How physics drove the design of the atomic bombs dropped on Japan

by VICTORIA JAGGARD

Workers prepare the Fat Man, the implosion bomb that was dropped on Nagasaki in August 1945 PHOTO/Atomic Heritage Foundation

For many scientists involved in the Manhattan Project, the race to build an atomic bomb was a grim battle between life and death. There was no denying the technology’s destructive force or its inevitable civilian toll. After the bombings of Hiroshima and Nagasaki, which took place 70 years ago this week, scientific director J. Robert Oppenheimer famously recalled his feelings upon hearing the news, quoting from a Hindu text: “Now I am become Death, the destroyer of worlds.”

But in the grip of World War II, with German scientists furtively working on the same technology, Oppenheimer and other physicists in the U.S. were keenly focused on the task of creating the world’s first nuclear weapon. And within the secret confines of Los Alamos National Laboratory, an internal battle was raging between two groups with opposing ideas for how to deliver the deadly payload.

Ultimately, two types of bomb using different radioactive materials fell on Japan just days apart, codenamed Little Boy and Fat Man. But if scientists had succeeded in their first attempts, both bombs could have been named Thin Man.

The nucleus of an atom is a more variable place than you might imagine. At its heart, an atom contains a mix of particles called protons and neutrons, which combine to give the atom its mass and its unique elemental personality. While all atoms of a given chemical element have the same number of protons, the neutron count can vary, yielding isotopes of different masses. But like an overcrowded raft, some isotopes teeter on the edge of stability and are prone to spontaneously tossing out excess energy and particles in the form of radiation. Over time, radioactive isotopes naturally decay into more stable configurations and even into new elements in a fairly predictable chain of events.