Einstein – Szilárd letter The Einstein – Szilárd letter was a letter sent to the United States President Franklin D . Roosevelt on August 2 , 1939 , that was signed by Albert Einstein . The letter was largely written by Leó Szilárd , in consultation with fellow Hungarian physicists Edward Teller and Eugene Wigner , and warned of the danger that Germany might develop atomic bomb s , and suggested that the United States should initiate 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 nuclear fission by Lise Meitner and Otto Hahn in December 1938 generated intense interest among physicists . 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 residing 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 bomb s . 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 , as Fermi maintained , the more abundant uranium - 238 isotope 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 was good at slowing 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 - dated the letter . They had known each other in Berlin in the early 1920s . The so - called Einstein - Szilárd refrigerator , an absorption refrigerator with no moving parts that runs at constant pressure and requires a heat source to operate , was jointly invented in 1926 by Einstein and Szilárd and patented in the US on November 11 , 1930 . The invention of freon as a fridge coolant limited the commercial success of this kind of refrigeration process . The Letter . On July 12 , 1939 , Szilárd and Wigner drove in Wigner ' s car to Peconic Bay , on Long Island , where Einstein was staying . When they explained about the possibility of atomic bombs , Einstein replied : " Daran habe ich gar nicht gedacht " ( I had not thought of that ) . Szilárd dictated a letter in German to the Belgian Ambassador to the United States to Wigner , who wrote it down , and Einstein signed it . At Wigner ' s suggestion , they also prepared a letter for the State Department explaining what they were doing and why , giving it two weeks to respond if it had any objections . This still left the problem of getting government support for uranium research . Another friend of Szilárd ' s , the Austrian economist Gustav Stolper , suggested approaching Alexander Sachs , who had access to President Franklin D . Roosevelt . Sachs told Szilárd that he had already spoken to the President about uranium , but that Fermi and Pegram had reported that the prospects for building an atomic bomb were remote . He told Szilárd that he would deliver a letter , but suggested that it come from someone more prestigious . For Szilárd , Einstein was again the obvious choice . Sachs and Szilárd drafted a letter riddled with spelling errors and mailed it to Einstein . Szilárd set out for Long Island again on August 2 . Wigner was unavailable , so this time Szilárd co - opted another Hungarian physicist , Edward Teller to do the driving . Einstein dictated the letter in German . On returning to Columbia University , Szilárd dictated the letter in English to a young departmental stenographer , Janet Coatesworth . She later recalled that when Szilárd mentioned extremely powerful bombs , she " was sure she was working for a nut " . Ending the letter with " Yours truly , Albert Einstein " did nothing to alter this impression . Both the letter and a longer explanatory letter were then posted to Einstein . The letter warned that : It also specifically warned about Germany : Delivery . The letter was signed by Einstein and posted back to Szilárd , who received it on August 9 . Szilárd gave both the short and long letters , along with a letter of his own , to Sachs on August 15 . Sachs asked the White House for an appointment to see the President , but before one could be set up , the administration became embroiled in a crisis due to Germany ' s invasion of Poland , which started World War II . Sachs delayed until October so that the President would give the letter due attention , securing an appointment on October 11 . On that date he met with the President , the President ' s secretary , Brigadier General Edwin " Pa " Watson , and two ordnance experts , the Army Lieutenant Colonel Keith F . Adamson and the Navy Commander Gilbert C . Hoover . Roosevelt summed up the conversation as : " Alex , what you are after is to see that the Nazis don ' t blow us up . " Roosevelt sent a reply thanking Einstein , and informing him that Einstein sent two more letters to Roosevelt , on March 7 , 1940 and April 25 , 1940 , which again urged action and urgency on nuclear research . A fourth letter , dated March 25 , 1945 , which was also drafted by Szilárd , urged the President to meet with Szilárd to discuss policy on nuclear energy . It did not reach Roosevelt before his death on April 12 , 1945 . Results . Roosevelt decided that the letter required action , and authorized the creation of the Advisory Committee on Uranium , chaired by Lyman James Briggs , the Director of the National Bureau of Standards with Adamson and Hoover as its other members . It met for the first time on October 21 , with the meeting also attended by Fred L . Mohler from the Bureau of Standards , Richard B . Roberts of the Carnegie Institution of Washington , and Szilárd , Teller and Wigner . Adamson was skeptical about the prospect of building an atomic bomb , but was willing to authorise $ 6 , 000 for the purchase of uraniumn and graphite for Szilárd and Fermi ' s experiment . The Advisory Committee on Uranium was the beginning of the US government ' s effort , but it did not vigorously pursue development of a weapon . It was superseded by the National Defense Research Committee in 1940 , and then the Office of Scientific Research and Development in 1941 , before Roosevelt authorized a full - scale development effort in January 1942 . The work of fission research was taken over by the Army ' s Manhattan Engineering District in June 1942 . It directed a full - scale bomb development program . Einstein did not work on the Manhattan Project . The Army denied him the clearance needed to work on it in July 1940 , because considered him to be a security risk because of his pacifist leanings . He did war work as a consultant for the United States Navy ' s Bureau of Ordnance . Einstein had no knowledge of the atomic bomb ' s development , and no influence on the decision for the bomb to be dropped . According to Linus Pauling , Einstein later regretted having signed the letter because it led to development and use of the atomic bomb against civilian populations . Einstein justified his decision because of the greater danger that Nazi Germany would develop the bomb first . He told " Newsweek " magazine in 1947 that " had I known that the Germans would not succeed in developing an atomic bomb , I would have done nothing . "