's possession of such a weapon, decided that the only sound policy was for America to develop such a weapon first. To this end he approached Albert Einstein, with whom he had worked earlier and who commanded sufficient authority to be heard by all, and invited him to write a letter to the President of the United States. This initiated the program that was to culminate in the dropping of the atomic bomb on Hiroshima six years later. During the war Szilard worked on the development of the bomb and, in particular, worked with Enrico Fermi on the development of the uranium–graphite pile.

If Szilard was one of the first to see the possibility and necessity to develop the bomb he was also one of the earliest to question the wisdom and justice of actually using it against the Japanese. He was the dominant spirit behind the report submitted by James Franck to the Secretary of War in 1945 forecasting the nuclear stalemate that would follow a failure to ban the bomb.

Although his early reputation was based on his work in physics he moved, after the war, into molecular biology. Szilard took his new subject seriously, attending classes at the Cold Spring Harbor laboratories in 1946. He was soon to develop a high degree of competence, designing an important new instrument, the chemostat, formulating new theories on the aging process, and stimulating Jacques Monod in his work on the operon and the repressor.


(1898–1964), physicist, molecular biologist, and arms control activist

Szilard was born in Budapest, Hungary. Educated at Budapest's Technical University, he earned a Ph.D. in physics at the University of Berlin in 1922. Fleeing to London in 1933, he conceived the nuclear chain reaction, which he patented in 1934 and assigned to the British Admiralty as a military secret. He pursued chain reaction research at Oxford until 1938, then emigrated to the United States.

At Columbia University in 1939, he codesigned with Enrico Fermi the world's first nuclear reactor, and drafted for Albert Einstein the 2 August 1939 letter to President Franklin D. Roosevelt that warned about German nuclear weapons research. This letter eventually led to the American effort in 1942, known as the Manhattan Project, to build the atomic bomb. Despite feuds over science and administration with the director, Gen. Leslie R. Groves, Szilard worked in 1942 and 1943 on reactor design, and by 1944 initiated postwar control schemes for atomic energy.

In 1945, Szilard organized an unsuccessful petition to President Harry S. Truman, urging that the atomic bomb be demonstrated before use against Japanese cities. He led the successful lobbying by scientists in 1945 to shift the atom's control from the army to the new, civilian Atomic Energy Commission, and thereafter worked against the proliferation of nuclear weapons.

In nuclear strategy, Szilard postulated in 1945 the concept of a “preventive” nuclear war, and in 1961 he proposed the balance of nuclear weapons necessary to assure minimal deterrence among armed states. He met privately in 1960 with Nikita S. Khrushchev, gaining the Soviet leader's assent to a Moscow‐Washington hot line. A founding participant from 1957 in the arms control and disarmament activities of the Pugwash Conferences on Science and World Affairs, Szilard created the first political action committee for arms control, the Council for a Livable World (1962). He also published both fiction and nonfiction positing wildly original and later useful techniques for nuclear arms control and verification.

[See also Atomic Scientists; Hiroshima and Nagasaki, Bombings of.]


  • Gertrud Weiss Szilard and Spencer Weart, eds., Leo Szilard: His Version of the Facts, 1978.
  • Helen Hawkins, G. Allen Greb, and Gertrud Weiss Szilard, eds., Toward a Livable World: Leo Szilard and the Crusade for Nuclear Arms Control, 1987.
  • William Lanouette, Genius in the Shadows: A Biography of Leo Szilard, the Man Behind the Bomb, 1993

Szilard, Leo (1898-1964) biologist and physicist. The Hungarian-born Szilard initially trained as an engineer but switched to chemistry and physics; from 1921 to 1922 he studied at the University of Berlin with Albert Einstein and did research in thermodynamics and in X-ray crystallography. He continued to work with Einstein and others in theoretical physics. In 1933 he moved to Britain to escape the Nazis and began the study of nuclear physics; he came to the United States in 1938 and worked with Enrico Fermi at Columbia University. In 1942 he became chief physicist at the Metallurgical Laboratory at the University of Chicago; in 1946 he became a half-time professor of biophysics at the Institute of Radiobiology and Biophysics and in 1956 a professor of biophysics at the Fermi Institute for Nuclear Studies at the same university. In the 1960s he worked with Jonas Salk at the Salk Institute for Biological Studies in California. In the three areas in which he worked—theoretical physics, nuclear physics, and theoretical biology-—his work was seminal and led to breakthroughs such as the electron microscope. He also understood early the implications of nuclear power for military strategy, and its dangers, and he was an early advocate of arms control.

See the Introduction, Abbreviations and Pronunciation for further details.

Biography: Leo Szilard

The Hungarian-American physicist - and later molecular biologist - Leo Szilard (1898-1964) helped initiate the atomic age and later worked for nuclear disarmament and world peace.

Leo Szilard was born in Budapest, Hungary, on February 11, 1898, the oldest of three children. His father was an engineer. "As far as I can see, " he wrote, "I was born a scientist." He received most of his instruction at home until the age of ten, learning German and French with governesses. From the age of ten to 18 he went to a public school. His attraction to physics began when he was 13.

In 1916, one year before his draft into the army, he entered the Hungarian Institute of Technology to study electrical engineering. He had returned there by the summer of 1919. At the end of 1919 he went to Berlin and registered at the Technische Hochschule, which he left in mid-1920 to complete his studies at the University of Berlin. He gave up engineering for physics. At the University of Berlin physics was thriving with Albert Einstein, Max Planck, Max von Laue, and Walter Nernst. Fritz Haber was director of one of the Kaiser Wilhelm institutes. Szilard was awarded a Doctor's degree in physics under von Laue in 1922. He served as Privatdozent (lecturer) at the University of Berlin, 1926 to 1933.

After the February 1933 Reichstag fire, Szilard left Germany. In 1934, in London, he joined the physics staff of the medical college of St. Bartholomew's Hospital. He also worked at the Clarendon Laboratory, Oxford University. Together with T. A. Chalmers, Szilard developed in 1934 the first method of separating isotopes of artificial radioactive elements.

In 1931 Szilard came to America on an immigrant visa. He stayed about four months. He immigrated to the United States on January 2, 1938, and became a naturalized citizen in 1943.

Launching the Atomic Age

The Albert Einstein letter to Peresident F. D. Roosevelt in 1939 initiated the atomic project. Szilard was the "ghost writer" [Julius Tabin]. Later Einstein acknowledged: "I made one great mistake in my life - when I signed the letter to President Roosevelt recommending that atom bombs be made, " he said in old age to Linus Pauling, "but there was some justification - the danger that the Germans would make them" [Donald Clark].

Between early 1939 and November 1940 Szilard had no formal affiliation. When Columbia University got a contract to develop the Enrico Fermi-Szilard system, Szilard was put on its payroll, on November 1, 1940.

From 1942 until the end of the war, Szilard conducted nuclear research at the University of Chicago. As recalled by Bernard Feld, Szilard had been "an indispensable factor in the successful achievement of the first man-made nuclear chain reaction and in the vast wartime enterprise known as the Manhattan Project, which culminated in the first man-made nuclear explosion." For Szilard, the "Father of the Bomb" [Donald Fleming], success was also a tragedy: "And on December 2, 1942, the chain reaction was actually started at Stagg Field on the campus of the University [of Chicago]. There was a crowd there and then Fermi and I stayed there alone. I shook hands with Fermi and I said I thought this day would go down as a black day in the history of mankind."

In October 1946 Szilard became professor of biophysics - with a joint appointment in social sciences - at the University of Chicago. He was seldom in residence. At the age of 65, in 1963, he became professor emeritus.

Two themes guided Szilard's life, as he noted in a letter to Niels Bohr on November 7, 1950: "Theoretically I am supposed to divide my time between finding what life is and trying to preserve it by saving the world." The man who "initiated the atomic age" [in the words of Edward Teller] was also the man who helped found the Pugwash conferences and pleaded for nuclear disarmament and world peace.

Although Szilard "always was a biologist at heart" [Jacques Monod], he made what he called "the switch to biology" in 1946. Together with Aaron Novick, he got his training in biology by attending summer courses given by Max Delbrück at Cold Spring Harbor on bacterial viruses and by C. B. Van Niel at Pacific Grove on bacterial biochemistry. Szilard and Novick developed the chemostat, a device used in growing bacterial populations in a stationary state. Szilard described himself as a "theoretical biologist."

Between 1923 and 1931, Szilard filed his earliest patent applications, several with Einstein as a joint inventor. The graphite-moderated nuclear reactor, listing Fermi and Szilard as co-inventors, received a patent in 1955.

On Szilard's influence, Teller said: "He was the most stimulating of all the people I have known. In a world in which conformity is almost a duty, Szilard remained a dedicated nonconformist." And further: "He [Szilard] played a unique role in American history. His ideas about atomic energy were ridiculed by Ernest Rutherford and doubted by Niels Bohr and Enrico Fermi, but accepted and acted upon by Albert Einstein and President Roosevelt." Monod remarked: "I have also recorded, in my Nobel lecture, how it was Szilard who decisively reconciled me with the idea (repulsive to me, until then) that enzyme induction reflected an antirepressive effect, rather than the reverse, as I tried, unduly, to stick to it."

A Man of Many, Many Interests

Szilard lived in a world of ideas. For Monod, "Indeed, he [Szilard] loved ideas, especially his own. But he felt that these lovely objects only revealed all their virtues and charms by being tossed around, circulated, shared, and played with." As noted by Teller: "Szilard was the originator of many ideas, ranging from information theory to the sexual life of bacteria, from how to release atomic energy to a proposal that people who inform about violations of disarmament treaties ought to receive international awards."

A paper he wrote in 1929 in which he showed a relationship between information and entropy, "to which for over 35 years nobody paid any attention, " claims Szilard, "is a cornerstone of modern information theory." He also stated: "I hit upon the idea of the cyclotron, may be a few years before Ernest Lawrence."

For Szilard "in science the greatest thoughts are the simplest thoughts" and "if you want to succeed in this world you don't have to be much cleverer than other people, you just have to be one day earlier than most people." He mentions his long walks or spending several months with the sole activity of dreaming about experiments.

Between 1957 and 1963 Szilard helped create the EMBO (European Molecular Biology Organization). He was visiting professor at the University of Colorado Medical Center and at Brandeis University. He was consultant to the National Institute of Mental Health, the World Health Organization, and the West German government. He helped create the Salk Institute, which he joined as non-resident fellow in July 1963 and as resident fellow on April 1, 1964.

Szilard was honored as fellow, American Academy of Arts and Sciences, 1954; as Humanist of the Year, American Humanist Association, 1960; as recipient of the Einstein Gold Medal of the Lewis and Rosa Strauss Memorial Fund, 1960, and of the Atoms for Peace Award, 1960; by election to membership in the National Academy of Sciences, 1961; and as recipient of an Honorary Doctor of Humane Letters from Brandeis University, 1961.

He was hospitalized for about a year in 1959-1960 and died in La Jolla, California, on May 30, 1964.

"To his friends, " wrote Monod, "his memory will remain as a unique image of a man to whom science was more than a profession, or even an avocation: a mode of being."

In 1970, a crater on the far side of the moon (34°N; 106°E) was named "Szilard" by the International Astronomical Union.

Further Reading

In over 40 years of scientific research, Szilard published only 29 articles in scientific journals; the last paper appeared posthumously. His only book of fiction, The Voice of the Dolphins (1961), is a collection of short stories of political satire. Szilard also wrote some autobiographical fragments. The collected works of Szilard were published by the MIT Press in 3 volumes: The Collected Works of Leo Szilard: Scientific Papers, Bernard T. Feld and Gertrud Weiss Szilard, editors, with Kathleen R. Winsor, with a foreword by Jacques Monod and introductory essays by Carl Eckart, Bernard T. Feld, Maurice Goldhaber, Aaron Novick, and Julius Tabin (1972); Leo Szilard: His Version of the Facts. Selected Recollections and Correspondence, Spencer R. Weart and Gertrud Weiss Szilard, editors (1978); and Toward a Livable World: Leo Szilard and the Crusade for Nuclear Arms Control, G. Allen Greb, Gertrud Weiss Szilard, and Helen S. Hawkins, editors, foreword by Norman Cousins, introduction by Barton J. Bernstein (1987).

On Szilard see William Lanouette, Genius in the Shadows: A biography of Leo Szilard, the man behind the bomb; Edward Teller, Better A Shield Than A Sword. Perspectives on Defense and Technology (1987); Richard Rhodes, The Making of the Atomic Bomb (1986); Emilio Segrè, "Historical Perspective. Refugee Scientists and Nuclear Energy, " in: Sixth International Conference on Collective Phenomena: Reports from the Moscow Refusnik Seminar, Inga Fischer-Hjalmars and Joel L. Lebowitz, editors, Annals of the New York Academy of Sciences, 452 (1985); Edward Teller, Energy. From Heaven and Earth (1979) (chapter 8); Ronald W. Clark, Einstein. The Life and Times (1971); Donald Fleming, "Émigré Physicists and the Biological Revolution, " and Leo Szilard, Reminiscences (edited by Gertrud Weiss Szilard and Kathleen R. Winsor) in: The Intellectual Migration. Europe and America, 1930-1960, Donald Fleming and Bernard Bailyn, editors. See also "Patent Is Issued On First Reactor. Fermi-Szilard Invention Gets Recognition - A.E.C. Holds Ownership, " The New York Times, May 19, 1955.


(born Feb. 11, 1898, Budapest, Hung., Austria-Hungary — died May 30, 1964, La Jolla, Calif., U.S.) Hungarian-born U.S. physicist. He taught at the University of Berlin (1922 – 33), then fled to England (1934 – 37) and the U.S., where he worked at the University of Chicago from 1942. In 1929 he established the relation between entropy and transfer of information, and in 1934 he helped develop the first method of separating isotopes of artificial radioactive elements. He helped Enrico Fermi conduct the first sustained nuclear chain reaction and construct the first nuclear reactor. In 1939 he was instrumental in establishing the Manhattan Project, in which he helped develop the atomic bomb. After the first use of the bomb, he promoted the peaceful uses of atomic energy and the control of nuclear weapons, founding the Council for a Livable World. In 1959 he received the Atoms for Peace Award.

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Columbia Encyclopedia: Szilard, Leo
('lärd) , 1898–1964, American nuclear physicist and biophysicist, born in Hungary. He was educated at the Budapest Institute of Technology and the Univ. of Berlin, receiving a doctorate from the latter in 1922. Working at the Univ. of Chicago with Enrico Fermi, he developed the first self-sustained nuclear reactor based on uranium fission. Szilard was one of the first to realize that nuclear chain reactions could be used in bombs and was instrumental in urging the U.S. government to prepare the first atomic bomb, but he later actively protested nuclear warfare and supported the use of nuclear energy for peaceful purposes.
Quotes By: Leo Szilard


"Don't lie if you don't have to."


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