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First edition, complete journal issue in original printed wrappers, of the second of Dirac's two 1927 papers (the first being 'The quantum theory of the emission and absorption of radiation') in which Dirac "laid the foundations of quantum electrodynamics" (Pais, p. 7). In this paper, "Dirac presented a complete theory of dispersion [the scattering of a photon by an electron], including derivations of the Kramers-Heisenberg formula and the Thomson formula for scattering of radiation by atoms. He was also able to treat the case of resonance, which theretofore had eluded quantum radiation theory" (Kragh, p. 125). It was also in this paper that the divergences which were to plague the development of quantum electrodynamics were first encountered. "Dirac's publications on quantum electrodynamics in 1927 completed the scheme of quantum mechanics. At the same time, they initiated a new field of research that soon was to move to the forefront of theoretical physics" (Kragh, p. 127). "Soon after arriving in Göttingen in February 1927, Dirac turned his attention to using field theory to understand what happens when light is scattered by an atom, normally visualized as being rather like a basketball bouncing off the hard rim of the basket. But, in the new field theory, things are not so straightforward. Dirac showed that, in the fleeting moment of a photon's scattering, it appears to pass through some strange, unobserved energy states. What makes these intermediate processes so odd is that they appear to flout the sacred law of conservation of energy. Although these subatomic virtual states cannot be seen directly, experimenters were later able to detect their subtle influences on fundamental particles. Dirac's calculations also threw up a more troubling artifact. He found that his new theory kept generating bizarre predictions: for example, when he calculated the probability that a photon had been emitted after a given interval, the answer was not an ordinary number but was infinitely large. This made no sense. The probability that an atom would emit a photon must surely be a number between zero (no chance) and one (complete certainty), so it seemed obvious that the prediction of infinity was wrong. But Dirac chose to be pragmatic. This difficulty is not due to any fundamental mistake in the theory, he wrote with more confidence than was warranted. The root of the problem, he speculated, was a simplistic assumption he had made in applying the theory; when he had identified his error and tweaked the theory, he implied, the problem would disappear. In the meantime, he dodged the difficulties using clever mathematical tricks, enabling him to use the theory to make sensible, finite predictions. But it would not be long before he saw that his optimism was misplaced: the lamb had caught its first sight of the wolf's tail" (Farmelo, p. 126). Farmelo, Dirac: A Scientific Biography, 1990; A. Pais, Paul Dirac: Aspects of his life and work, in Paul Dirac: The Man and his Work, ed. P. Goddard, 1998, pp. 1-45. 8vo, pp. 619-748, vi. Original printed wrappers.
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