Enrico Fermi

I

INTRODUCTION

Enrico Fermi (1901-1954), Italian-born American physicist and Nobel Prize winner, who made important contributions to both theoretical and experimental physics. Fermi’s most well-known contribution was the demonstration of the first controlled atomic fission reaction. Atomic fission occurs when an atom splits apart (see Atom). Fermi was the first scientist to split an atom, although he misinterpreted his results for several years. He also had an important role in the development of fission for use as an energy source and as a weapon (see Nuclear Energy; see Atomic Bomb). He won the 1938 Nobel Prize in physics for his work in bombarding atoms with neutrons, subatomic particles with no electric charge. Initially, Fermi believed that this process created new chemical elements heavier than uranium (see Transuranium Elements), but other scientists showed that he actually split atoms to create fission reactions.

II

FERMI’S LIFE

Fermi was born in Rome, Italy. At age 17 he earned a scholarship to the prestigious Scuola Normale Superiore in Pisa by writing an essay on the characteristics of sound. He went on to the University of Pisa, where he earned his doctorate in 1922. Fermi studied with German physicist Max Born in Göttingen, Germany, from 1922 to 1924.

In 1924 Fermi returned to Italy to teach mathematics at the University of Florence. He became professor of theoretical physics at the University of Rome in 1927. He was 26 years old—the youngest professor in Italy since 16th-century Italian scientist Galileo. In the 1930s dictator Benito Mussolini introduced anti-Semitic laws to Italy and Fermi feared for the safety of his wife, who was Jewish. In 1938, after traveling to Sweden to accept the Nobel Prize, Fermi immigrated to the United States rather than return to Italy. Fermi became a professor at Columbia University in New York in 1939, and in 1941 moved to Chicago, Illinois, for a professorship at the University of Chicago. During World War II (1939-1945) he was involved in the Manhattan Project, the American effort to develop an atomic bomb. In 1945 Fermi became a U.S. citizen and returned to Chicago, where he remained until his death.

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.