Wolfgang Pauli

Wolfgang Pauli the Austrian theoretical physicist (1900-1958), one of the founders of quantum mechanics and the author of the exclusion principle that governs the behaviour of electrons in atoms. He also predicted the existence of the neutrino, and his demand for rigorous consistency in physics made him a legendary critic of sloppy reasoning.

Early Life and Education

Wolfgang Ernst Pauli was born on 25 April 1900 in Vienna, the son of a physician and a writer. He was a precocious student with an extraordinary gift for mathematics and physics. He entered the University of Munich in 1918 and completed his doctorate in 1921 under Arnold Sommerfeld, publishing a substantial paper on the hydrogen molecule while still a student.

Sommerfeld recognised his brilliance and invited him to write the article on relativity for a great German reference work on the mathematical sciences. The resulting monograph, published in 1921 when Pauli was only twenty-one, drew praise from Albert Einstein himself and remains a classic exposition of the theory. Pauli then worked briefly with Max Born at Goettingen and with Niels Bohr in Copenhagen before taking up teaching posts in Hamburg and Zurich.

The Exclusion Principle

In 1924 and 1925, studying the anomalous behaviour of electrons in atomic spectra and the structure of the shells of electrons in atoms, Pauli was led to a fundamental rule. He proposed that no two electrons in an atom can occupy the same quantum state, a principle that became known as the exclusion principle. Its statement required the assignment to the electron of a fourth quantum number, later identified as spin. The principle explained the regularities that had been observed in the periodic table but never fully understood.

The exclusion principle explained why electrons fill successive shells rather than collapsing into the lowest energy level, and it therefore underlies the whole periodic table of the elements, the stability of matter and the nature of chemical bonding. It is one of the few principles of modern physics that applies without modification to systems of enormous complexity, from atoms to white dwarf stars. The pressure that supports a white dwarf against gravitational collapse is a direct consequence of the rule that no two of its electrons may share a quantum state.

Quantum Mechanics and Spin

Pauli was deeply involved in the development of quantum mechanics in the 1920s and formulated a description of the spinning electron in terms of matrices before the spin concept had been fully clarified. He combined special relativity and quantum mechanics in the Pauli equation, which describes the behaviour of a particle with spin in an electromagnetic field. He also co-authored early papers on the quantum theory of fields and was among the first to recognise the importance of the new mechanics for the theory of solids.

He also proved the spin-statistics theorem, which shows that particles of half-integer spin obey the exclusion principle and follow Fermi-Dirac statistics, while particles of integer spin do not. This result established the great division of matter into fermions and bosons and gave a profound explanation of why the exclusion principle holds. The distinction between the two classes of particles governs the behaviour of matter from the structure of atoms to the phenomenon of superconductivity.

The Neutrino and Later Work

In 1930, to account for the apparent loss of energy in beta decay, Pauli proposed that a very light, electrically neutral particle was emitted along with the electron, carrying away the missing energy. He called it the neutron, but after Chadwick's discovery the name neutrino was adopted for Pauli's particle. It was detected experimentally in 1956 by Clyde Cowan and Frederick Reines, by which time Pauli had received the 1945 Nobel Prize in Physics for his exclusion principle. The neutrino is now known to be abundant throughout the universe and to play a central role in the energy balance of stars.

Pauli spent the war years at Princeton and at the Institute for Advanced Study, refusing to work on nuclear weapons, and returned afterwards to Zurich. He turned to the theory of elementary particles, attempting without success to find a relation between the charge and the mass of a particle. His later years were marked by a continuing search for a unified account of the fundamental forces, and he remained convinced that such a unification must exist even though he could not discover it.

Influence and Legacy

Pauli's insistence on logical rigour and his quickness in detecting error made him a famous and feared presence at scientific gatherings; his verdict that a colleague's idea was not even wrong passed into legend. Yet his severity was in the service of clarity, and his criticism helped shape the standards of theoretical physics. Colleagues prized his judgement even when they dreaded it. He was also a warm friend and correspondent, and his long letters to colleagues record the intellectual life of the quantum generation in remarkable detail.

His exclusion principle remains a foundation of atomic, solid-state and nuclear physics, and his prediction of the neutrino opened a field that is still immensely active in astrophysics and particle physics. Pauli ranks among the central figures of the quantum revolution, remembered equally for his deep physical insight and for the austere intellectual honesty that was the mark of his character. He died in Zurich on 15 December 1958, and the exclusion principle that carries his name remains one of the permanent pillars of modern physics.

See also

• Pauli exclusion principle
• Neutrino
• Spin
• Fermi-Dirac statistics