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Friday, January 2, 2015
Reading List: The Strangest Man
 Farmelo, Graham. The Strangest Man. New York: Basic Books, 2009. ISBN 9780465022106.

Paul Adrien Maurice Dirac was born in 1902 in Bristol, England. His father,
Charles, was a SwissFrench immigrant who made his living as a French teacher at a
local school and as a private tutor in French. His mother, Florence (Flo), had
given up her job as a librarian upon marrying Charles. The young Paul and his
older brother Felix found themselves growing up in a very unusual, verging upon
bizarre, home environment. Their father was as strict a disciplinarian at home
as in the schoolroom, and spoke only French to his children, requiring them to
answer in that language and abruptly correcting them if they committed any
faute de français. Flo spoke to the
children only in English, and since the Diracs rarely received visitors at home,
before going to school Paul got the idea that men and women spoke different
languages. At dinner time Charles and Paul would eat in the dining room,
speaking French exclusively (with any error swiftly chastised) while Flo,
Felix, and younger daughter Betty ate in the kitchen, speaking English.
Paul quickly learned that the less he said, the fewer opportunities for error
and humiliation, and he traced his famous reputation for taciturnity to his
childhood experience.
(It should be noted that the only account we have of Dirac's childhood experience comes from himself, much later in life. He made no attempt to conceal the extent he despised his father [who was respected by his colleagues and acquaintances in Bristol], and there is no way to know whether Paul exaggerated or embroidered upon the circumstances of his childhood.)
After a primary education in which he was regarded as a sound but not exceptional pupil, Paul followed his brother Felix into the Merchant Venturers' School, a Bristol technical school ranked among the finest in the country. There he quickly distinguished himself, ranking near the top in most subjects. The instruction was intensely practical, eschewing Latin, Greek, and music in favour of mathematics, science, geometric and mechanical drawing, and practical skills such as operating machine tools. Dirac learned physics and mathematics with the engineer's eye to “getting the answer out” as opposed to finding the most elegant solution to the problem. He then pursued his engineering studies at Bristol University, where he excelled in mathematics but struggled with experiments.
Dirac graduated with a firstclass honours degree in engineering, only to find the British economy in a terrible postwar depression, the worst economic downturn since the start of the Industrial Revolution. Unable to find employment as an engineer, he returned to Bristol University to do a second degree in mathematics, where it was arranged he could skip the first year of the program and pay no tuition fees. Dirac quickly established himself as the star of the mathematics programme, and also attended lectures about the enigmatic quantum theory.
His father had been working in the background to secure a position at Cambridge for Paul, and after cobbling together scholarships and a gift from his father, Dirac arrived at the university in October 1923 to pursue a doctorate in theoretical physics. Dirac would already have seemed strange to his fellow students. While most were scions of the upper class, classically trained, with plummy accents, Dirac knew no Latin or Greek, spoke with a Bristol accent, and approached problems as an engineer or mathematician, not a physicist. He had hoped to study Einstein's general relativity, the discovery of which had first interested him in theoretical physics, but his supervisor was interested in quantum mechanics and directed his work into that field.
It was an auspicious time for a talented researcher to undertake work in quantum theory. The “old quantum theory”, elaborated in the early years of the 20th century, had explained puzzles like the distribution of energy in heat radiation and the photoelectric effect, but by the 1920s it was clear that nature was much more subtle. For example, the original quantum theory failed to explain even the spectral lines of hydrogen, the simplest atom. Dirac began working on modest questions related to quantum theory, but his life was changed when he read Heisenberg's 1925 paper which is now considered one of the pillars of the new quantum mechanics. After initially dismissing the paper as overly complicated and artificial, he came to believe that it pointed the way forward, dismissing Bohr's concept of atoms like little solar systems in favour of a probability density function which gives the probability an electron will be observed in a given position. This represented not just a change in the model of the atom but the discarding entirely of models in favour of a mathematical formulation which permitted calculating what could be observed without providing any mechanism whatsoever explaining how it worked.
After reading and fully appreciating the significance of Heisenberg's work, Dirac embarked on one of the most productive bursts of discovery in the history of modern physics. Between 1925 and 1933 he published one foundational paper after another. His Ph.D. in 1926, the first granted by Cambridge for work in quantum mechanics, linked Heisenberg's theory to the classical mechanics he had learned as an engineer and provided a framework which made Heisenberg's work more accessible. Scholarly writing did not come easily to Dirac, but he mastered the art to such an extent that his papers are still read today as examples of pellucid exposition. At a time when many contributions to quantum mechanics were roughedged and difficult to understand even by specialists, Dirac's papers were, in the words of Freeman Dyson, “like exquisitely carved marble statues falling out of the sky, one after another.”
In 1928, Dirac took the first step to unify quantum mechanics and special relativity in the Dirac equation. The consequences of this equation led Dirac to predict the existence of a positivelycharged electron, which had never been observed. This was the first time a theoretical physicist had predicted the existence of a new particle. This “positron” was observed in debris from cosmic ray collisions in 1932. The Dirac equation also interpreted the spin (angular momentum) of particles as a relativistic phenomenon.
Dirac, along with Enrico Fermi, elaborated the statistics of particles with halfintegral spin (now called “fermions”). The behaviour of ensembles of one such particle, the electron, is essential to the devices you use to read this article. He took the first steps toward a relativistic theory of light and matter and coined the name, “quantum electrodynamics”, for the field, but never found a theory sufficiently simple and beautiful to satisfy himself. He published The Principles of Quantum Mechanics in 1930, for many years the standard textbook on the subject and still read today. He worked out the theory of magnetic monopoles (not detected to this date) and speculated on the origin and possible links between large numbers in physics and cosmology.
The significance of Dirac's work was recognised at the time. He was elected a Fellow of the Royal Society in 1930, became the Lucasian Professor of Mathematics (Newton's chair) at Cambridge in 1932, and shared the Nobel Prize in Physics for 1933 with Erwin Schrödinger. After rejecting a knighthood because he disliked being addressed by his first name, he was awarded the Order of Merit in 1973. He is commemorated by a plaque in Westminster Abbey, close to that of Newton; the plaque bears his name and the Dirac equation, the only equation so honoured.
Many physicists consider Dirac the second greatest theoretical physicist of the 20th century, after Einstein. While Einstein produced great leaps of intellectual achievement in fields neglected by others, Dirac, working alone, contributed to the grand edifice of quantum mechanics, which occupied many of the most talented theorists of a generation. You have to dig a bit deeper into the history of quantum mechanics to fully appreciate Dirac's achievement, which probably accounts for his name not being as well known as it deserves.
There is much more to Dirac, all described in this extensivelydocumented scientific biography. While declining to join the British atomic weapons project during World War II because he refused to work as part of a collaboration, he spent much of the war doing consulting work for the project on his own, including inventing a new technique for isotope separation. (Dirac's process proved less efficient that those eventually chosen by the Manhattan project and was not used.) As an extreme introvert, nobody expected him to ever marry, and he astonished even his closest associates when he married the sister of his fellow physicist Eugene Wigner, Manci, a Hungarian divorcée with two children by her first husband. Manci was as extroverted as Dirac was reserved, and their marriage in 1937 lasted until Dirac's death in 1984. They had two daughters together, and lived a remarkably normal family life. Dirac, who disdained philosophy in his early years, became intensely interested in the philosophy of science later in life, even arguing that mathematical beauty, not experimental results, could best guide theorists to the best expression of the laws of nature.
Paul Dirac was a very complicated man, and this is a complicated and occasionally selfcontradictory biography (but the contradiction is in the subject's life, not the fault of the biographer). This book provides a glimpse of a unique intellect whom even many of his closest associates never really felt they completely knew.
Posted at January 2, 2015 14:29