# New Books of Note

### Heisenberg and the Interpretation of Quantum Mechanics: The Physicist as Philosopher

**By Kristian Camilleri,** Cambridge University Press, 2009, 199 pp., $81.00

### Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay Conference

**By Guido Bacciagaluppi and Antony Valentini**, Cambridge University Press, 2009, 532 pp., $126.00

**Reviewed by Cathryn Carson**

Adepts of twentieth-century physics might be forgiven for thinking that the history of quantum mechanics is all wrapped up. After all, the origin of the quantum theory is one of the most celebrated stories in physics. The postulations of Planck and Einstein, the invention of Bohr's atomic model, the steady accumulation of spectral data, the breakthrough into quantum mechanics: this narrative has been with us since the new theory's authors put it into circulation. And yet there still seems to be infinite richness waiting to be discovered. A first wave of historical studies came in the 1960s through the 1980s, when the treasure troves of the archives were exploited and a round of in-depth interviews was analyzed and put to work. Now a new wave of studies is opening up new questions. This wave includes two significant books: one on Heisenberg's interpretative commitments by Kristian Camilleri; and one on the famous Solvay Conference on quantum theory by Guido Bacciagaluppi and Antony Valentini. Each takes us into the interpretative thickets of the new theory, with a spotlight on 1925-27, and each rewards its readers handsomely.

Camilleri, a historian and philosopher of science at Melbourne, starts from a thesis that has recently become attractive to historians and philosophers of science: There is no single coherent Copenhagen Interpretation—or at least there was none in the 1920s. Explicating Bohr's thought has occupied an army of interpreters, with less than satisfactory results. Camilleri's book *Heisenberg and the Interpretation of Quantum Mechanics* takes up another main architect of the Copenhagen Interpretation. Heisenberg's interpretative opinions have, strangely enough, garnered less critical attention. Camilleri carefully traces how his ideas unfolded, disentangling them from Bohr's. The result is a masterful and enriching account that does much to re-complicate the story of quantum mechanics and the uncertainty relations.

Camilleri works through Heisenberg's *oeuvre* largely chronologically, analyzing his published papers and supplementing them with his letters. This is useful for storytelling, and it also helps with analytical clarity. He makes the point stick that Heisenberg's views changed significantly. This is true in the larger sense, as Heisenberg moved from an early strategic operationalism (or positivism), via a rethinking of Immanuel Kant's critical philosophy, to something Camilleri describes as a late-Wittgensteinian “linguistic turn.” The same point about shifting views can be made at a finer level of detail, as Heisenberg's interpretative positions changed from 1925 to 1926, 1926 to 1927, 1927 to 1929, and so on.

While this emphasis on Heisenberg's changeable opinions could seem like punctiliousness, it is central to how Camilleri makes his case. His Heisenberg is not the late Heisenberg, committed to defending an interpretative common ground with Bohr. Rather, he is the working physicist of the 1920s engaged with the details of technical problems. Camilleri neatly demonstrates how not just Heisenberg's views, but his approach, differed from Bohr's, giving substance to the familiar idea that Heisenberg started his thinking from the theory's mathematical formalism. Along the way, Camilleri shows how to understand important steps in Heisenberg's reasoning: his strategic use of observability, his understanding of the wave-particle duality and early matter waves, his “uncertainty” paper (which really deals with the possibility of quantum concepts) and the thought experiment on the gamma-ray microscope, and his take on Bohr's notion of complementarity.

Some of these arguments have already been advanced by other historians and philosophers. Camilleri adds important new insights and makes a case for the whole. He goes on to explore themes and topics that have received less attention, including Heisenberg's views on language, which may or may not be taken from Bohr. As an important source he points to Heisenberg's unpublished philosophical manuscript of 1942 (now translated into English, available at http://werner-heisenberg.unh.edu).

The result is a manageable-sized book that covers significant terrain. Camilleri does not address recent controversies over Heisenberg's role in the Third Reich, nor does he take up the whole course of Heisenberg's physical theorizing. Rather, we get a focused, in-depth account of important issues in his interpretative engagements. While the overall themes are of general interest, the details of the chapters are written for specialists, and the references to other historians and philosophers bear this out.

Bacciagaluppi and Valentini's *Quantum Theory at the Crossroads* also pushes us to rethink familiar tales. The focus is the monumental Fifth Solvay Conference in 1927, where the most important interpretative disputes over quantum mechanics were supposedly hashed out. The book is half a translation of the conference proceedings (originally issued in French, perhaps one reason it has not received adequate attention) and half a commentary on the scientific issues raised there. Both authors work on the foundations of physics, Bacciagaluppi as a philosopher, Valentini as a physicist, and their commentaries are scientifically well-informed and historically trenchant. They single-handedly retrieve the 1927 Solvay Conference from its undeserved fate, of always being gestured at but rarely understood.

Bacciagaluppi and Valentini make good on the claim that there was much more to the conference than what it is usually remembered for—the famous Bohr-Einstein debate. The mythology of that exchange, which took place outside the formal conference discussions, has crowded out nearly everything else. In fact, the conference's reports and discussions are a remarkable condensation of so much of what was stirring in quantum physics in these exceptionally rich years. The disputes of the day were physical, not just philosophical, as Bacciagaluppi and Valentini's commentary makes plain. And they were not resolved in the simple triumph of a Copenhagen Interpretation. Here the book's larger ambitions come into play.

Along with much else, the book is a brief for the intelligibility of interpretative positions that were marginalized after 1927. Louis de Broglie's pilot-wave theory is presented on equal footing with Born and Heisenberg's quantum mechanics and Schrödinger's wave mechanics—as it was, quite plainly, at the conference itself. Bacciagaluppi and Valentini put particular weight on the observation that de Broglie proposed a multiparticle theory, not just a one-electron one (as David Bohm later suggested) and that his picture was extensively discussed at the meeting (contrary to what de Broglie later recalled). They also argue that de Broglie had answers to at least some of the criticisms he encountered, so the rejection of the pilot-wave theory was not foreordained. The authors start from the position that there is no longer an established interpretation of quantum mechanics; the pilot wave theory is very much in their sights. All the same, their account is historically sensitive, thoroughly grounded, and conspicuously fair. If it is mainly written for people working on the foundations of quantum theory, its introductory material and some of its commentary will be useful for all interested general readers.

It is a real service that Camilleri has explicated Heisenberg's thinking with such attention and that Bacciagaluppi and Valentini have brought the 1927 Solvay Conference back to light. These books go a long way toward something that has only recently seemed plausible—disassembling the appearance of early consensus around the interpretation of quantum mechanics. Rooting interpretative debates in the physics and providing critical historical context, the two books ought to open up far more interest in the consolidation of quantum mechanics.

*Cathryn Carson is Associate Professor of History and Director of the Office for History of Science and Technology at the University of California, Berkeley. She is author of Heisenberg in the Atomic Age: Science and the Public Sphere (Cambridge, 2010).*

Note Added: This article represents the views of the author, which are not necessarily those of the FHP or APS.