Leo Szilard
Leo Szilard was a nuclear physicist and a close friend of Einstein. In 1920 he studied in Berlin where he witnessed the rise of Nazism,
Early in the race to build a nuclear bomb, scientists had identified the property a substance needed to have in order to turn mass into energy. It was radioactivity. Radioactivity is E=mc2 in action. A tiny amount of mass in the nucleus of an unstable atom is converted to energy as the nucleus splits into two stable nuclei. Neutrons are also ejected from the splitting nucleus, as shown on the right.
Radioactive material releases energy slowly. Few atoms split at any one time and the amount of energy released is minimal. In order for the radioactive material to be used as a fuel source the energy release must be accelerated.
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Early attempts to split the atom involved bombardment of nuclei with alpha particles. Alpha particles are composed of 2 neutrons and 2 protons and have an overall positive charge. More energy was needed to split the atom than was yielded during the process.
In 1933 Leo Szilard realised that splitting the atom with alpha particles was wrong. Repulsion of the positive alpha particles by the positive nucleus was preventing the alpha particle from blasting apart the nucleus. Szilard proposed that a neutral particle, such as the newly found neutron, be used to split the atom. His calculations revealed that if a neutron strikes the nucleus, not only will the nucleus split into two smaller, lighter nuclei but two neutrons will be released to continue the reaction. This has the potential for an exponential reaction and thus the conception of the nuclear bomb.
In 1938 news came of a German breakthrough in splitting the atom. German scientists achieved nuclear fission, which up to now was just a theory. Within months of this announcement the Nazis stockpiled uranium.
In July of 1939, Szilard demonstrated the possibility of a nuclear chain reaction. Concerned of the prospect that the Nazis could also produce a nuclear bomb, Szilard called on Einstein to explain his work. During their meeting Szilard convinced Einstein to write a letter to president Roosevelt outlining his concern of the possibility that the Nazis may win the race to develop a nuclear bomb.
As a direct result of this letter Roosevelt brought the US into the race to build a nuclear bomb. Three years later the top secret Manhattan Project was set up at a cost of 2.2 billion dollars. Science and the military were now working closely, driven by the fear the Nazis would succeed first.
Franklin Roosevelt