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A Relative Success MILESTONES Image courtesy of Lida Lopes Cardozo Kindersley. Cardozo Lopes Lida of courtesy Image M iles Tone 4 DOI: 10.1038/nphys859 A relative success The idea of the spinning electron, beyond position and momentum they were the two spin states of the as proposed by Samuel Goudsmit in the physical description of the electron. But the other two solutions and George Uhlenbeck in 1925 electron. Inspection revealed them to seemed to require particles exactly (Milestone 3), and incorporated into be extensions of the two-dimensional like electrons, but with a positive the formalism of quantum mechanics spin matrices introduced by Pauli in charge. by Wolfgang Pauli, was a solution his earlier ad hoc treatment. Applied Dirac did not immediately and of expediency. Yet this contrivance to an electron in an electromagnetic explicitly state the now-obvious threw up a more fundamental ques- field, the new formalism delivered conclusion — out of “pure cowardice”, tion: as a 25-year-old postdoctoral the exact value of the magnetic he explained later. But when, in 1932, fellow at the University of Cambridge moment assumed in the spinning Carl Anderson confirmed the exist- formulated the problem in 1928, why electron model. ence of the positron, Dirac’s fame was should nature have chosen this par- What had emerged was an equa- assured. He shared the 1933 Nobel ticular model for the electron, instead tion that, in its author’s words, Prize in Physics — its second‑ of being satisfied with a point charge? “governs most of physics and the youngest-ever recipient — and his The young postdoc’s name whole of chemistry”. Dirac was a equation went on to become the bed- was Paul Dirac, and in two papers famously modest man, and was not rock of quantum electrodynamics, published in the Proceedings of the wont to exaggerate. The Dirac equa- the quantum field theory of the elec- Royal Society of London, he set out to tion is still today the best description tromagnetic interaction. Following explain why. Using the clear, austere not just of the electron, but of all his death in 1984, a stone was set prose and adroit mathematics that spin-1/2 particles — including all the into the floor of Westminster Abbey were his hallmarks, he showed how quarks and leptons from which in London. It was inscribed with spin emerged as a natural conse- matter is made. his name and iγ · дΨ = mΨ — the quence of the correct application When asked what had led him shortest and sweetest rendering of his of special relativity to the quantum to his formula, Dirac replied simply extraordinary brainchild. mechanics of the electron. “I found it beautiful”. His equation Richard Webb, Senior Editor, Dirac was able to remove the is indeed a powerful example of the Nature News & Views nonlinearities in space and time deep and mysterious connection derivatives that had confounded between the language of mathematics ORIGINAL RESEARCH PAPERs Dirac, P. A. other attempts to marry those two and the expressions of the physical M. The quantum theory of the electron. Proc. great new physical theories. The world. R. Soc. Lond. A 117, 610–624 (1928); ibid. 118, 351–361 (1928) corollary of his logic was that the Yet, however much beauty might FUrTher reading Dirac, P. A. M. The wavefunction of the electron must be indicative of rightness, a physical Principles of Quantum Mechanics (Int. Ser. Monograph. Phys. 27) 4th edn (Oxford Univ. have four components, and must be theory is judged on its predictive Press, Oxford, UK, 1982) | Pais, A., Jacob, M., operated on by four-dimensional power. The Dirac equation did not Olive, D. I. & Atiyah, M. F. Paul Dirac: The matrices. These matrices required disappoint. The interpretation of Man and his Work (Cambridge Univ. Press, Cambridge, UK, 1998) an additional degree of freedom two of its four solutions was clear: natuRE MILESTONES | SPIN maRch 2008 © 2008 Nature Publishing Group .
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