Paul Dirac

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**Paul Dirac**
Paul Adrien Maurice Dirac
Born	August 8, 1902 Bristol, England
Died	October 20, 1984 Tallahassee, Florida, USA
Residence	UK USA
Nationality	English- Swiss (to 1919) English (after 1919)
Field	Physicist
Institution	Cambridge University Florida State University
Alma Mater	University of Bristol Cambridge University
Doctoral Advisor	Ralph Fowler
Doctoral Students	Homi Bhabha Harish Chandra Mehrotra Dennis Sciama Behram Kurşunoğlu John Polkinghorne Per-Olov Löwdin
Known for	Quantum physics
Notable Prizes	Nobel Prize in Physics (1933)
Religion	Atheist

Paul Adrien Maurice Dirac, OM, FRS (IPA: [dɪ'ræk]) (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 the University of Cambridge. Among other key discoveries, he formulated the so-called "Dirac equation," which describes the behavior of fermions and which led to the prediction of the existence of antimatter. Dirac shared the Nobel Prize in physics for 1933 with Erwin Schrödinger, "for the discovery of new productive forms of atomic theory."

1 Biography
2 Honours and tributes
3 Personality
4 Selected bibliography
5 See also
6 References
7 External links

[edit] Biography

[edit] 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.

[edit] 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 wavefunction of 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.

The necessity of electron matter being created and destroyed in Enrico Fermi's 1934 theory of beta decay, however, led to a reinterpretation of Dirac's equation as a "classical" field equation for any point matter of spin ħ/2, itself subject to quantization conditions involving anti-commutators. Thus reinterpreted, 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 1930s.

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, which are now universally used.

Guided by a comment in Dirac's textbook and by Dirac's 1933 article "The Lagrangian in quantum mechanics" (published in the Soviet journal Physikalische Zeitschrift der Sowjet Union), 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 it is based on the Lagrangian, whose relativistic invariance is explicit, while the invariance is only implicit 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.

He married Eugene Wigner's sister, Margit, in 1937. He adopted Margit's two children, Judith and Gabriel. Paul and Margit Dirac had two natural children together, daughters Mary Elizabeth and Florence Monica.

[edit] 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 would write, "I am very disturbed by the situation because the so-called good theory [quantum theory] does involve neglecting infinities in an arbitrary way.This is not sensible. Sensible Mathematics involves neglecting a quantity when it's small not because it's infinitely great and we do not want it" ^[1] Dirac became unsatisfied with the renormalization approach to dealing with these 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 ten years (both of life and as a physicist) at Florida State University (FSU) in Tallahassee, Florida.

Amongst his many students was John Polkinghorne who recalls that Dirac, "was once asked what was his fundamental belief. he strode to a blackboard and wrote that the laws of nature should be expressed in beautiful equations"^[2]

[edit] Death and afterwards

In 1984 Dirac died in Tallahassee, Florida 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, Switzerland, the town of origin of his father's family.

[edit] Honours and tributes

Paul Dirac shared the 1933 Nobel Prize for physics in with Erwin Schrödinger "for the discovery of new productive forms of atomic theory." ^{[citation needed]} Dirac was also awarded the Royal Medal in 1939 and both the Copley Medal and the Max Planck Medal in 1952.

He was elected a Fellow of the Royal Society in 1930, and of the American Physical Society in 1948.

Immediately after his death, two organizations of professional physicists established annual awards in Dirac's memory. The Institute of Physics, the United Kingdom's professional body for physicists, awards the Paul Dirac Medal and Prize for "outstanding contributions to theoretical (including mathematical and computational) physics". The first three recipients were Stephen Hawking (1987), John Bell (1988), and Roger Penrose (1989). The Abdus Salam International Centre for Theoretical Physics (ICTP) awards the Dirac Medal of the ICTP each year on Dirac's birthday (August 8).

The street on which the National High Magnetic Field Laboratory in Tallahassee, Florida, is located was named Paul Dirac Drive. The BBC named its video codec Dirac in his honour. And in the popular British television show Doctor Who, the character Adric was named after him (Adric is an anagram of Dirac).

[edit] Personality

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 replied, "I was taught at school never to start a sentence without knowing the end of it." ^{[citation needed]} 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". ^{[citation needed]} When asked about his views on poetry, he responded, "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". ^{[citation needed]}

Eugene Wigner, Dirac's brother-in-law, once described Richard Feynman as "another Dirac, only this time human". ^{[citation needed]}

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". ^{[citation needed]}

Wolfgang Pauli, when asked about Dirac's religious views, replied, "If I understand Dirac correctly, his meaning is this: there is no God, and Dirac is his Prophet," ^{[citation needed]} a wry reference to the Islamic profession of faith.

Dirac once "re-invented" the purling stitch in knitting. While speaking with a collegue about physics, he saw the colleague's wife knitting. Afterward, he considered the topology involved and realized that there was another way to do it. He also concluded that the stitch that he saw and the alternative that he conceived were the only stitches possible.^{[citation needed]}

[edit] 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.

[edit] See also

[edit] 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.
"Paul Adrien Maurice Dirac"