III.

Fermi’s Work

Fermi’s first important contributions to physics were theoretical. In 1926 he devised a method for calculating the behavior of a system composed of particles that obeyed the Pauli exclusion principle. The Pauli exclusion principle, developed by Austrian-born Swiss physicist Wolfgang Pauli, states that no two particles can have identical quantum numbers. Quantum numbers identify properties of a particle such as energy, angular momentum, magnetic properties, and spin, or direction of rotation. The method that Fermi developed became known as Fermi statistics, and the particles that obey the Pauli exclusion principle became known as fermions. Fermions include all three of the particles that make up atoms (electrons, protons, and neutrons) as well as many other particles. British physicist P. A. M. Dirac independently developed an equivalent theory with a different approach several months later.

In 1933 Fermi published a theory that explained beta decay, or the transformation of a neutron into a proton, an electron, and a neutrino. Neutrinos are neutral particles related to electrons. Beta decay is a form of radioactivity, a process in which particles in atoms release energy and other particles. Fermi’s explanation of beta decay introduced a fundamental force called the weak force, or weak nuclear interaction. Scientists recognized three fundamental forces of interactions at that time: The gravitational force controls interactions between masses, the electromagnetic force controls the interaction of electric charges, and the strong force controls the interaction of particles in the nucleus of an atom. The weak force is more obscure and removed from everyday experience than the other forces. It allows particles to change into other particles under certain circumstances.

Fermi then turned to experimental physics. In 1933 French physicists Irène Joliot-Curie and Frédéric Joliot-Curie had artificially produced radioactive elements by bombarding aluminum and boron with alpha particles. Radioactive elements are elements composed of atoms that decay, or easily release particles and energy. Alpha particles are the nuclei of helium atoms, which contain two protons and two neutrons. In 1934 Fermi showed that single neutrons were even more effective than alpha particles at creating radioactive elements and isotopes. Isotopes of an element are atoms that contain the same number of protons (the number of protons in an atom determines which element it is), but different numbers of neutrons. Fermi discovered that shooting neutrons through paraffin wax at a sample of atoms slowed the neutrons down and increased the intensity of the radioactivity. He bombarded uranium samples with these slow neutrons and interpreted the results as the creation of elements heavier than uranium, or transuranium elements. In 1938, however, Austrian-born Swedish physicist Lise Meitner and Austrian-born British physicist Otto Frisch proposed and confirmed a theory that the uranium atoms were actually splitting apart instead of forming heavier elements. Fermi won the 1938 Nobel Prize in physics for his work with neutrons and radioactivity.

Fermi and other scientists realized the potential power of fission, or the splitting of atoms. Atoms release energy in the form of heat and radiation when they split. Because fission is triggered by neutrons, and atoms release neutrons when they split, one fission reaction can start more reactions, creating a self-sustaining, or chain, reaction. The more fission reactions that occur, the more energy the system releases. In 1939 a group of physicists warned U.S. President Franklin D. Roosevelt that fission chain reactions could be used as weapons, and that Germany might be developing such a weapon—an atomic bomb. In 1942, the Manhattan Project, the American effort to develop an atomic bomb, officially began. By the end of the year Fermi had designed and presided over the first controlled fission reaction, which occurred in an unused squash court in the basement of Stagg Field at the University of Chicago. In July 1945 the United States tested the first atomic bomb, and in August of that year the United States dropped atomic bombs on two cities in Japan, Hiroshima and Nagasaki.

Fermi eventually returned to the University of Chicago and continued to research radioactivity and neutrons. He also consulted on the construction of the synchrocyclotron, a large particle accelerator at the University of Chicago, completed in 1951. Particle accelerators increase the speed of subatomic particles to allow scientists to study the particles at high energies. Fermi used the particle accelerator to study what happens to atoms when they break up under great force. In 1954 Fermi received the Atomic Energy Commission Award, which was later renamed the Fermi Award. In 1955, a year after his death, the element fermium was named in his honor.