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Monday, October 1, 2012

paul dirac

Paul Adrien Maurice Dirac was one of the twentieth century's leading theoretical physicists. Instrumental in developing quantum mechanics , the theoretical study of atomic structure and properties, and quantum electrodynamics, the study of the electrical interactions between atomic particles, Dirac also postulated the existence of the positron, a positive-charge electron, which led to later discoveries concerning antimatter. For his work on the development of quantum mechanics, Dirac shared the 1933 Nobel Prize in physics with Austrian physicist Erwin Schrödinger .
Dirac was born in Bristol, England, on August 8, 1902. His father, Charles Adrien Ladislas Dirac, was a Swiss immigrant, and his mother, Florence Hannah (Holten) Dirac, was British. Charles Dirac took a position as a teacher of French at the Merchant Venturer's Technical College, where young Dirac enrolled for his early schooling. After graduating in 1918, Dirac entered Bristol University, where he majored in electrical engineering and received his bachelor's degree in 1921. His hopes for employment after graduation were postponed, however, by Great Britain's postwar depression, and he decided instead to return to school, accepting a two-year scholarship from the department of mathematics at Bristol.
When Dirac's scholarship at Bristol came to an end, he was accepted at St. John's College, Cambridge, as a research student in mathematics. He soon became familiar with the latest developments in atomic theory , partly through his class work and partly by reading the works of Danish physicist Niels Henrik David Bohr , German physicist Max Born , German physicist Arnold Sommerfeld, English astronomer Arthur Stanley Eddington, and other leaders in the field. In addition, he had the opportunity to hear lectures by Bohr, German physicist Werner Karl Heisenberg , and others during their visits to Cambridge. Dirac's first research papers were published in 1925, while he was still a student. He received his Ph.D. in physics from Cambridge in 1926. His thesis involved an elaboration of quantum mechanical concepts that had originally been developed by Heisenberg.
In the fall of 1926, Dirac traveled to Copenhagen, where he spent much time talking with Bohr. In February 1927 he moved on to Göttingen, where he came into contact with Born, American physicist J. Robert Oppenheimer , American physicist James Franck, and Russian physicist Igor E. Tamm , among others. In late 1927, Dirac returned to England where he was elected a fellow at St. John's College, Cambridge. It was there during the winter of 1927-28 that he made an improvement on Schrödinger's wave equation.
Develops Wave Equation
Schrödinger's wave equation sought to explain the behavior of an electron in an atom, but Schrödinger chose to ignore relativistic effects in his calculations. Dirac's approach was to begin with only the simplest information known about the electron--its mass and charge--and devise a mathematical theory that would describe the electron's properties more fully than Schrödinger's equation did. He was successful in this effort, producing equations that accurately predicted the electron's spin, magnetic charge, and other properties, as measured in experimental work. In recognition of their work on the wave equation, Dirac and Schrödinger shared the 1933 Nobel Prize in physics.
One consequence of Dirac's mathematical analysis led to his hypothesis of the positive electron. In working with his equations for the electron, Dirac discovered that two solutions were possible, suggesting that the electron could either have positive or negative kinetic energy. Dirac's theory also suggested experiments a researcher might use to look for 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 later named, the positron.
By extension, Dirac's theory also suggested the existence of other forms of antimatter, including the antiproton and antineutron. Those predictions took much longer to be fulfilled, but were eventually confirmed. Today, considerable speculation exists as to the possibility of a whole universe of matter made out of antiprotons, positrons, and antineutrons, as well as to the ultimate consequences of the collision of such a universe with our own.
Appointments at Cambridge and Worldwide Travel after 1929
In 1929, Dirac was appointed to the post of university lecturer and praelector in mathematical physics at St. John's. The appointment carried with it very few specific duties, allowing him to spend as much time as he liked on research, writing, and travel, the latter of which had become a great passion for him. In fact, he spent five months abroad in 1929, during which time he taught briefly at the University of Michigan and the University of Wisconsin. At the end of this trip, he returned to England in 1930 by way of Japan and Siberia. Upon his return, he was elected to the Royal Society and, two years later, was chosen for the post of Lucasian Professor of Mathematics at Cambridge, a position once held by the seventeenth-century English physicist and mathematician Isaac Newton.
In 1934, Dirac returned to the United States, spending most of the 1934-35 academic year at the Institute for Advanced Studies at Princeton University. While there, he met his future wife, Margit ("Manci") Wigner, sister of the physicist Eugene Wigner. Dirac and Margit returned to Cambridge and were eventually married in London in January 1937. They had two daughters, Mary Elizabeth, born in 1940, and Florence Monica, born in 1942. Dirac remained at Cambridge during World War II, 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, although none of his suggestions was specifically incorporated into later weapons development programs.
During the last half century of his life, Dirac became increasingly interested in problems of cosmology , a branch of astronomy that is concerned with the origins of the universe. One of the first topics in this field to capture his attention was that of "large numbers." During the 1920s, Eddington had become intrigued by the realization that certain fundamental physical constants had relationships to each other that fall in the range of 1039 -40 . For example, the ratio of the gravitational attraction between an electron and proton to their electrostatic attraction is about 1040 :1, as is the ratio of the radius of the universe to the radius of the electron. Dirac argued that this recurring ratio was variable over time, a hypothesis not yet proven by experiment.
In 1969, Dirac retired from his post at Cambridge and moved to the United States. He stayed briefly at the Center for Theoretical Studies of the University of Miami before accepting an appointment as professor of physics at Florida State University in Tallahassee in 1972. He continued to travel, write, and speak during the next decade. After 1982, however, his health began to deteriorate, and he died in Tallahassee on October 20, 1984.