Leo Szilard (; Hungarian: Leó Szilárd [ˈlɛoː ˈsilaːrd]; born Leó Spitz; February 11, 1898 – May 30, 1964) was a Hungarian-born American physicist, biologist, and inventor who made numerous important discoveries in nuclear physics and the biological sciences. He conceived the nuclear chain reaction in 1933, and patented the idea in 1936. In late 1939 he wrote the Einstein–Szilard letter for Albert Einstein's signature that resulted in the Manhattan Project that built the atomic bomb, and then in 1945 wrote the Szilard petition asking president Harry S. Truman to demonstrate the bomb without dropping it on civilians. According to György Marx, he was one of the Hungarian scientists known as The Martians.
Szilard initially attended Palatine Joseph Technical University in Budapest, but his engineering studies were interrupted by service in the Austro-Hungarian Army during World War I. He left Hungary for Germany in 1919, enrolling at Technische Hochschule (Institute of Technology) in Berlin-Charlottenburg (now Technische Universität Berlin), but became bored with engineering and transferred to Friedrich Wilhelm University, where he studied physics. He wrote his doctoral thesis on Maxwell's demon, a long-standing puzzle in the philosophy of thermal and statistical physics. Szilard was the first scientist of note to recognize the connection between thermodynamics and information theory.
Szilard coined and submitted the earliest known patent applications and the first publications for the concept of the electron microscope (1928), the cyclotron (1929), and also contributed to the development of the linear accelerator (1928) in Germany. Between 1926 and 1930, he worked with Einstein on the development of the Einstein refrigerator. After Adolf Hitler became chancellor of Germany in 1933, Szilard urged his family and friends to flee Europe while they still could. He moved to England, where he helped found the Academic Assistance Council, an organization dedicated to helping refugee scholars find new jobs. While in England, he discovered a means of isotope separation known as the Szilard–Chalmers effect, alongside Thomas A. Chalmers.
Foreseeing another war in Europe, Szilard moved to the United States in 1938, where he worked with Enrico Fermi and Walter Zinn on means of creating a nuclear chain reaction. He was present when this was achieved within the Chicago Pile-1 on December 2, 1942. He worked for the Manhattan Project's Metallurgical Laboratory at the University of Chicago on aspects of nuclear reactor design, where he was the chief physicist. He drafted the Szilard petition advocating a non-lethal demonstration of the atomic bomb, but the Interim Committee chose to use them in a military strike instead.
Together with Enrico Fermi, he applied for a nuclear reactor patent in 1944. He publicly sounded the alarm against the possible development of salted thermonuclear bombs, a new kind of nuclear weapon that might annihilate mankind. His inventions, discoveries, and contributions related to biological science are also equally important; they include the discovery of feedback inhibition and the invention of the chemostat. According to Theodore Puck and Philip I. Marcus, Szilard gave essential advice which made the earliest cloning of the human cell a reality.
Diagnosed with bladder cancer in 1960, he underwent a cobalt-60 treatment that he had designed. He helped found the Salk Institute for Biological Studies, where he became a resident fellow. Szilard founded Council for a Livable World in 1962 to deliver "the sweet voice of reason" about nuclear weapons to Congress, the White House, and the American public. He died in his sleep of a heart attack in 1964.
He was born as Leó Spitz in Budapest, Kingdom of Hungary, on February 11, 1898. His middle-class Jewish parents, Lajos (Louis) Spitz, a civil engineer, and Tekla Vidor, raised Leó on the Városligeti Fasor in Pest. He had two younger siblings, a brother, Béla, born in 1900, and a sister, Rózsi, born in 1901. On October 4, 1900, the family changed its surname from the German "Spitz" to the Hungarian "Szilárd", a name that means "solid" in Hungarian. Despite having a religious background, Szilard became an agnostic. From 1908 to 1916 he attended Föreáliskola (high school) in Budapest's 6th District . Showing an early interest in physics and a proficiency in mathematics, in 1916 he won the Eötvös Prize, a national prize for mathematics.
With World War I raging in Europe, Szilard received notice on January 22, 1916, that he had been drafted into the 5th Fortress Regiment, but he was able to continue his studies. He enrolled as an engineering student at the Palatine Joseph Technical University, which he entered in September 1916. The following year he joined the Austro-Hungarian Army's 4th Mountain Artillery Regiment, but immediately was sent to Budapest as an officer candidate. He rejoined his regiment in May 1918 but in September, before being sent to the front, he fell ill with Spanish Influenza and was returned home for hospitalization. Later he was informed that his regiment had been nearly annihilated in battle, so the illness probably saved his life. He was discharged honorably in November 1918, after the Armistice.
In January 1919, Szilard resumed his engineering studies, but Hungary was in a chaotic political situation with the rise of the Hungarian Soviet Republic under Béla Kun. Szilard and his brother Béla founded their own political group, the Hungarian Association of Socialist Students, with a platform based on a scheme of Szilard's for taxation reform. He was convinced that socialism was the answer to Hungary's post-war problems, but not that of Kun's Hungarian Socialist Party, which had close ties to the Soviet Union. When Kun's government tottered, the brothers officially changed their religion from "Israelite" to "Calvinist", but when they attempted to re-enroll in what was now the Budapest University of Technology, they were prevented from doing so by nationalist students because they were Jews.
Convinced that there was no future for him in Hungary, Szilard left for Berlin via Austria on December 25, 1919, and enrolled at the Technische Hochschule (Institute of Technology) in Berlin-Charlottenburg. He was soon joined by his brother Béla. Szilard became bored with engineering, and his attention turned to physics. This was not taught at the Technische Hochschule, so he transferred to Friedrich Wilhelm University, where he attended lectures given by Albert Einstein, Max Planck, Walter Nernst, James Franck and Max von Laue. He also met fellow Hungarian students Eugene Wigner, John von Neumann and Dennis Gabor.
Szilard's doctoral dissertation on thermodynamics Über die thermodynamischen Schwankungserscheinungen (On The Manifestation of Thermodynamic Fluctuations), praised by Einstein, won top honors in 1922. It involved a long-standing puzzle in the philosophy of thermal and statistical physics known as Maxwell's demon, a thought experiment originated by the physicist James Clerk Maxwell. The problem was thought to be insoluble, but in tackling it Szilard recognized the connection between thermodynamics and information theory.
Szilard was appointed as assistant to von Laue at the Institute for Theoretical Physics in 1924. In 1927 he finished his habilitation and became a Privatdozent (private lecturer) in physics. For his habilitation lecture, he produced a second paper on Maxwell's demon, Über die Entropieverminderung in einem thermodynamischen System bei Eingriffen intelligenter Wesen (On the reduction of entropy in a thermodynamic system by the intervention of intelligent beings), that had actually been written soon after the first. This introduced the thought experiment now called the Szilard engine and became important in the history of attempts to understand Maxwell's demon. The paper also introduced the first equation linking negative entropy and information. This work established Szilard as a foundational figure in information theory; however, he did not publish it until 1929 and chose not to pursue the topic further. Cybernetics, via the work of Norbert Wiener and Claude E. Shannon, would later develop the concept into a general theory in the 1940s and 1950s—though, during the time of the Cybernetics Meetings, John Von Neumann pointed out that Szilard first equated information with entropy in his review of Wiener's Cybernetics book.