Albert Einstein
The physicist whose 1905 equation E=mc2 explained why splitting atoms releases such power, and who signed the letter that launched the US bomb program.
The most famous equation in the world is really a bookkeeping rule. E=mc2 says that mass and energy are two currencies for the same thing, and the exchange rate is the speed of light squared, an almost unimaginably large number. Because that multiplier is so enormous, converting even a sliver of mass releases a staggering amount of energy. When Albert Einstein wrote the relation down in 1905 it was pure theory. Four decades later it was the reason a lump of uranium could level a city: fission turns a fraction of an atom's mass directly into the blast.
The miracle year
Einstein produced the equation during his 1905 'miracle year,' a burst of work he did as an unknown clerk in a Swiss patent office. In months he reimagined space and time with special relativity, explained the jitter of particles suspended in fluid, and showed that light behaves as discrete packets, the insight that helped found quantum theory and won him the 1921 Nobel Prize. A decade later his general theory of relativity recast gravity itself as the bending of spacetime. No single scientist has ever remade physics so completely, and he did the deepest of it before he turned forty.
The letter he signed
Einstein was a lifelong pacifist, but in the summer of 1939 the physicist Leo Szilard convinced him the danger was real: recent discoveries meant Nazi Germany might build an atomic bomb. Einstein signed a letter, largely drafted by Szilard, warning President Roosevelt that uranium could yield extremely powerful bombs of a new type and urging the United States to act. Delivered that October, it helped set in motion the research that became the Manhattan Project. Einstein himself never worked on the bomb; deemed a security risk for his politics, he was never cleared to see the program he had helped launch.
One great mistake
The weapon he warned about was built, and it was used on Japan, not Germany. Einstein spent his last decade campaigning against it, calling for world government and warning that the bomb had changed everything except our way of thinking. Of the 1939 letter he told a colleague, 'I made one great mistake in my life, when I signed the letter to President Roosevelt recommending that atom bombs be made.' He added the caveat that had always haunted the choice: there had been some justification, the fear that the Germans would get there first.
They never did. The German program barely progressed, and the man whose equation opened the atomic age lived just long enough to see it and regret his single push to build the weapon. He died in 1955, still doing physics, still signing appeals against the bomb his arithmetic had made possible.