The history of nuclear reactors gives us a singular opportunity to study what happens when the world’s leading physicists, faced with the same problem, find solutions in complete isolation from one another. This paper takes as an example an elementary part of reactor theory, the four‐factor formula. It was discovered independently at least six times (in France, Germany, the Soviet Union, and the United States). But the groups, separated by wartime secrecy, used the formula in very different ways. Usually it was simply ignored. In only two cases was the formula integrated with experimental work—by the French and, in an entirely different way, by Fermi’s team in Chicago. Thus even though simultaneous discovery occurred, we need not conclude that the physics developed inevitably in a unique pattern.