Paul Dirac

Paul Adrien Maurice Dirac, OM FRS (IPA: ^*) (August 8, 1902 – October 20, 1984) was a British theoretical physicist and a founder of the field of quantum physics. He held the Lucasian Professorship of Mathematics at Cambridge, where he discovered the Dirac equation.

Biography

Early years

Paul Dirac grew up in Bishopston, in the English city of Bristol. His father, Charles Dirac, was an immigrant from Saint-Maurice in the Canton of Valais, Switzerland and taught French for a living. His mother was originally from Cornwall and the daughter of a mariner. Paul had an elder brother, Felix, who committed suicide in March 1925, and a younger sister, Beatrice. His early family life appears to have been unhappy on account of his father's unusually strict and authoritarian nature. He was educated first at Bishop Road Primary School and then at Merchant Venturers' Technical College (later Cotham Grammar School), where his father was a teacher. The latter was an institution, attached to the University of Bristol, that emphasized scientific subjects and modern languages. This was an unusual arrangement at a time when secondary education in Britain was still dedicated largely to the classics, and something for which Dirac would later express gratitude.

Dirac studied electrical engineering at the University of Bristol, completing his degree in 1921. He then decided that his true calling lay in the mathematical sciences and, after completing a degree in mathematics at Bristol in 1923, he received a grant to conduct research at St. John's College, Cambridge, where he would remain for most of his career. At Cambridge, Dirac pursued his interests in the theory of general relativity (an interest he gained earlier as a student in Bristol) and in the nascent field of quantum physics, working under the supervision of Ralph Fowler.

Middle years

Dirac noticed an analogy between the old Poisson brackets of classical mechanics and the recently-proposed quantization rules in Werner Heisenberg's matrix formulation of quantum mechanics. This observation allowed Dirac to obtain the quantization rules in a novel and more illuminating manner. For this work, published in 1926, he received a Ph.D. from Cambridge.

In 1928, building on Wolfgang Pauli's work on non-relativistic spin systems, he proposed the Dirac equation as a relativistic equation of motion for the wave function describing the electron. This work led Dirac to predict the existence of the positron, the electron's antiparticle, which he interpreted in terms of what came to be called the Dirac sea. The positron was subsequently observed by Carl Anderson in 1932. Dirac's equation also contributed to explaining the origin of quantum spin as a relativistic phenomenon. However, the existence of the positron , predicted from Dirac's equation meant to describe a single electron, the positing of an infinite sea and the necessity of electron matter being created and destroyed in Fermi's 1934 theory of beta decay, led to the view that the fundamental role of the Dirac equation was as a "classical" field equation, itself subject to quantization conditions involving commutators. With this reinterpretation as the fundamental quantum field equation of any point matter of spin ħ/2, the Dirac equation is as central to theoretical physics as the Maxwell, Yang-Mills and Einstein field equations. Dirac is regarded as the founder of quantum electrodynamics, being the first to use that term. He also introduced the idea of vacuum polarization in the early 30s.

Dirac's Principles of Quantum Mechanics, published in 1930, is a landmark in the history of science. It quickly became one of the standard textbooks on the subject and is still used today. In that book, Dirac incorporated the previous work of Werner Heisenberg on “Matrix Mechanics” and of Erwin Schrödinger on “Wave Mechanics” into a single mathematical formalism that associates measurable quantities to operators acting on the Hilbert space of vectors that describe the state of a physical system. The book also introduced the bra-ket notation and the delta function.

Guided by a comment in Dirac's textbook and by Dirac's 1933 Soviet physics journal article entitled "The Lagrangian in quantum mechanics", Richard Feynman developed the path integral formulation of quantum mechanics in 1948. This work would prove exceedingly useful in relativistic quantum field theory, in part because the Lagrangian (difference between Kinetic and Potential energy), or rather its density in 3-dimensional space, can be integrated over a 4 space-time-dimensional volume so that its relativistic invariance is explicit, rather than implicit when commutators of field quantities are used, in the Hamiltonian formulation.

In 1931 Dirac showed that the existence of a single magnetic monopole in the universe would suffice to explain the observed quantization of electrical charge. This proposal received much attention, but there is to date no convincing evidence for the existence of magnetic monopoles.

Paul Dirac shared the Nobel Prize for physics in 1933 with Erwin Schrödinger "for the discovery of new productive forms of atomic theory." He married Eugene Wigner's sister, Margit, in 1937.

Later years

Dirac was the Lucasian Professor of Mathematics at Cambridge from 1932 to 1969. During World War II, he conducted important theoretical and experimental research on uranium enrichment by gas centrifuge. In 1937, he proposed a speculative cosmological model based on the so called "large numbers hypothesis." Dirac was unsatisfied with the renormalization approach to dealing with the infinities in quantum field theory and his work on the subject moved increasingly out of the mainstream. After having relocated to Florida in order to be near his elder daughter, Mary, Dirac spent his last years at Florida State University (FSU) in Tallahassee, Florida.

Death and afterwards

Dirac died in Tallahassee, where he is buried. The Dirac-Hellman Award at FSU was endowed by Dr Bruce Hellman (Dirac's last doctoral student) in 1997 to reward outstanding work in theoretical physics by FSU researchers. The Dirac Prize is also awarded by the International Centre for Theoretical Physics in his memory. In 1995, a plaque in his honour bearing his equation was unveiled at Westminster Abbey in London with a speech from Stephen Hawking. A commemorative garden, in his honour, has been established opposite the railway station in Saint-Maurice, the town of origin of his Dirac forefathers. Despite being born in Bristol, England of an English mother, the plaque in the garden describes him as "originating" in Saint-Maurice. Under Swiss law, every child born to a father who is a Swiss national also has Swiss citizenship, in the case of Diracs, they are citizens of Saint-Maurice in Canton Valais. Since Paul Dirac's father, Charles, only renounced his Swiss citizenship in 1919, Paul also had Swiss citizenship until 1919, when he was 17.

Views

Dirac was known among his colleagues for his precise and taciturn nature. When Niels Bohr complained that he didn't know how to finish a sentence in a scientific article he was writing, Dirac famously replied, "I was taught at school never to start a sentence without knowing the end of it." While visiting the U.S.S.R., he was invited to lecture on his philosophy of physics. He merely stood up and wrote on the board, "Physical laws should have mathematical beauty and simplicity." When asked on some occasion about his views on poetry, he replied, "In science one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in poetry, it's the exact opposite."

When asked to describe Richard Feynman, Eugene Wigner, Dirac's brother-in-law, described him as "another Dirac, only this time human".

Dirac was also noted for his personal modesty. He called the equation for the time-evolution of a quantum-mechanical operator, which Dirac was in fact the first to write down, the "Heisenberg equation of motion". Most physicists speak of Fermi-Dirac statistics for half-integer spin particles and Bose-Einstein statistics for integer spin particles. While lecturing later in life, Dirac always insisted on calling the former "Fermi statistics". He referred to the latter as "Einstein statistics" for reasons, he explained, of "symmetry".

After being asked about his thoughts on Dirac's religious views, Wolfgang Pauli remarked, "If I understand Dirac correctly, his meaning is this: there is no God, and Dirac is his Prophet," a reference to the Islamic profession of faith.

Trivia

Dirac re-discovered the purling stitch in knitting, after observing knitting and deducing that there was an alternative stitch. ^*

The character of Adric in the popular British television show Doctor Who was named after Paul Dirac; "Adric" is an anagram of "Dirac."

Selected bibliography

Principles of Quantum Mechanics (1930): This book summarizes the ideas of quantum mechanics using the modern formalism that was largely developed by Dirac himself. Towards the end of the book, he also discusses the relativistic theory of the electron (see Dirac equation) which was also pioneered by him. This work does not refer to any other writings then available on quantum mechanics.
Lectures on Quantum Mechanics (1966): Much of this book deals with quantum mechanics in curved space-time.
General Theory of Relativity (1975): This 68 page work summarizes Einstein's general theory of relativity.

References

The Second Creation: makers of the revolution in twentieth century physics by Robert P. Crease and Charles C. Mann, 1986 Macmillan Publishing, New York, 1996 (revised), Rutgers University Press. Entertaining personality-based history of particle physics/quantum mechanics in the twentieth century.
QED and the men who made it: Dyson, Feynman, Schwinger, and Tomonaga by Silvan S. Schweber, Princeton University Press, 1994. Includes a chapter on Dirac as the founder of quantum electrodynamics.

External links

Dirac Medal of the International Centre for Theoretical Physics
Paul Adrien Maurice Dirac Biography
Dirac Medal of the World Association of Theoretically Oriented Chemists (WATOC)
Photographs of Dirac
The Paul Dirac Collection at Florida State University
The Paul A. M. Dirac Collection Finding Aid at Florida State University
discovery of new productive forms of atomic theory.
Annotated bibliography for Paul Dirac from the Alsos Digital Library for Nuclear Issues

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