The idea that matter is composed of indivisible fundamental units — atoms — proposed by Greek philosophers, abandoned for two millennia, revived by chemists in the 19th century, transformed by physicists in the 20th, and now the foundation of all chemistry, much of physics, and the technology of nuclear weapons and nuclear power.
In 1945, two bombs destroyed two cities and demonstrated, with terrible clarity, what was inside an atom. The Manhattan Project had taken three years, employed 130,000 people, and cost $2 billion to turn a theoretical insight — that atoms could be split, releasing enormous energy — into a weapon. The theoretical insight was seven years old. The Greek philosopher Democritus had proposed the existence of atoms twenty-five centuries earlier. The distance from philosophical speculation to nuclear weapon illustrates both the extraordinary fertility of the atomic idea and the distance between knowing something is true and knowing what it means.