In December 1938 two German physicists, Otto Hahn and Fritz Strassmann, performed a revolutionary experiment. Working at the Kaiser Wilhelm Institute in Berlin, Hahn and Strassmann bombarded uranium with neutrons and, in the process, split the uranium atom into two substances nearly equal in atomic weight, one of which they originally believed was radium. At first the two scientists could not explain what had happened to the uranium; it was inconceivable to them that an atom could be divided. Seeking an explanation of the experiment, Hahn in early January 1939 wrote a letter to his former colleague, the well-known physicist Lise Meitner, who, because she was Jewish, fled to Sweden when the Nazis occupied her native Austria.
Meitner soon realized that what the German scientists had produced was barium, not radium, since only barium could have been produced by splitting the uranium atom, a process called "nuclear fission." She also realized that a small fraction of the original mass of uranium in the Hahn-Strassmann experiment had entirely vanished. As stipulated by the law of the conservation of matter and energy, the missing mass must have been transmitted into pure energy. Hahn and Strassmann had proven the validity of the theoretical relationship between mass and energy first stated by Albert Einstein in 1905. Using Einstein's equation, E = mc2, Meitner calculated that a pound of uranium, if completely fissioned, would yield as much energy as burning several million pounds of coal. She also realized an ominous possibility: if the proper conditions were created, the fission of uranium could yield a weapon of unprecedented explosive power.
Alarmed by the prospect of German atomic bombs, Meitner informed her nephew, the physicist Otto R. Frisch, about the Hahn-Strassmann experiment. Frisch, in turn, rushed off to Copenhagen to alert the Danish physicist and Nobel laureate Niels Bohr, who was about to set sail for America to attend a conference of theoretical physicists in Washington, D.C., on January 26, 1939. The foreboding news of the Hahn-Strassmann discovery that Bohr brought with him spread quickly among the scientists attending the Washington conference. Enrico Fermi, a brilliant physicist and refugee from Mussolini's Italy, was the first to realize that uranium, if present in sufficient quantity (called its "critical mass"), could generate enough neutrons to produce a chain reaction, without which nuclear fission could not be sustained. In an experiment performed on March 3, 1939, Leo Szilard, the Hungarian expatriate, demonstrated that Fermi's assessment was correct. He bombarded uranium with neutrons and produced more neutrons. He concluded correctly that, if sufficient uranium could be accumulated, an atomic bomb theoretically might be produced.
Szilard, Fermi, and other like-minded refugees from fascist Europe were frightened by the possibility that the Hahn-Strassmann breakthrough could lead to the creation of a German atomic bomb. In fact, Fermi and Szilard feared that German physicists already enjoyed a substantial lead in nuclear research. This view was reinforced by news that Hitler had banned the export of uranium from Czechoslovakia after that country was occupied by the Nazis in March 1939. In the same month, Fermi and Szilard launched an effort to alert the American government to the potential danger of a German atomic bomb.
The initial results of their effort, however, were not auspicious. In March 1939 they were able to persuade George B. Pegram, the dean of the Graduate Faculties at Columbia University, to arrange a meeting with Rear Admiral S. C. Hooper, the director of Technical Operations for the Navy. The Navy, however, was not impressed. Fermi and Szilard received a Naval Research Laboratory grant of only $1,500 for fission research.The two physicists decided to try another tack. They persuaded Albert Einstein, America's best-known scientist, to send a letter, drafted by Szilard, to President Franklin Roosevelt warning him of the potential danger of a German atomic bomb. The letter, which Einstein signed on August 2, stated in part "that a single bomb of this type carried by boat and exploded in port might very well destroy the whole port together with some surrounding territory." Perhaps the most alarming and persuasive part of the letter warned: "I understand Germany has actually stopped the sale of uranium from Czechoslovakian mines which she has taken over. "
To make sure the letter was not lost in bureaucratic channels, Szilard arranged to have it delivered personally to Roosevelt by Dr. Alexander Sachs, a director of the Lehman Brothers Investment Corporation, who Szilard thought was sufficiently close to the President to gain an adequate hearing. Because of the pressure of other presidential business, especially the outbreak of war in September, Roosevelt was unable to see Sachs until October 11. After Sachs read Einstein's letter to him that day, and as a result of a subsequent conversation with Sachs the following day, Roosevelt was impressed by its implications. "Alex," he responded, "what you are after is to see that the Nazis don't blow us up." After Sachs nodded his head affirmatively, Roosevelt answered: "This requires action." And with these words, the United States had entered the nuclear arms race.
March to Armageddon, Ronald E. Powaski