Of all the rules that govern the way the physical world works, one of the most far-reaching, but also one of the most mysterious, is Wolfgang Pauli's "exclusion principle". Its discovery in 1924 earned Pauli the 1945 Nobel prize for physics, following his nomination by Einstein. By then, Pauli had long been recognised as one of the major figures in 20th-century physics, not only because of his own seminal contributions, but also because of the sharpness of his critical judgements of the work of nearly all his distinguished contemporaries. He was called "the conscience and criterion of truth for a large part of the community of theoretical physicists", but also "God's scourge".
Yet, despite his central role, little has been written about Pauli since his early death in 1958. His voluminous correspondence has remained largely inaccessible to those without a command of the German language. A full-length scientific biography is long overdue, and that gap has now been most handsomely filled by Pauli's last research assistant, Charles Enz. No Time to Be Brief is a complex narrative, woven from extracts of the correspondence, technical commentary on all Pauli's important papers, discussion of the family, political and economic backgrounds, and anecdotes. It should remain a standard reference work for years to come.
Pauli was born in Vienna on April 25 1900, and he burst on the physics scene in 1921, with the publication of a magisterial monograph on relativity theory that received a rave review from Einstein. Twenty-five years later at the Institute for Advanced Study at Princeton University, where Pauli had found refuge from an Axis-encircled Switzerland in 1940, Einstein offered an impromptu tribute at the dinner to celebrate Pauli's Nobel. "It was like a king who abdicates and appoints me as a sort of 'chosen son' successor," Pauli later wrote. He was indeed offered (in preference to Robert Oppenheimer) a permanent position at Princeton after Einstein's retirement. But although he had been relatively happy there ("The Chianti is now replaced by a quite serviceable California wine, not too expensive''), he was too much of a European and he returned to Zurich in 1946.
Europe is where most of the physics action had been during that remarkable quarter-century, and Pauli had been right in the thick of it. From his secondary school in Vienna he had gone to Munich in 1918 to do graduate work with Arnold Sommerfeld. There he was joined in 1920 by Werner Heisenberg. The two young prodigies developed a strong bond that remained unbroken until Pauli's death. The language of the letters they exchanged gives a sense of the intoxicating excitement of those magical few years in the mid-1920s, when the New Land of quantum physics was being discovered.
In October 1921, Pauli moved to Gottingen, to become the assistant of Max Born, who reported to Einstein that he was sure he would never find such a good one again. But he revised his opinion after Pauli's departure for Hamburg in April 1922 - he found his new assistant, Heisenberg, "as intelligent and more conscientious besides: him we do not have to waken in the mornings or otherwise remind him of his duties''. Enz includes a photo of Born pinching a grinning Pauli's ear in punishment for his lazy habits.
October 1922 found Pauli with Niels Bohr in Copenhagen, at the start of the other most influential association of his life. He struggled for more than a year with a nagging experimental anomaly in atomic spectroscopy before finally making his first real breakthrough. Back in Hamburg in late 1924, he proposed that the data could be understood only by supposing that valence electrons possessed a mysterious new quantum attribute: "A peculiar, not classically describable two-valuedness." Without at that stage offering any physical interpretation for this new property, he pressed on with what would become the Nobel paper. In it, he proposed that the new attribute should apply to all electrons, thus extending to four the list of three quantum numbers (one for energy and two for angular momentum) introduced by Bohr. He then found that the numbers of electrons in each atomic "shell", as already given by Edmund Stoner, could all be accounted for by the following simple rule: no two electrons can have identical values for all four of the quantum numbers.
Originally referred to as "Pauli's prohibition", or veto, this rule is the crucial ingredient for an understanding not only of the periodic table of elements, but also of the stability of matter in bulk (it is the reason, in effect, why we do not fall through the floor).
But, what was the reason for the rule? In his paper, Pauli admitted that he could not provide one, and he suggested that justification might be forthcoming only after a deeper understanding of quantum theory had been arrived at. The words had scarcely appeared in print when he received an advance copy, in July 1925, of Heisenberg's revolutionary article, in which the elements of the new quantum mechanics were first delineated. But it turned out that, unlike many other recipes of the old quantum theory, Pauli's rule could not be deduced from the standard axioms of quantum mechanics. Even starting from the more powerful formalism of quantum field theory, the foundations of which were laid by the two friends in 1928-29 in their only joint publications, investigations by Pauli and others showed that the best one could obtain was a kind of "negative" result: if the rule was not imposed, the theory developed all kinds of bad diseases. Thus it was that Einstein, in his nominating telegram to the Nobel committee, could press the case on the ground that the exclusion principle was "independent of the other axioms of quantum mechanics''. Nevertheless, when Pauli made it to Stockholm in 1946, he still regretted, in his Nobel lecture, the absence of a true derivation: "The impression seems to me unavoidable that the shadow of some incompleteness fell here on the bright light of success of the new quantum mechanics."
Pauli's reaction to any new idea, even one of his own, was often sharply hostile, but he was always prepared to change his mind. He had almost not published his note about the "two-valuedness". When, in early 1925, Ralph Kronig proposed to him that it could be interpreted as an intrinsic angular momentum (soon to be called "spin"), he brushed the suggestion aside as a "witty idea but having no connection with reality''. Poor Kronig did not publish his idea, leaving others beyond the range of Pauli's veto to reap (rightfully) the credit. Two years later, however, Pauli provided the mathematical description of spin, in the form of the Pauli matrices known to every physics undergraduate since.
In December 1930, driven once more by the necessity to make reasonable sense of obstinate data, Pauli proposed the existence of a new particle, which became known as the neutrino. But he made the suggestion only in a private letter to a group of experimentalists meeting in Tubingen. "Dear Radioactive Ladies and Gentlemen," it famously began, "I have hit upon a desperate remedy... Unfortunately,'' he went on, "I cannot appear personally in Tubingen, since on account of a dance which takes place in Zurich on the night of December 6-7 I cannot get away from here...'' It was another three years before the cautious Pauli allowed his speculation to appear in official print.
He lived long enough to learn, in 1956, that neutrinos had finally been directly detected. In 1958, just two months before his death, he called the neutrino "that foolish child of the crisis of my life''. Eight days before sending the "Radioactive" letter, he had divorced his first wife after less than a year of marriage. His mother had committed suicide in 19, and after some time his father had remarried. On the advice of his father, Pauli consulted Carl Gustav Jung, who took over his analysis in 1932. By 1935 Jung had careful accounts by Pauli of some 1,300 of his dreams, the analysis of which (keeping Pauli's name secret) Jung published. Pauli's treatment was successfully concluded after two years, but he remained fascinated by his dreams and by broader questions concerning the psychological aspect of concept formation in the natural sciences, and their archetypal foundations.
Enz's account is a work of impressive scholarship, allied to a sympathetic insight into the character and milieu of his subject. He is owed a debt of gratitude by all physicists, historians of science and by anyone interested in the story of those heroic years when our understanding of the physical world underwent such radical change.
Ian Aitchison is professor of physics, University of Oxford.
No Time to Be Brief: A Scientific Biography of Wolfgang Pauli
Author - Charles P. Enz
ISBN - 0 19 856479 1
Publisher - Oxford University Press
Price - £35.00
Pages - 573