Albert Einstein did not build the atomic bomb. But, with his Theory of Relativity, he published the formula which constitutes the basic principle of the atomic bomb, or how a huge amount of energy can be released from a small amount of matter. This theory was summarised by the equation:

E=mc² (energy = mass times the speed of light squared)

Einstein did not want to use his discovery to build bombs, but he changed his mind during the Second World War as Nazi Germany was growing more powerful. In addition, in 1938, Germany succeeded in splitting the uranium atom. In 1939, this led a group of physicists, including Einstein, Leo Szilard and Eugene Wigner, to write a letter to President Roosevelt to urge him to build the atomic bomb in order to counter Nazi aggression.

But Einstein never thought the bomb would actually be used, especially three months after the surrender of Germany. A year afterward, he expressed his view on the bombing, saying that “he was sure that President Roosevelt would have forbidden the atomic bombing of Hiroshima had he been alive”. He also said: "I have always condemned the use of the atomic bomb against Japan."