However, the letter was largely written by Leó Szilárd in consultation with fellow Hungarian physicists Edward Teller and Eugene Wigner. The letter suggested that the U.S. should begin its own research because of the potentially vast destructive power of atomic bombs. Einstein, Szilárd, Teller and Wigner were among a number of concerned scientists who initially feared Nazi Germany would develop the weapon first.
The connection between Einstein and Szilárd predated this now famous letter, but has largely been missed by many World War II historical researchers. The so called Einstein 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 Albert Einstein and Leó Szilárd (his former grad student) and patented in the US on 11 November 1930. The invention of freon as a fridge coolant limited the commercial success of this kind of refrigeration process.
The letter warned that:
In the course of the last four months it has been made probable — through the work of Joliot in France as well as Fermi and Szilard in America — that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future. This new phenomenon would also lead to the construction of bombs, and it is conceivable — though much less certain — that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air.
It also specifically warned about Germany:
I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated.
The letter was signed by Einstein on August 2 and delivered to Roosevelt by economist Alexander Sachs. But Sachs was delayed until October 11 because of the president's preoccupation with Adolf Hitler's invasion of Poland, which started World War II. After hearing Sachs' summary of the letter, Roosevelt stated that the letter required action, and authorized the creation of the Advisory Committee on Uranium.
The Committee first met on October 21 and was headed by Lyman James Briggs, Director of the National Bureau of Standards. Six thousand dollars were budgeted for neutron experiments performed by Fermi at the University of Chicago.
The Advisory Committee on Uranium did not vigorously pursue development of a weapon, and two other organizations superseded it (the National Defense Research Committee and the Office of Scientific Research and Development) before the work of fission research was taken over by the Manhattan Engineering District (MED) in 1942. It directed a full-scale bomb development program.
Einstein did not work on the Manhattan Project, the project to develop atomic bombs, because General Leslie Groves considered him to be a security risk because of his pacifist leanings, so he was assigned to work on improving conventional artillery for the U.S. Navy. Einstein had no knowledge of the atomic bomb's manufacturing, 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.
See also 
|Wikisource has original text related to this article:|
- "Albert Einstein's Letters to President Franklin Delano Roosevelt". E-World. 1997.
- Reproduction of 1939 Einstein-Szilard letter
- President Roosevelt's response to Dr. Einstein Letter, Atomic Archive
- Roosevelt correspondence with Einstein and Szilard, FDR library, Marist University