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Szilard, Leo


(b. Budapest, Austria-Hungary, 11 February 1898; d. La Jolla, California, 30 May 1964), nuclear physics.

For the original article on Szilard see DSB, vol. 13.

The nuclear physicist Szilard was one of the world’s most original problem solvers in the broad realm of sciences. He is known mostly for his role in discovering nuclear chain reaction and his participation in the Manhattan Project that led to the development of the A-bomb. He is recognized in the early twenty-first century as a pioneer of thermodynamics and nuclear physics whose conscience catalyzed international efforts to fight against a nuclear war.

Szilard was born in Budapest, Austria-Hungary, to a middle-class Jewish-Hungarian family. As a student in Budapest, he won a prestigious second prize in the 1916 national competition in physics, and went on to study engineering at the Technical University of Budapest. Feeling insecure during the reign of the White Terror in 1919, Szilard left for Berlin, Germany.

Berlin Szilard completed his studies in physics at the University of Berlin, with Max von Laue, Max Planck, and Albert Einstein. His doctoral dissertation showed that the second law of thermodynamics permits conclusions pertaining to the relationship between entropy and information, and developed ideas that were to contribute control thermodynamic fluctuations. He investigated to modern information theory. His papers were accepted as a Habilitationsschrift at the University of Berlin, where Szilard became a Privatdozent.

In the late 1920s Szilard cooperated with Albert Einstein in developing a method for pumping liquid metals through tubes, which found its practical application in the United States after the introduction of atomic reactors during World War II.

Nuclear Scientist Szilard started work in nuclear physics in 1934, at St. Bartholomew’s Hospital, London, England, and by the late 1930s, he had become part of the distinguished group of top atomic scientists.

In London, Szilard started to experiment with Thomas A. Chalmers on radioactive elements. They produced a method for the separation of a radioactive element from the mass of the stable element. They also separated photo neutrons from beryllium, a process that ultimately resulted in the possibility for inducing the fission process that was of critical importance for war-related nuclear research. This discovery later provided the key to the problem of the chain reaction. Szilard also found that radium-beryllium photo neutrons represented a useful tool in nuclear research. His British experiments proved of value for the discovery and investigation of neutron emission of uranium on which a chain reaction is based. Szilard was invited to the Clarendon Laboratory in Oxford in 1935.

In 1941 the Nobel laureate Eugene P. Wigner recalled that Szilard’s patent applications of 1934–1935 contained references to pure neutron chains, in which the links of the chain are formed by neutrons of the mass number 1 alone. In spring 1934 Szilard applied for a provisional British patent on a chain-reacting system based on the concept that beryllium may give off two neutrons when reacting with one slow neutron. A year later he filed a patent application, a part of which was assigned to the British Admiralty as a sealed secret. Szilard’s patents described methods of production of fast protons, one of which became the cyclotron, as well as the production of radioactive elements by bombardment of fast protons and alpha-particles by neutrons. His patents also included a method for the artificial production of radioactive bodies based on a process discovered by Enrico Fermi.

Szilard migrated to the United States in 1938. He was a source of inspiration for the letter to President Franklin D. Roosevelt signed by Einstein on 2 August 1939, in which scientists urged the U.S. government to give particular attention and provide adequate funding for research on nuclear chain reaction, which they understood could lead to the construction of extremely powerful bombs. Like other Hungarian refugees arriving from Adolf Hitler’s Germany, Szilard keenly recognized that the Nazis would use scientific findings to further Nazi global ambitions, a recognition made vivid by the years of escape into exile since 1919.

In July 1939 Szilard reached the conclusion that a self-sustaining chain reaction is likely to be supported by a graphite-uranium system, a discovery that was first presented in an article submitted to the Physical Review in 1940. This became a key to developing nuclear weapons. Szilard faced the dilemma of publishing a scientific invention that could be used by Nazi Germany, and decided to withhold the article without losing his scientific credit.

Joining the War Effort Szilard entered Columbia University’s division contracted by the National Defense Research Committee in 1940. From 1942 he was intermittently employed by the Metallurgical Laboratory of the University of Chicago, as a member of the group led by Fermi. As “chief physicist” in the laboratory he faced a multiple handicap: he was not able to publish his research related to the chain reaction in the carbon-uranium system, its control, or its cooling; and it seemed equally undesirable to make money by creating a destructive weapon to be used in the war against Germany and Japan.

Based on the carbon-uranium system, the chain reaction was first demonstrated at the University of Chicago’s Stagg Field, on 2 December 1942. In 1943 Szilard decided to file patent applications to acquire the rights to basic inventions underlying his Chicago work with Fermi. In 1944 Szilard applied for a patent describing the system, but by that time he had no choice but to assign to the government “any and all inventions, discoveries, methods and ideas relating to nuclear fission, which are not covered by issued patents or abandoned patent applications” (Leo Szilard to A. H. Compton, 9 August 1943, Box 6, Folders 29–30, Leo Szilard Papers, Mandeville Special Collections Library, University of California, San Diego). Szilard ultimately signed a patent agreement allowing the U.S. government the right to all of his inventions in the field of nuclear fission. The patent was granted in 1955 to the U.S. Atomic Energy Commission, naming Fermi and Szilard as inventors.

Szilard often complained about the compartmentalization of information, the lack of trust on behalf of the War Department, and the slowdown of the nuclear project that established a dangerous atmosphere in war-related science. Not only did it cause a significant delay in producing the bomb, but it set the pattern and the discourse of Cold War politics as well.

Turning the Tide By spring 1945, Szilard became convinced, along with other scientists, that the bomb should not be used, even on Japan, or at least its use should be delayed. Szilard alerted the president to the dangers of a nuclear arms race, and called for the establishment of an international system of controls in the field of atomic bombs. His plea reached President Harry Truman, who directed Szilard to James F. Byrnes, the director of war mobilization.

Joined by the Nobel laureate Harold C. Urey, Szilard visited the future secretary of state on 28 May 1945, attempting to persuade Byrnes that an arms race between the United States and the Soviet Union could be averted only if control over atomic energy were not kept an American monopoly but were vested in an international organization. Szilard prophetically warned that the “competition between the United States and Russia … would lead to a rapid accumulation of vast quantities of atomic bombs in both countries” (“Atomic Bombs and the Post-War Position of the United States in the World,” Leo Szilard Papers, Box 10, Folder 9).

Byrnes believed that the Soviet Union had no uranium, and that it might take many years for it to make the bomb, time enough for the United States to create its own world order. American military power, he suggested, rather than U.S. diplomacy would keep the Soviets out of occupied Eastern Europe. The visit was a tremendous disappointment for Szilard: he sensed that the nuclear program was moving beyond the control of scientists.

Joined by sixty-seven fellow scientists, Szilard drew up a petition requesting no use of the new weapons in World War II. By then (24 July 1945), however, the Scientific Panel of the secretary of war’s Interim Advisory Committee had expressed their opinion “that military use of such weapons should be made in the Japanese War” (A. H. Compton to K. D. Nichols, 24 July 1945, in Szilard, 1978, p. 215).

Szilard suffered the consequences of his moral stance. General Leslie Groves’s personal dislike of him was soon translated into action: he was not to be offered a position in the new laboratory that was formed after mid-1946.

Continuing to work against his own interests, in 1947 he published “A Letter to Stalin,” in the Bulletin of the Atomic Scientists. Szilard naively suggested that Joseph Stalin speak to the American people directly every month, and allow the president of the United States to address the Russian people. He invited Stalin to discuss “the framework of a post-war reconstruction of the world”(reprinted in Szilard, 1987, pp. 29–32).

Throughout the 1950s and early 1960s, Szilard became involved in a number of social and political organizations, such as the Scientists for Survival, the Emergency Committee of Atomic Scientists, the Council of Inquiry into the Conditions of Peace, the Council to Abolish War, the Campaign for World Government, and the Alliance for Progress.

A self-appointed volunteer-ambassador, he continued to mediate between the United States and the Soviet Union, attending all meetings to discuss the international political implications of “the bomb.” He attended the Pugwash Conferences, sponsored by Cyrus Eaton, believing that the problem was not disarmament but the stability of a stalemate between the strategic atomic striking forces of the superpowers. As he wrote to Congressman Chester Bowles in 1955, “a full scale atomic war, no matter who gets in the first blow, would end with the devastation of both Russia and the United States to the point where the continued existence of either of them as a nation would be in serious jeopardy” (“Memo,” May 24, 1955, Leo Szilard Papers, Box 4, Folder 36). Launching a crusade, Szilard founded the Council to Abolish War (later the Council for a Livable World) and reinforced his message of ending the Cold War in a number of ways.

Szilard used his connections with Senator John F. Kennedy to brief him on his conversations with Nikita Khrushchev in 1960. Though Szilard was critical toward Soviet policies in his native Hungary during and after the revolution of 1956, Khrushchev was willing to enter into a correspondence with him on atomic stalemate.

In 1956–1957 Szilard became a candidate for the directorship of the newly founded Hahn-Meitner Institute in West Berlin, but ultimately refrained from accepting the position.

Toward Microbiology At forty-seven, Szilard took the risk of starting a new career in microbiology. He teamed with the physical chemist Aaron Novick from the Argonne National Laboratory, and began working for the Institute of Radiobiology and Biophysics at the University of Chicago, and as a professor of biophysics at the Enrico Fermi Institute for Nuclear Studies.

He built a new scientific tool, the Chemostat, which was described in Science as well as in the Proceedings of the National Academy of Sciences of the United States of America. Szilard’s main contribution to biology, however, was a theory of aging. Published in 1959, the theory postulated that “different individuals age at different rates, and the rate is determined by the number of ‘faults’ inherited.” For Szilard “faults” were mutants that he dubbed “vegetative” genes viewed as of critical importance to the healthy life of an adult. His conclusions included the belief that the theory of the aging process will help explain the practice of birth control and the optimal fertility period of the female population. Studies on enzyme induction in bacteria and on antibody formation made him part of modern molecular biology.

Szilard ended his career at the Salk Institute in La Jolla, California, where he died of a heart attack in 1964. His papers are preserved in the University of California, San Diego, library. He was a politician among scientists, a scientist among politicians, and a supranational humanist who embraced a universal vision of mankind. In an age of disasters he was a man of conscience trying to save the world.


The Leo Szilard Papers can be found at the Mandeville Special Collections Library, University of California, San Diego, La Jolla, California.


The Collected Works of Leo Szilard: Scientific Papers. Edited by

Bernard T. Feld and Gertrud Weiss Szilard. Cambridge, MA, and London: MIT Press, 1972.

Leo Szilard: His Version of the Facts: Selected Recollections and

Correspondence. Edited by Spencer R. Weart and Gertrud Weiss Szilard. Cambridge, MA, and London: MIT Press, 1978.

Toward a Livable World: Leo Szilard and the Crusade for Nuclear

Arms Control. Edited by Helen S. Hawkins, G. Allen Greb, and Gertrud Weiss Szilard. Cambridge, MA, and London: MIT Press, 1987.


Hargittai, István. The Martians of Science: Five Physicists Who

Changed the Twentieth Century. New York: Oxford University Press, 2006.

Lanouette, William, with Bela Silard. Genius in the Shadows: A

Biography of Leo Szilard, the Man behind the Bomb. New York: Charles Scribner’s Sons, 1992.

Frank, Tibor. “Ever Ready to Go: The Multiple Exiles of Leo

Szilard.” Physics in Perspective 7 (2005): 204–252.

Tibor Frank

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Leo Szilard

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. □

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Szilard, Leo


(b. Budapest, Hungary, 11 February 1898; d. La Jolla, California, 30 May 1964)

physics, biology.

Szilard, one of the most profoundly original minds of this century, contributed significantly to statistical mechanics, nuclear physics, nuclear engineering, genetics, molecular biology, and political science.

The oldest of three children of a successful Jewish architect-engineer, he was a sickly child and received much of his early education at home, from his mother. His electrical engineering studies were interrupted by World War I; drafted into the Austro-Hungarian army, he was still in officers’ school at the end of the war. In 1920 he went to Berlin to continue his studies at the Technische Hochschule. The attraction of physics proved too great, however, and he soon transferred to the University of Berlin, where he received the doctorate in 1922. His dissertation, written under the direction of Max von Laue, showed that the second law of thermodynamics not only covers the mean values of thermodynamic quantities but also determines the form of the law governing the fluctuations around the mean values. The continuation of this work led to his famous paper of 1929, which established the connection between entropy and information, and foreshadowed modern cybernetic theory.

During this period in Berlin, as a research worker at the Kaiser Wilhelm Institute and then as Privatdozent at the university, Szilard undertook experimental work in X-ray crystallography with Herman Mark. He also began to patent his long series of pioneering discoveries, including devices anticipating most modern nuclear particle accelerators. With Albert Einstein he patented an electromagnetic pump for liquid refrigerants that now serves as the basis for the circulation of liquid metal coolants in nuclear reactors.

Hitler’s assumption of power caused Szilard to leave Germany for England in 1933. There he conceived the idea that it might be possible to achieve a nuclear chain reaction. Szilard’s search for an appropriate nuclear reaction (he early realized that the neutron was the key), while a guest at St. Bartholomew’s Hospital in 1934 and at the Clarendon Laboratory, Oxford, after 1935, led to the establishment of the Szilard-Chalmers reaction and the discovery of the γ-ray-induced emission of neutrons from beryllium. It was only after he came to the United States, in 1938, that he learned of the discovery of fission in Germany by Hahn and Strassmann.

Szilard instantly recognized–as did nuclear physicists in other countries–that fission would be the key to the release of nuclear energy, and he immediately undertook experiments at Columbia University to demonstrate the release of neutrons in the fission process and to measure their number. With Fermi he organized the research there that eventually led to the first controlled nuclear chain reaction, on 2 December 1942, at Chicago. Probably more than any other individual, Szilard was responsible for the establishment of the Manhattan Project; it was he who arranged for the letter from Einstein to President Roosevelt that brought it about. His contributions to the success of its plutonium production branch, both in physics and in engineering, were manifold, especially in the earliest stages. The basic patent for the nuclear fission reactor was awarded jointly to Fermi and Szilard in 1945, but Szilard never realized any financial profit from it.

The last months of the war found Szilard, with James Frank and other Manhattan Project scientists, engaged in a futile effort to convince President Truman to use the first atomic bomb in a nonlethal demonstration to the Japanese of its destructive power.

After the war Szilard turned to biology. With Aaron Novick he invented and constructed a device for studying growing bacteria and viruses in a stationary state by means of a continuous-flow device, called the chemostat, in which the rate of bacteria growth can be changed by altering the concentration of one of the controlling growth factors. He used it for a number of years in fundamental studies of bacterial mutations and various biochemical mechanisms.

In the late 1950’s Szilard became increasingly interested in theoretical problems of biology; his 1959 paper “On the Nature of the Aging Process” still stimulates research and controversy. His last paper, “On Memory and Recall,” was published posthumously.

Throughout his life Szilard had a profoundly developed social consciousness. On fleeing Nazi Germany to England, one of his first acts was to inspire the organization of the Academic Assistance Council, to help find positions in other countries for refugee scientists. He was one of the leaders of the successful postwar Congressional lobbying effort by Manhattan Project alumni for a bill establishing civilian control over peaceful development of nuclear energy. Szilard was one of the instigators and active early participants in the international Pugwash Conferences on Science and World Affairs, and he wrote extensively on questions of nuclear arms control and the prevention of war. In 1962 he founded the Council for a Livable World, a Washington lobby on nuclear arms control and foreign policy issues.

Szilard was a fellow of the American Physical Society, the American Academy of Arts and Sciences, the American Academy of Arts and Sciences, and the National Academy of Sciences. He received the Einstein Award in 1958 and the Atoms for Peace Award in 1959.


Many of Szilard’s works are being brought together in Collected Works of Leo Szilard: Scientific Papers, Bernard T. Feld and Gertrude W. Szilard, eds, (Cambridge, Mass., 1972– ). His writings include “über die Ausdehnung der phänomenologischen Thermodynamik auf die Schwankungserscheinungen,” in Zeitschrift für Physik, 32 (1925), 753–788, his diss.; “über die Entropieverminderung in einem thermodynamischen System bei Eingriffen intelligenter Wesen,” ibid., 53 (1929), 840–856, translated as “On the Decrease of Entropy in a Thermodynamic System by the Intervention of Intelligent Beings,” in Behavioral Science, 9 (1964), 301–310; “Chemical Separation of the Radioactive Element From Its Bombarded Isotope in the Fermi Effect,” in Nature, 134 (1934), 462, written with T. A. Chalmers: The Voice of the Dolphins, and Other Stories (New York, 1961); a report to the secretary of war (June 1945), written with James Frank, Donald J. Hughes, J. J. Nickson, Eugene Rabinowitch, and Joyce C. Stearns, and a petition to the president of the United States (17 July 1945), in Bulletin of the Atomic Scientists, The Atomic Age, M. Grodzins and E. Rabinowitch, eds. (New York, 1963); and reminiscences, in Donald Fleming and Bernard Bailyn, eds., The Intellectual Migration: Europe and America, 1930–1960 (Cambridge, Mass., 1969).

On his life and work, see the notice by Eugene P. Wigner, in Biographical Memoirs, National Academy of Sciences, 40 (1969), 337–341.

Bernard T. Feld

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Szilard, Leo

Szilard, Leo (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.

William Lanouette

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Szilard, Leo

Leo Szilard (sĬ´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.

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"Szilard, Leo." The Columbia Encyclopedia, 6th ed.. . Retrieved April 29, 2017 from

Szilard, Leo

Szilard, Leo (1898–1964) US physicist, b. Hungary. Szilard's early work established the relation between information transfer and entropy. He devised a means of separating radioactive isotopes. He was active in the development of the nuclear bomb and, with Enrico Fermi, created the first sustained nuclear chain reaction based on uranium fission.

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"Szilard, Leo." World Encyclopedia. . 29 Apr. 2017 <>.

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"Szilard, Leo." World Encyclopedia. . Retrieved April 29, 2017 from