The physicist Albert Einstein did not directly participate in the invention of the atomic bomb. But as we shall see, he was instrumental in facilitating its development.
In 1905, as part of his Special Theory of Relativity, he made the intriguing point that a large amount of energy could be released from a small amount of matter. This was expressed by the equation E=mc2 (energy = mass times the speed of light squared). The atomic bomb would clearly illustrate this principle.
But bombs were not what Einstein had in mind when he published this equation. Indeed, he considered himself to be a pacifist. In 1929, he publicly declared that if a war broke out he would "unconditionally refuse to do war service, direct or indirect... regardless of how the cause of the war should be judged." (Ronald Clark, "Einstein: The Life and Times", pg. 428). His position would change in 1933, as the result of Adolf Hitler's ascent to power in Germany. While still promoting peace, Einstein no longer fit his previous self-description of being an "absolute pacifist".
Einstein's greatest role in the invention of the atomic bomb was signing a letter to President Franklin Roosevelt urging that the bomb be built. The splitting of the uranium atom in Germany in December 1938 plus continued German aggression led some physicists to fear that Germany might be working on an atomic bomb. Among those concerned were physicists Leo Szilard and Eugene Wigner. But Szilard and Wigner had no influence with those in power. So in July 1939 they explained the problem to someone who did: Albert Einstein. According to Szilard, Einstein said the possibility of a chain reaction "never occurred to me", altho Einstein was quick to understand the concept (Clark, pg. 669+; Spencer Weart & Gertrud Weiss Szilard, eds., "Leo Szilard: His Version of the Facts", pg. 83). After consulting with Einstein, in August 1939 Szilard wrote a letter to President Roosevelt with...