Wolfgang Pauli
Pauli, Wolfgang (1900-1958) was an Austrian theoretical physicist who won the 1945 Nobel Prize in physics for his proposal in 1925 of a rule explaining the behavior of electrons in atoms. Such electrons orbit the nucleus of the atom. The rule, now called the Pauli exclusion principle, holds that no two electrons in an atom can have the same quantum numbers. An electron in an atom has four such numbers. They define the energy of the electron in terms of the distance of its orbit from the nucleus, the orbit's shape, the orientation of the axis of the orbit, and the electron's spin on its own axis.
Pauli also became the first to propose the existence of the subatomic particle the neutrino. He was also one of the first to fully understand the theory of general relativity.
Pauli was born April 25, 1900, in Vienna, Austria, to an accomplished family. His father, Wolfgang Joseph Pauli, was a noted physician and biochemist who later became a professor of biochemistry at the University of Vienna. His mother was Bertha (Schütz) Pauli, a writer with ties to the theater and journalism communities in Vienna. Pauli's younger sister, Hertha, became an actress and author. She later moved to the United States and wrote a number of books in English, including Toward Peace; The Nobel Prizes and Man's Struggle for Peace (1969).
Pauli's godfather, Ernst Mach, was the renowned Austrian physicist and psychologist who studied the action of bodies moving at high speeds through gases, and developed an accurate method for measuring their speeds in terms of the speed of sound.
At secondary school in Vienna, Pauli showed a talent for mathematics. He independently studied advanced calculus and read the newly published general theory of relativity of German-born physicist Albert Einstein. Pauli would later show his brilliance as a physicist when he wrote an exceptional article on the theory of relativity, while still a 21-year-old university student. It was done upon invitation and intended for publication in an encyclopedia of the mathematical sciences. Einstein read Pauli's work and praised it. In later years, Pauli's exposition of relativity would be published in book form, most recently published as Theory of Relativity in 1981.
Pauli studied at the University of Munich, obtaining his doctor's degree in 1921. He then spent a year at the University of Göttingen as assistant to Max Born, the German physicist who played a major role in developing quantum mechanics. Pauli spent another year at Copenhagen with Niels Bohr, the noted Danish physicist who developed a theory about the structure of the atom.
Paul's studies under Born, Bohr, and other top scientists brought Pauli to center stage in the new field of physics called quantum mechanics, which deals with the forces inside an atom and the motions of subatomic particles. This field began in 1913, when Bohr used the quantum theory to explain the motion of electrons in atoms. In 1913, Bohr published a theory describing how the single electron of the hydrogen atom moves around the nucleus. Quantum mechanics also explains how atoms absorb and give off energy as light, and it clarifies the nature of light. Quantum mechanics goes beyond the limits of classical mechanics, which is based on the physical laws formulated by the British scientist Sir Isaac Newton, and the Scottish scientist James Clerk Maxwell. It ranks as one of the major scientific achievements of the 1900's.
From 1923 to 1928, Pauli worked as assistant professor of theoretical physics at the University of Hamburg in Germany. There he conducted theoretical research in atomic structure and the behavior of atoms in magnetic fields, doing the work that would win him the Nobel Prize. The Pauli exclusion principle was to become essential for the understanding of the properties of atomic nuclei. In the following two decades, atomic research focused on the properties of the atomic nuclei. Further discoveries confirmed that the Pauli principle could be considered a fundamental law of physics. Although the principle was first discovered for electrons, it proved to be valid for the nuclei of hydrogen and also for the neutrons that are formed in many nuclear reactions.
In 1928, Pauli became professor of theoretical physics at the Federal Institute of Technology in Zurich, Switzerland. It was during this time that Pauli made one of his greatest contributions to physics. He explained the loss of energy and angular momentum (a measurement of spin) in certain atom-smashing experiments. In 1930, he proposed the existence of a subatomic particle, now called a neutrino, that has the “missing” energy and momentum. A neutrino is any of three types of subatomic particles that have no electric charge. Though some experiments indicate that neutrinos may carry some mass, the masses of neutrinos are as yet too small to measure directly. Neutrinos travel at or near the speed of light. Two processes produce neutrinos: (1) the breaking apart of unstable atomic nuclei or subatomic particles and (2) the fusion (joining) of atomic nuclei. Such fusion occurs in the core of the sun and other stars.
Neutrinos belong to the lepton family of particles. The other leptons carry an electric charge and have mass. They are, from the lightest to the heaviest, the electron, the muon, and the tau. A neutrino can change into a charged lepton by interacting with an atomic nucleus. Each type of neutrino can change into only one kind of charged lepton. The names of the three types of neutrinos come from the names of the corresponding charged leptons. Thus, an electron-neutrino can change into an electron, a muon-neutrino into a muon, and a tau-neutrino into a tau.
The neutrinos also have antimatter counterparts called antineutrinos that differ from neutrinos in the direction of their spin. There are electron-antineutrinos, muon-antineutrinos, and tau-antineutrinos.
Over 25 years after Pauli's prediction of the existence of neutrinos, the first neutrinos discovered were electron-neutrinos announced in 1956 by American physicists Frederick Reines and Clyde Cowan. The discovery earned Reines a share of the 1995 Nobel Prize in physics.
Neutrino detectors must be built deep underground. A detector senses a neutrino interaction indirectly, by sensing a later event produced by the interaction. For example, one kind of detector senses a flash of light that occurs after a neutrino interacts with a nucleus or an electron in the detector. But other kinds of particles can also produce the flashes and other events sensed by the detectors. There are tremendous numbers of such particles at and near the earth's surface. Unlike neutrinos, however, most of these particles cannot penetrate deep into the earth. So, to prevent such particles from interacting with matter in detectors, researchers build the detectors deep underground.
In 1935-1936. Pauli was visiting professor at the Institute for Advanced Study, Princeton, New Jersey. He held similar appointments at the University of Michigan in 1931 and 1941 and at Purdue University in 1942. Pauli was elected to the chair of theoretical physics at Princeton in 1940. He finally returned to Zurich at the end of World War II (1939-1945).
Pauli's discovery of the exclusion principle enabled scientists to picture the atomic structure and the behavior of the electrons of every known chemical element. Because the behavior of an atom's electrons determines how it will take part in chemical reactions, the principle also explained the chemical characteristics of the elements. Pauli helped to lay the foundations of the quantum theory of fields and he participated actively in the great advances made in this area of physics in the mid-1900's.
Pauli was a Foreign Member of the Royal Society of London, the Swiss Physical Society, the American Physical Society, and the American Association for the Advancement of Science. He was awarded the Lorentz Medal in 1930. He wrote General Principles of Quantum Mechanics, most recently reprinted in 1990. A series called Pauli Lectures on Physics, from Volume 1 through 6, was published in 1973. They included: 1. Electrodynamics, 2. Optics and the Theory of Electrons, 3. Thermodynamics and the Kinetic Theory of Gases, 4. Statistical Mechanics, 5. Wave Mechanics, and 6. Selected Topics in Field Quantization.
Pauli died in Zurich on Dec. 15, 1958.