Paul Dirac

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Paul Adrien Maurice Dirac

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 Cambridge, where he discovered the Dirac equation.

Biography

Early years

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

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

Death and afterwards

Views

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. [link]

• 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.

See also

• Dirac sea
• Dirac notation
• Dirac equation
• Dirac delta function, Dirac comb
• Dirac large numbers hypothesis
• Dirac Prize
• Fermi-Dirac statistics
• Quantum Mechanics
• Gabriel Andrew Dirac

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