Einstein's letter to FDR pictured a bomb too heavy to fly

On 2 August 1939, Albert Einstein signed a letter to US President Franklin D. Roosevelt warning that a nuclear chain reaction in uranium could make “extremely powerful bombs of a new type” possible. It was drafted with the help of the Hungarian émigré physicist Leo Szilard, who with Eugene Wigner and Edward Teller persuaded Einstein to lend his famous name, and it was sent from a summer address at Nassau Point on Long Island.

Did the letter predict the atomic bomb?

Yes, though cautiously, and it guessed wrong about delivery. Such a bomb was “conceivable—though much less certain”, and Einstein pictured one “carried by boat and exploded in a port”, where it might destroy the port and some of the surrounding territory. Such bombs, he added, “might very well prove to be too heavy for transportation by air.” Six years later, the 9,700-pound Little Boy was dropped on Hiroshima from a B-29.

Its requests were modest: a trusted go-between to keep the government informed and secure uranium ore (the United States had only poor ores; the best source was the Belgian Congo), and help funding university experiments. It closed by noting that Germany had stopped selling uranium from the Czechoslovakian mines it had seized.

What did Roosevelt do with the letter?

He set up a committee, though not at once. Delayed by the outbreak of war in Europe, the economist Alexander Sachs briefed Roosevelt in person on 11 October 1939. The president was noncommittal and worried about funding, but over breakfast the next morning he was persuaded, telling his military aide, General Edwin Watson: “This requires action.”

On 19 October 1939, Roosevelt wrote back to Einstein that he had set up a committee of civilian and military representatives to study uranium. That committee was the first in a long chain of decisions that grew into the Manhattan Project and its first bomb test.