- Farmelo, Graham.
The Strangest Man.
New York: Basic Books, 2009.
ISBN 978-0-465-02210-6.
-
Paul Adrien Maurice Dirac was born in 1902 in Bristol, England. His father,
Charles, was a Swiss-French 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 first-class honours degree in engineering, only
to find the British economy in a terrible post-war 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 rough-edged
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 positively-charged 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 half-integral 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 extensively-documented 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
self-contradictory 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.
January 2015 