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The Restless universe
Introduction to The restless Universe

1 The lawful Universe

2 The clockwork Universe

3 The irreversible Universe

4 The intangible Universe

5 The uncertain Universe

6 Closing items

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Other titles in the Physical World series

Describing motion

Predicting motion

Classical physics of matter

Static fields and potentials

Dynamic fields and waves

Quantum physics: an introduction

Quantum physics of matter

Featured Physicists

Paul Adrien Maurice Dirac (1902-1984)

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Paul Adrien Maurice Dirac was born in Bristol, England, in 1902. His father was a Swiss-born teacher of French, his mother a librarian. Dirac's first degree, obtained at the Merchant Venturer's Technical College, was in electrical engineering, but he had no real interest in the subject and after graduating spent two years studying mathematics at the University of Bristol. In 1923 he left Bristol for Cambridge where he remained for most of his working life.
figure 1.31, Paul DiracFigure 1.31
Paul Dirac
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Dirac's achievements in Cambridge were prodigious. In 1925, while working for his doctorate, he became one of the founders of quantum mechanics when he produced an elegant extension of Heisenberg's work. A little over a year later he presented a very general formulation of quantum mechanics that has remained the basis of the subject ever since. During the next year he essentially founded quantum electrodynamics. In 1928 Dirac took an important step towards bringing quantum physics into conformity with Einstein's special theory of relativity by devising an equation (now called the Dirac equation) that could describe the behaviour of electrons at any speed up to the speed of light. This equation provided a natural explanation of one of the electron's intrinsic properties - its spin. Taking the mathematical form of his equation seriously, and searching for a way of interpreting it, Dirac was led, in 1931, to propose that there should exist a class of 'anti-electrons', particles with the same mass and spin as the electron but with the opposite electrical charge (Figure 1.32). By correctly predicting the existence of these antiparticles, now called positrons, Dirac became recognized as the 'discoverer' of antimatter - one of the most important discoveries of the century.
Figure 1.32 Tracks left by fundamental particles.
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figure 1.32a, An electron and a positron
(a) An electron and a positron (a particle-antiparticle pair) reveal their opposite charges by spiralling in different directions in a magnetic field.
figure 1.32a, particles created from the energy released when an electron and a positron collide
(b) A variety of particles created from the energy released when an electron and a positron collide at high speed and annihilate.
From 1932 to 1969 Dirac held the Lucasian Chair of Mathematics in Cambridge, the post that Newton himself had once occupied. During this period Dirac worked on a variety of topics including magnetic monopoles (hypothetical magnetic charges) and the speculation that the fundamental constants of physics might be gradually changing in a co-ordinated way. However he became disenchanted with some of the detailed developments that occurred in quantum field theory and became increasingly distanced from what others regarded as the scientific mainstream.

In 1971, following his retirement from Cambridge, Dirac moved to the USA where he became a professor of physics at Florida State University. He died there in 1984. Throughout his life Dirac was renowned for his economy of speech and lack of social awareness. His book Principles of Quantum Mechanics (1930) is regarded as a classic of clear and elegant exposition. When a correspondent asked him to clarify a certain result in the text, Dirac is said to have replied that he knew of no clearer way of expressing the point. No rudeness would have been intended, just an honest statement of fact. Dirac preferred to work by himself, and had few collaborators or research students.

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S207 The Physical World
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