Nuclear fission

Nuclear fission was discovered in Berlin on the eve of World War II, by scientists who didn't understand what they had found until a physicist fleeing the Nazis explained it to them from Sweden. The discovery opened the adjacent possible for both nuclear weapons and nuclear power—the most consequential dual-use technology ever created.

Otto Hahn and Fritz Strassmann, chemists at the Kaiser Wilhelm Institute, had been bombarding uranium with neutrons for years, hoping to create elements heavier than uranium. What they found instead made no chemical sense: barium, an element with roughly half uranium's atomic mass. On December 17, 1938, they confirmed the puzzling result but couldn't explain it.

Hahn wrote to his longtime collaborator Lise Meitner, who had fled Berlin just months earlier when Nazi racial laws caught up with her Jewish ancestry. Meitner was now at the Nobel Institute in Stockholm, safe but isolated from her laboratory. Over Christmas, her nephew Otto Frisch—a physicist working with Niels Bohr in Copenhagen—came to visit.

They went for a walk in the snow, Frisch on skis, Meitner keeping up on foot. They stopped at a tree stump to work through the physics. Using Niels Bohr's model of the nucleus as a liquid droplet, they realized what had happened: the uranium nucleus had split in two. When Meitner calculated the mass difference between the original uranium and the two daughter nuclei, it came to about one-fifth of a proton's mass. Plugged into Einstein's E=mc², this yielded 200 million electron volts—an enormous amount of energy.

Frisch named the process "fission" after learning that biologists called cell division "binary fission." He returned to Copenhagen and told Bohr, who immediately grasped the implications. If fission released neutrons as well as energy, those neutrons could trigger more fissions, creating a chain reaction. A sustained chain reaction would release energy continuously—a power source. An uncontrolled chain reaction would release it all at once—a bomb.

Hahn and Strassmann published their chemical findings on December 22, 1938, in Naturwissenschaften, without waiting for Meitner's physics explanation. Meitner and Frisch published their theoretical analysis separately in Nature. The Nobel Committee later awarded the 1944 Chemistry Prize to Hahn alone, ignoring Meitner's essential contribution—a injustice that historians still debate.

The cascade from December 1938 reshaped everything that followed. Within seven years, the Manhattan Project had built atomic bombs. Within fifteen, nuclear reactors were generating electricity. Fission enabled plutonium production, which enabled smaller bombs, which enabled the arms race that defined the Cold War. Every nuclear power plant, every nuclear submarine, every piece of medical isotope equipment traces back to that Berlin laboratory and that walk in the snow.