Quantum theory at the Crossroads: Reconsidering the 1927 Solvay Conference ,

Cambridge U. Press
New York
, 2009. $126.00 (530 pp.). ISBN 978-0-521-81421-8

According to standard accounts, the 1927 Solvay conference represents the pinnacle of the debate over the interpretation of quantum mechanics—a debate that was launched after the developments of matrix and wave mechanics. After the conference the quantum mechanics community split into two camps: those who adhered to Niels Bohr and Werner Heisenberg’s position known as the Copenhagen Interpretation — which, among other things, describes what happens when an observer attempts to make a measurement—and those like Albert Einstein and Erwin Schrödinger who were not willing to accept that “pillow of complementarity.”

Recent scholarly efforts by Mara Beller, Don Howard, and Kristian Camilleri have considerably revised this “myth of the Copenhagen Interpretation.” With Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay Conference, philosopher Guido Bacciagaluppi and theoretical physicist Antony Valentini add an essential piece to this revision. As they see it, Solvay was not the event that settled the dispute, even among those who belonged to the Copenhagen School, but rather the moment of maximal dissent.

Although the book takes aim at this revision and includes the first English translation of the conference proceedings, it is not meant to be a purely historical study of the proceedings; according to the authors, the main purpose of Quantum Theory at the Crossroads is to render the historical arguments advanced at Solvay accessible to today’s physicists and thereby to “contribute to a revival of sharp and informed debate on the meaning of quantum theory.”

Quantum Theory at the Crossroads is divided into three parts: a presentation of the early history of quantum mechanics, a discussion of the ideas on foundations of quantum mechanics presented at Solvay, and the aforementioned translation of the proceedings. As a collection of primary sources, the book is indisputably of great importance for historians and philosophers of science, especially because its editors have made careful and transparent translations of ambiguous terms.

In the historical section, the authors identify speakers on the theoretical side of quantum mechanics: Louis de Broglie, Heisenberg and Max Born, and Schrödinger as the proponents of three major formulations of quantum mechanics, each of which had a different research program and different interpretational agendas. The authors characterize the development of those approaches and discuss how they were presented at the Solvay conference. To accommodate physicists, the book’s primary target audience, formulas are often re-expressed in the language of modern quantum mechanics.

The authors discuss the development of matrix and wave mechanics and how the interactions among Born, Heisenberg, and Schrödinger from 1926 to 1927 shaped those physicists’ interpretations. Although the book does not discuss the uncertainty principle or complementarity, it nonetheless provides a good introduction to the historical development of the two ideas and to the secondary literature. Bacciagaluppi and Valentini mostly focus on de Broglie’s pilot-wave theory, tracing the minute details of its development from 1923 to 1927. They argue that de Broglie’s theory was more systematic than the often reported tinkering with formulas and stumbling upon matter waves. Hence, historical studies that dismiss the work of de Broglie and the French and Dutch communities do not cover the whole story.

As it relates to the development of quantum mechanics, Quantum Theory at the Crossroads takes a very stimulating point of view. First, by their focus on the reasons de Broglie’s theory was disregarded, Bacciagaluppi and Valentini present the interpretational debate as the establishment of an orthodox interpretation. However, de Broglie’s case offers another perspective that may be more fruitful: unlike wave and matrix mechanics, the pilot-wave theory did not play a role in shaping quantum mechanics because it was not applied further to a wide range of phenomena.

Second, to limit the scope of their 530-page book, the authors reasonably chose to focus on the speakers at Solvay. However, it would be incorrect for the reader to assume that the position a speaker took as proponent of a particular research program was the full story; even within a research group, members held different ideas and opinions. Finally, I agree with the authors’ assertion that historical studies can be used as a basis for reflection on current ideas, but I believe one has to compare historical and present ideas in their respective conceptual frameworks. Important similarities and differences are lost when historical arguments are transposed into a modern formalism.

Overall, Quantum Theory at the Crossroads provides an important and inspiring history of quantum mechanics, being most valuable for its translation of the conference proceedings and its account of de Broglie’s often neglected wave theory.

Martin Jähnert is completing his doctoral research on the history and foundations of quantum physics at the Max Planck Institute for the History of Science in Berlin.