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The Einstein–Szilárd letter was a letter written by Leó Szilárd and signed by Albert Einstein that was sent to the United States President Franklin D. Roosevelt on August 2, 1939. Written by Szilárd in consultation with fellow Hungarian physicists Edward Teller and Eugene Wigner, the letter warned that Germany might develop atomic bombs and suggested that the United States should start its own nuclear program. It prompted action by Roosevelt, which eventually resulted in the Manhattan Project developing the first atomic bombs. ==Origin== The discovery of uranium fission in December 1938, reported in the January 6, 1939 issue of ''Die Naturwissenschaften'' by Otto Hahn, and Fritz Strassmann, and its correct identification as nuclear fission by Lise Meitner in the February 11, 1939 issue of ''Nature'', generated intense interest among physicists. Even before publication, the news was brought to the United States by Danish physicist Niels Bohr, who opened the Fifth Washington Conference on Theoretical Physics with Enrico Fermi on January 26, 1939. The results were quickly corroborated by experimental physicists, most notably Fermi and John R. Dunning at Columbia University. The Hungarian physicist Leó Szilárd, who was living in the United States at the time, realized that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction that could yield vast amounts of energy for electric power generation or atomic bombs. Such a reaction using neutrons was an idea he had first formulated in 1933, upon reading Ernest Rutherford's disparaging remarks about generating power from his team's 1932 experiment using protons to split lithium. However, Szilárd had not been able to achieve a neutron-driven chain reaction with neutron-rich light atoms. In theory, if in a neutron-driven chain reaction the number of secondary neutrons produced was greater than one, then each such reaction could trigger multiple additional reactions, producing an exponentially increasing number of reactions. Szilárd teamed up with Fermi to build a nuclear reactor from natural uranium at Columbia University, where they were fortunate in having a sympathetic head of the physics department in George B. Pegram. At the time there was disagreement about whether it was uranium-235, which made up less than 1% of natural uranium, or the more abundant uranium-238 isotope, as Fermi maintained, that was primarily responsible for fission. Fermi and Szilárd conducted a series of experiments, and concluded that a chain reaction in natural uranium could be possible if they could find a suitable neutron moderator. They found that the hydrogen atoms in water slowed neutrons, but tended to capture them. Szilárd then suggested using carbon as a moderator. They now needed large quantities of carbon and uranium to create a reactor. Szilárd was convinced that they would succeed if only they could get the materials. Szilárd was concerned that German scientists might also attempt this experiment. The German nuclear physicist Siegfried Flügge published two influential articles on the exploitation of nuclear energy in 1939. After discussing this prospect with fellow Hungarian physicist Eugene Wigner, they decided that they should warn the Belgians, as the Belgian Congo was the best source of uranium ore. Wigner suggested that Albert Einstein might be a suitable person to do this, as he knew the Belgian Royal Family. The connection between Einstein and Szilárd pre-dates the letter. They knew each other in Berlin in the early 1920s, and in 1926 jointly invented the Einstein-Szilárd refrigerator.〔(【引用サイトリンク】title=This Month in Physics History - November 11, 1930: Patent granted for Einstein-Szilard Refrigerator )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Einstein–Szilárd letter」の詳細全文を読む スポンサード リンク
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