Wolfgang Ernst Pauli was born in 1900, in Vienna, in what was then Austria-Hungary. His father was Wolfgang Joseph Pauli, a medical doctor and biochemist who later became a professor at the University of Vienna. His mother was the former Bertha Schültz, an author. In 1918, Pauli entered the University of Münich. He chose Münich because it was the home of German theoretical physicist Arnold Sommerfeld, then one of the greatest teachers of theoretical physics alive. Pauli excelled, earning his Ph.D. in the shortest time allowed by the university.
After receiving his degree, Pauli was offered a job as assistant in theoretical physics at the University of Göttingen. There he came into contact not only with English physicist Max Born, professor of theoretical physics, but also with Danish physicist Niels Bohr, who was guest lecturer at Göttingen in 1922. It was through Bohr's lectures that Pauli began to think about some of the fundamental difficulties that still remained in Bohr's quantum theory of the atom. One of these problems was the existence of various electron energy levels within an atom. Nothing in classical physics could explain the fact that electrons are distributed in various energy levels outside the nucleus, with each energy level having a maximum permitted number of electrons.
Over the next three years, Pauli worked on this question. He also began to think about another puzzle in atomic theory, the Zeeman effect. In 1896, the Dutch physicist Pieter Zeeman had found that the presence of a strong magnetic field causes the lines of an atomic spectrum to split. More than twenty years later, no one had yet devised an explanation for this phenomenon, but, early in 1925, Pauli found a possible explanation. He suggested that a fourth quantum number --in addition to the three already known--was needed to describe completely the energy state of an electron. In addition to its principle quantum number (n), its azimuthal quantum number (script l ), and its magnetic quantum number (m), an electron must also have a fourth quantum number, Pauli said. This fourth number could have one of two (but only two) possible values.
At the time, Pauli had no idea as to how this fourth quantum number could be interpreted in physical terms. That problem was solved three years later when Dutch physicists Samuel Goudsmit and George Uhlenbeck discovered electron spin. Goudsmit and Uhlenbeck suggested that an electron could spin in one of two directions, clockwise or counter-clockwise. Their designation of these spins as +1/2 or -1/2 corresponded precisely to the two-valuedness of the fourth quantum number that Pauli had predicted.
As significant as the discovery of the fourth quantum number was, it was perhaps even more important in terms of what it led to: the exclusion principle. Prompted by a 1924 paper by the English physicist E. D. Stoner, Pauli was able to develop an explanation for the fact that all electrons in an atom do not occupy the lowest energy level. The reason, he said, is that no two electrons can have exactly the same set of quantum numbers. An electron in the first energy level (n = 1) is restricted to an azimuthal quantum number of zero (because script l = n - 1) and a magnetic quantum number of 0 (because m = +mn script l ). It can have spin quantum numbers of +1/2 or -1/2. These restrictions mean that electrons in the first energy level can have quantum numbers of 1,0,0,+1/2 or 1,0,0,-1/2, but no others. Therefore, since the exclusion principle says that there can be no more than one electron of each of these two kinds, only two electrons can occupy that first energy level. By a similar argument, it is possible to show how the second energy level can hold no more than eight electrons, the third level no more than eighteen, and so on. In this way, a theoretical basis is provided for the electron orbital capacities originally devised empirically by Bohr more than a decade earlier.
In 1928, Dutch American chemical physicist Peter Debye retired as professor of theoretical physics at the Eidgennössische Technische Hochschule (ETR, or Federal Institute of Technology) in Zürich and Pauli was appointed as his successor. With brief interruptions, Pauli would remain at this post until his death. During his first years in Zürich, Pauli turned to another problem troubling physicists: beta decay. According to quantum theory at the time, the loss of a beta particle by a radioactive particle should be accompanied by the loss of a discrete quantity of energy. The spectrum produced by beta decay should, therefore, be characterized by a series of lines. Instead, the spectra associated with beta decay were always continuous spectra. In 1930, Pauli proposed a solution for this dilemma. He suggested that the loss of a beta particle by a nucleus was accompanied by the loss of a second particle. The characteristics of beta decay required that this particle have no electrical charge and no--or almost no--mass. At first, Pauli referred to the particle as a "neutron," a name later given to the chargeless nuclear particle discovered by English physicist James Chadwick in 1932. After Chadwick's discovery, American physicist Enrico Fermi rechristened Pauli's particle as the "neutrino," or "little neutron." A year later, Fermi had also incorporated the neutrino into an elegant and totally satisfactory mathematical theory of beta decay. Because of the characteristics of this elusive particle, the neutrino itself was not actually discovered for more than two decades after the work of Fermi and Pauli. Shortly before his return to Switzerland in 1945, Pauli was informed of his selection as the winner of the Nobel Prize in physics.
Pauli was married twice, first to Kate Depner, then to Franciska Bertram in 1934. Pauli's work was acknowledged not only by the Nobel Prize but also by the Lorentz Medal of the Royal Dutch Academy of Sciences in 1930, the Franklin Medal of the Franklin Institute in 1952, and the Max Planck Medal of the German Physical Society in 1958. Pauli died in 1958.
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