PAULI, WOLFGANG

The Austrian-Swiss physicist Wolfgang Ernst Pauli was born in Vienna on April 25, 1900, the son of Bertha (Schütz) and Wolfgang Joseph Pauli. His father, originally from Prague, became a professor of chemistry at the University of Vienna in 1922 and was one of the founders of the science of colloid chemistry. Bertha Pauli was a writer, as was her daughter Hertha who was also an actress, and they belonged to the cultural elite of Vienna. The family was originally of Jewish origin, but Wolfgang Sr. became a Catholic, and his son was baptized—his godfather being the famous physicist and philosopher Ernst Mach.

In high school, Pauli was an outstanding student, with a strong interest in mathematics and astronomy. In 1918, he enrolled at the University of Munich, Germany, to study with Arnold Sommerfeld, a famous expert on relativity and atomic physics and the teacher of future Nobel Prize winners, including Werner Heisenberg and Hans Bethe. Pauli completed his Ph.D. in only three years, writing a dissertation on the quantum theory of the hydrogen molecule ion. In 1920, while still a student, Pauli wrote a 250-page article on relativity for the 1921 Encyclopedia of Mathematical Physics at Sommerfeld's request. It was highly praised by Einstein and is still regarded as a major treatise on the subject.

The Exclusion Principle

After receiving his Ph.D., Pauli spent a year at the University of Göttingen, with James Franck and Max Born. He then worked for a year at Copenhagen with Niels Bohr, who had originated the quantum theory of the atom. It was at this time that he first took up the problem of the Zeeman effect, the splitting of spectral lines in the presence of a magnetic field, which was a subject of major interest because it seemed to be an insoluble problem in the Bohr-Sommerfeld quantum theory that dominated atomic physics. This theory, an extensive elaboration of Bohr's 1913 hydrogen model, placed the atomic electrons in classical orbits that were restricted by a general set of quantum conditions. For example, angular momenta and their vector components were restricted to being integer multiples of ? = h /2?, where h = 6.63 × 10^-34 J/s is Planck's constant.

However, the number of atomic states in a magnetic field was double the number that the theory predicted. In the simple case of sodium, for example,

there is one electron outside of a closed shell of electrons (the core) that Bohr-Sommerfeld theory predicts should have zero angular momentum. To account for the extra atomic states, it was proposed that the core should instead have an angular momentum of ½?, an idea that Pauli rejected. His solution was to say that the electron itself has a "non-classically describable two-valuedness," so that it was not the core but the external valence electron that was responsible for the doubling of the number of atomic states

This was a suggestion of the greatest importance, for it played an essential role in explaining the periods in Mendeleev's table of chemical elements. Bohr had already made a start in this direction with his building-up principle, which asserted that in passing from one atom in the table (characterized by the atomic number Z, or number of electrons) to the next one, the inner electrons kept the same quantum numbers. However, to complete this picture, Pauli's new two-valuedness was needed: each set of the old quantum numbers labeled not one, but two states. The new form of the principle became known as the Pauli Exclusion Principle, and it was for this discovery that Pauli was awarded the Nobel Prize in Physics in 1945.

Quantum Mechanics and Quantum Electrodynamics

In 1925 Heisenberg, Pauli's close friend and collaborator, replaced the Bohr-Sommerfeld orbit theory with a new quantum mechanics from which the modern theory of physics and chemistry has originated. Pauli, however, was the first to apply Heisenberg's theory to a real physical problem, namely, the hydrogen atom, which he solved completely. Meanwhile, also in 1925, two young Dutch physicists, Samuel Goudsmit and George Uhlenbeck, identified Pauli's quantum number as belonging to electron spin. That is, every electron has an intrinsic spin angular momentum of ½?, and an associated intrinsic magnetic moment e ?/mc , which can take up one of two orientations in a magnetic field. Pauli resisted this rotating electron interpretation for almost a year, but in March 1926 he wrote to Bohr that he would "capitulate completely" (Mehra and Rechenberg 1982, p. 709). He then applied the spin and the exclusion principle to explain the magnetic properties of normal metals (paramagnetism) and thus initiated in 1927 a new research field, the quantum electron theory of metals.

In that same year, the English quantum theoretician Paul Dirac made a quantum theory of the electromagnetic field and also a relativistic generalization of the wave function, introduced by the Austrian physicist Erwin Schrödinger in his version of quantum mechanics. Dirac's theory predicted the existence of a positive electron (positron) that could be produced together with an ordinary negative electron, providing enough energy was available (at least 2mc², with m being the electron mass and c the velocity of light). Pauli and Heisenberg then wrote two important papers providing a relativistic treatment of the interaction between radiation and matter. They discovered important difficulties in their theory, which had to wait until the late 1940s for a satisfactory resolution. Problems of quantum field theory, as it came to be called, occupied Pauli for the rest of his life, especially the relation between spin and quantum statistics, which is crucial for the collective behavior of identical particles (whether they form shells, for example, or collapsed states, as in a laser).

As professor at the Swiss Federal Institute of Technology (ETH) from 1928, after serving at the University of Hamburg, Pauli continued his research on wave mechanics. This led in 1933 to another remarkable treatise, published as an encyclopedia article.

Nuclear Beta Decay and the Neutrino

In December 1930, convinced that a puzzling situation in nuclear beta decay required a "desperate solution," Pauli suggested that a new extremely penetrating neutral particle of very small (perhaps zero) mass accompanied each electron emitted in beta decay. Pauli took this step to account for what appeared to be energy "missing" from the process. Now called the neutrino, Pauli's particle became an ingredient of a new and successful quantum field theory of beta decay worked out by the Italian physicist Enrico Fermi at the end of 1933. (A generalized version of Fermi's theory forms part of the so-called Standard Model of elementary particle interactions developed in the 1970s.)

In 1940 Pauli, fearing a possible German invasion of Switzerland, moved to the Institute for Advanced Study in Princeton, New Jersey, where Einstein was also in residence. He returned in 1945 to the ETH in Zurich, where he remained until his death on December 14, 1958. Pauli had many important accomplishments in physics, and he was also a philosopher. In studying the psychology of creativity, he collaborated with the Swiss psychoanalyst Carl Gustav Jung. Because of the profoundly high standards that he brought to his work, Pauli is sometimes referred to as the "conscience of physics."

See also:Neutrino, Discovery of

Bibliography

Enz, C. P. "W. Pauli's Scientific Work" in The Physicist's Conception of Nature, edited by J. Mehra (Reidel, Boston,1973).

Enz, C. P. No Time to Be Brief: A Scientific Biography of Wolfgang Pauli (Oxford University Press, New York, 2000).

Fierz, M., and Weisskopf, V.F. Theoretical Physics in the Twentieth Century (memorial volume to Pauli)(Interscience, New York, 1960).

Mehra, J., and Rechenberg, H. The Historical Development of Quantum Mechanics, Vol. 1, Part 2 (Springer-Verlag, New York, 1982).

Pauli, W. "Remarks on the History of the Exclusion Principle." Science103 , 213–215 (1946).

Pauli, W. "Relativitätstheorie" in Encyclopädie der Mathematis-chen Wissenschaften 5, Part 2, 539–775 (B.G. Teubner, Leipzig, 1921). English translation: Theory of Relativity (Pergamon, New York, 1958).

Pauli, W. Lectures on Physics, Vols. 1–6 (MIT Press, Cambridge, MA, 1973).

Pauli, W. "Die allgemeine Prinzipien der Wellenmechanik" in Handbuch der Physik, Vol. 24, Part 1, edited by H. Geiger and K. Scheel (Springer-Verlag, Berlin, 1980). English translation: General Principles of Quantum Mechanics (Springer-Verlag, New York, 1990).

Peierls, R. E. "Wolfgang Ernst Pauli." Biographical Memoirs of Fellows of the Royal Society5 , 175–192 (1959).

Weisskopf, V. F. "Personal Memories of Pauli." Physics Today38 , 36–41 (1985).

Laurie M. Brown