EINSTEIN, Albert (1879-1955). Typed letter signed ("A. Einstein") to Dr. F. G. Garcia, Princeton, 14 August 1950.

One page, 280 x 215mm on his blind embossed stationery (some foxing and mild toning). With the original transmittal envelope.

Underlining the importance of his equation E=mc2 to the development of the atomic bomb: "To the theory of relativity is due a formula informing us about the energy which can be freed by nuclear reactions. If you are interested in this matter you may read the popular book "Explaining the Atom" by Selig Hecht which is really excellent."

Although Einstein does not cite it explicitly, the "formula" which reveals "the energy which can be freed by nuclear reactions' is none other than E=mc2, the equation derived from special relativity which expresses the fact that mass and energy are equivalent and can be changed into each other. In the book Einstein recommends, Selig Hecht describes E=mc2 as: "probably the most important equation in history ... In 1905 Einstein wondered how to test this equation experimentally, and suggested that it might apply to the enormous energies released in radioactivity, which had only recently been discovered ... little did Einstein imagine then that his equation would be demonstrated forty years later on so large a scale as was done at Hiroshima [and] Nagasaki" (p.111). It is worth noting that at this date nuclear devices had been applied solely to military purposes: the first experimental use of nuclear power to generate electricity was not until the following year. Although inextricably linked to the development of atomic weapons both scientifically and practically (through the 1939 Einstein-Szilard letter to F.D. Roosevelt which prompted the Manhattan Project), Einstein deeply regretted the connection, and was notably reluctant to discuss it; he also extended this reluctance to quoting his most famous theory in correspondence. Letters connecting E=mc2 to the bomb are therefore of extreme rarity. Even writing to his son Hans Albert immediately after the war, Einstein wrote only obliquely "I showed (39 years ago already) that according to the special theory of relativity there exists an equivalence between the mass and energy of a system [and] that the energies released by radioactive decay are great enough to be emitted in a nuclear reaction when there is an imbalance of mass. That is all" (letter of 2 September 1945). Selig Hecht (1892-1947) was a professor of biophysics at Colombia University: his Explaining the Atom (1947) was a much-praised popularization of the science behind the development of atomic weapons.