Spinning a good scientific yarn

The Story of Spin
September 25, 1998

THE STORY OF SPIN. By Sin-itiro Tomonaga. University of Chicago Press, 258pp, Pounds 39.95. ISBN 0 226 80793 2

The Story of Spin by Nobel laureate Sin-itiro Tomonaga is a most enjoyable and instructive review of the development of quantum mechanics in the second quarter of the 20th century, focusing on the emergence of the concept of "spin", the intrinsic angular momentum of the electron and other elementary particles. It was originally written in Japanese in 1974 and it has had to wait for over 20 years for an English translation. For today's readers, who have learned about quantum mechanics largely from textbooks, this book provides a very accessible account of how experimental measurements guided theoretical developments (and vice-versa) towards our current understanding of this fundamental subject, which plays a central role in so much of modern science and technology. The author's evident excitement and enthusiasm, even after so many years since the developments he describes occurred, are contagious and stimulating.

Tomonaga (1906-1979) received the 1965 Nobel prize in physics, together with Julian Schwinger and Richard Feynman, "for their fundamental work in quantum electrodynamics (QED), with deep-ploughing consequences for the physics of elementary particles". In the presentation speech, Ivar Waller cites Tomonaga's contribution to the development of what we now know as renormalisation, the cancellation of infinities in QED predictions for experimental quantities (such as the Lamb shift) when they are expressed in terms of the physical charges and masses. Much of what is described occurred during the author's early career, so that he not only contributed to and experienced many of the scientific developments, but he also knew many of the other physicists. For example, he was an undergraduate with Hideki Yukawa at the Kyoto Imperial University in the late 1920s and spent two years in Leipzig from 1937-39 in the theoretical group of Werner Heisenberg. Detailed descriptions of scientific progress are interwoven with stories concerning human aspects of scientific research and the book is richer for this.

Since it was written in 1974, the book naturally cannot describe more recent developments. But even after more than 70 years "spin physics'' is still at the forefront of fundamental scientific research. For example, the European Muon Collaboration discovered in 1987 that only a small fraction of the spin of the proton is due to the spins of the quarks. This has opened up an active area of investigation trying to understand how the spins of the proton and other elementary particles arise from combining the spins and orbital angular momenta of their constituent quarks and gluons, and how the spins are passed on from parent to daughter particles in scattering and decay processes. Our understanding of this subject is very unsatisfactory. Another exciting recent development in which the spins of elementary particles play a key role is the emergence of the subject of "quantum information". It is still too early to be confident that some of the optimistic expectations of the eventual implications for quantum computation, cryptography and even teleportation can be realised, but it is likely that we are at the birth of a new class of technologies in which fundamental properties of quantum mechanics (and spin in particular) will be key ingredients. "Spin physics" is very much alive.

Although probably not suitable as the textbook for an introductory course in quantum mechanics, The Story of Spin should be readily accessible to all scientists who have already followed such a course. Its emphasis on the historical development of the subject will provide much insight into the frustrations and excitement of scientific research. This is well illustrated in the second chapter, which describes how:

* Alfred Lande was forced to abandon his model for the level splitting in alkali atoms, which was based on the interaction between the magnetic moments of the atomic "core" and the "radiant electron", after finding that the predictions disagreed with the experimental data

* In order to explain the data Wolfgang Pauli postulated a "classically indescribable two-valuedness to the electron''

* This two-valuedness was interpreted in terms of a self-rotating electron, with angular momentum of self-rotation of 1/2-h, by Ralph Kronig and the following year by George Uhlenbeck and Samuel Goudsmit. Kronig did not publish his idea because of a factor of 2 discrepancy between the prediction and data (and difficulties in the theoretical interpretation) while Uhlenbeck and Goudsmit tried to withdraw their paper from publication but were too late

* Llewellyn Thomas explained the discrepancy of the factor of 2, and gradually the idea of the self-rotating electron was accepted.

This is just one example from several in the book, demonstrating that progress in scientific understanding is not always straightforward and obvious.

The material is presented as a series of 12 lectures, starting from the early attempts to understand the details of atomic spectra and proceeding to the emergence of the quantum mechanical explanation of the electron's spin, to the discovery of the proton's spin, to the derivation of the relation between spin and statistics and finally to the early days of nuclear physics and to the concept of iso-spin, mathematically similar to spin, but which is not due to space-time properties of particles. Although written largely from a historical perspective, the scientific formalism is presented in substantial detail, so student readers will also consolidate and expand their understanding of quantum mechanics. One chapter, for example, which is intriguingly entitled "The quantity which is neither vector nor tensor", provides an excellent introduction to covariance, to vectors and tensors and to what Ehrenfest calls "the mysterious tribe by the name of the spinor family".

Our understanding of quantum mechanics appears to provide us with a reliable "recipe" for predicting and interpreting a wide variety of physical processes. It is, nevertheless, incomplete and frequently unintuitive and therefore unsatisfactory. This stimulating account of a particularly exciting period in the development of fundamental physics should serve as an inspiration to young scientists.

Christopher Sachrajda is professor of physics, University of Southampton.

The Story of Spin

Author - Sin-itiro Tomonaga
ISBN - 0 226 80793 2
Publisher - University of Chicago Press
Price - £ 39.95
Pages - 258

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