Dirac, Paul

NAME:  Dirac, Paul Adrien Maurice

ACHIEVEMENT(S):  Involved in the development of quantum theory, and the

reconciliation of relativity with quantum theory resulting in the

formulation of the relativistic wave theory for the electron

NICKNAME/ALIAS:  None

DATE AND PLACE OF BIRTH:  August 8, 1902, Bristol, Gloucestershire,

England

DATE/PLACE OF DEATH/AGE AT DEATH:  October 20, 1984, Tallahassee,

FL.  Aged 82

DISCIPLINE: Quantum mechanics

FIELD:  Physicist

LANGUAGE:  English

PARENTS:  Charles Adrien Ladislas Dirac; Florence Hannah Holten

SIBLINGS:  1 brother, 1 sister

SPOUSE:  Margit Wigner

CHILDREN:  Mary Elizabeth; Florence Monica

EDUCATION:  Merchant Venturers' Technical College, Bristol, England;

University of Bristol;  University of Cambridge, England

MAJOR WORK: “Principles of Quantum Mechanics”

LIFE AND TIMES:

In 1925, Dirac attended lectures by Bohr* and German physicist Heisenberg*, who were visiting Cambridge, and later that year,  received a proof copy of Heisenberg's seminal paper on quantum mechanics (the laws of motion that govern atomic particles). 

The paper contained an equation, not fully understood by Dirac at the time.  [In ordinary mathematics, two quantities, x and y can be multiplied to form xy or yx -- then xy or yx are the same, and the quantities are said to commute; quantum mechanics deals with quantities (or objects) where xy is different from yx, and the difference (xy - yx) is called the commutator of the two quantities] 

After several weeks’ work, Dirac was able to demonstrate a fundamental link between commutators in quantum mechanics and certain expressions  in classical mechanics. This understanding led Dirac to an independent and highly original form of (and approach to) quantum mechanics (different from the “matrix mechanics” developed by Heisenberg and the “wave mechanics” developed by Erwin Schrodinger*).  

In 1926, Cambridge awarded him his Ph.D. for his work and world scientists took note of him.  

In late 1926, he travelled to the Institute  of Theoretical Physics at Copenhagen, where he consulted with Bohr, and in early 1927, visited the University of Gottingen, where he met physicists Born*, Oppenheimer*, Pauli* and Heisenberg, with whom he had already established a correspondence.

In Copenhagen in 1926, Dirac derived the quantum mechanical properties of a collection of particles, such as electrons, that obey the Pauli exclusion principle:  i.e. no two identical particles may occupy the same state.  Dirac found that these particles also obeyed statistical rules different from other particles.  Unknown to Dirac these rules had been obtained slightly earlier by Italian physicist Enrico Fermi*,but Dirac went on to show the connection between these statistical properties and the antisymmetry of the wave function in quantum mechanics.

These later became known as the Fermi-Dirac statistics, which are of great value in nuclear and solid-state physics and are used,  foe example, to determine the distribution of electrons at different energy levels.

In 1927, Dirac developed his “transformation theory”, which showed that the matrix  mechanics of Heisenberg and the wave mechanics of Schrodinger were not at odds with each other or with his own formulations, but were, in fact, special cases of his own more general theory.  

The “transformation theory” was Dirac’s own favorite piece of work. Back in England from Gottingen in late 1927, Dirac returned to  St.John's College at Cambridge, where he had been offered a fellowship. That winter, he made his famous improvement on Schrodinger's wave equation, which combined quantum mechanics with Einstein’s* special theory of relativity.  

Five years earlier, de Broglie* had proposed a model of the electron which said it also had wave-like properties  (a major break from the Bohr model of the atom).  Schrodinger then extended de Broglie's work by considering the movement of a particle in an electromagnetic field -- thereby establishing the subject now known as wave mechanics -- and developed his wave equation, which sought to explain the behavior of an electron in an atom.  However, Schrodinger chose to ignore relativistic effects in his calculations (as did Heisenberg, who also developed a non-relativistic model of the electron).  

Dirac’s approach was to begin with only the simplest information known about the electron -- its mass and charge -- and to develop a mathematical theory that would describe the electron's properties more fully than Schrodinger's theory.  The result, the Dirac equation, described the behavior of a single electron, and was completely consistent with relativity. 

His model described many quantitative aspects of the electron, including properties that were entirely new.  These included the half-quantum spin and magnetic charge of the electron, and other properties, as measured in experimental work. 

In recognition of their work on the wave equation, Dirac and Schrodinger shared the 1933 Nobel Prize in physics.

Dirac's analyses of electrons led to his hypothesis of the positive electron.  Using his electron equations, he discovered that the electron could have either positive or negative energy.  His theory also suggested experiments that a researcher might do to seek the positive electron, examining situations in which positively-charged electrons would be produced (always in connection with negatively-charged electrons and always in such a way that the two would annihilate each other).

Exactly these conditions were noted in 1932, when American physicist Carl David Anderson first observed the positive electron, or, as it was named later, the “positron." By extension, Dirac's theory also suggested the existence of other forms of antimatter, including the antiproton and antineutron. Those predictions were eventually confirmed, but not until 1955.  

In 1929, Dirac was appointed lecturer at Cambridge, and made his first of many visits to the United States.  It was a five-month trip during which he taught at the University of Michigan and the University of Wisconsin.  He continued westward, travelling around the world by way of Japan and Siberia.  Heisenberg accompanied him part of the way. Upon his return to England Dirac was elected to the Royal Society and published his seminal text, “The Principles of Quantum Mechanics”. 

In 1932, he was chosen for the post of Lucasian Professor of Mathematics at Cambridge.  In 1933, he was awarded the Nobel Prize. In 1934, Dirac returned to the United States, spending the following academic year  at the Institute for Advanced Studies at Princeton University. 

During the Second World War, he remained at Cambridge, but became involved in a number of government projects related to the development of atomic energy.  He was especially interested in techniques for the separation of uranium isotopes by diffusion and centrifuge processes, although none of his suggestions was specifically incorporated into later weapons development programs.

In 1969, Dirac retired from his post at Cambridge and moved to the United States.  After a brief stay at the Center for Theoretical Studies of the University of Miami, he was appointed professor of physics  at Florida State University in 1972.

ASSESSMENT:  One of the 20th century's leading theoretical physicists

EXTRA  CONNECTIONS:  His work was influenced by that of Rutherford*

His work related to that of Planck*

ONELINER: Paul Dirac, major figure in quantum physics

BIBLIOGRAPHY:

EB, DSB, DNB

Collier's Encyclopedia.

Kragh, Helge, Dirac: A Scientific Biography, Oxford & New York:  OUP,1990

Newton, David E., "Paul Dirac", in Notable Twentieth Century Scientists, ed. Emily J. McMurray, New York & London: Gale  Research Inc., 1995