Conundrums in Copenhagen

Creativity is inherently elusive. In science, its definition is beset by an intriguing interplay between the individual and the group. This challenge is taken up in Mara Beller's book with regard to the early development of quantum mechanics (QM), and in particular the evolution of QM's so-called "Copenhagen interpretation" by Niels Bohr and his colleagues.

The book is a bold attempt to analyse carefully the complex process by which the insights and predilections of individuals contributed to the emergence of a consensus. Of course, one may not agree with all the conclusions. Nevertheless, Quantum Dialogue is an intellectually stimulating piece of work, energised by a distinct point of view that should appeal to physicists and philosophers alike.

The central theme is what Beller calls "dialogical flux": a dialectical process of "creative" conversations seeking to resolve tension between conflicting approaches by synthesis. That the evolution of the Copenhagen interpretation was agitated by such a flux of dialogues is brought out emphatically. The account results from a painstaking, comprehensive study of primary sources: correspondence, notebooks and, of course, original scientific papers.

The first seven chapters roughly cover the period 1925-35, during which both matrix and wave-mechanical versions of QM, though conceived independently, turned out to be equivalent representations giving rise to Heisenberg's uncertainty relation and Bohr's complementarity. Beller argues that the uncertainty relation and complementarity appeared not so much because of a prior commitment either to indeterminism or positivism, but rather as a response to Schrodinger's wave mechanics and to ensure consistency with some crucial experiments. Complementarity then sparked off the Bohr-Einstein debates, which culminated in the Einstein-Podolsky-Rosen (EPR) paper of 1935 challenging the completeness of QM.

The high point of the analysis is the way Beller shows how the Copenhagen school shifted its focus from arguing for consistency to arguing for inevitability. This transformation was prompted by Einstein changing his position from finding a contradiction with the uncertainty relation to showing that the QM description of the state of a system was incomplete. Beller illustrates the shift by pointing out differences between Bohr's reply to the 1935 EPR paper and his 1928 Como lecture. His response to EPR was particularly apocalyptic, with its strong statements such as that there was "no question of any unambiguous interpretation of the symbols of quantum mechanics other than that embodied in the well-known rules".

Prior to EPR, concepts such as uncertainty and complementarity were attributed essentially to the uncontrollable and inevitable "disturbance" caused by a measurement. This notion of "disturbance" was crucial to the way in which EPR defined both "elements of reality" and the "locality condition". As Beller correctly points out, Bohr cleverly avoided questioning the locality condition. Instead he introduced the "wholeness" of the composite system, comprising the spatially separated parts together with the measuring set-up. This led to what Bohr criticised as an "ambiguity" in the EPR argument that hinged on inferring a value "without disturbing the system in question". However, the way a measurement on one sub-system specifies the experimental context of the other remained ambiguous in Bohr's reply. This was reflected in his introduction by fiat of the "unanalysability" of measurement interactions, a strange notion that restricts the universality of QM.

Beller quite clearly brings out the above weakness in Bohr's response to EPR. The book stresses that in the post-EPR period Bohr's mode of argumentation increasingly followed Louis Pasteur's maxim for a persuasive strategy: "Make it look inevitable." Bohr asserted as a "simple logical demand" the "indispensability" of the classical description of an apparatus, while leaving the boundary between quantum and classical behaviour unspecified. Interestingly, soon after his seminal 1925 paper, Heisenberg had become aware of the problem of reconciling quantum mechanics with the everyday macroscopic world. Ironically, this classical-limit problem remains one of the unresolved issues in QM.

As an example of the usefulness of "dialogical analysis", Beller undertakes a comparative study of Heisenberg's "seemingly confident" published paper suggesting the uncertainty relation and a draft he sent in a letter to Pauli. This brings out hidden ambiguities in the published paper. Another application of the method is Beller's decipherment of apparently obscure texts like Bohr's Como lecture. Beller seeks to demystify it by viewing it as a juxtaposition of several arguments directed towards different issues raised by Bohr's colleagues. Internal divergences within the Copenhagen school were skilfully smoothed over and an apparently monolithic structure with a sense of "finality" presented to the physics community as a whole. Beller discusses in depth, within this framework of "dialogical flux", how these rhetorical strategies adopted by Bohr and his colleagues worked to achieve triumph for the Copenhagen interpretation.

However, the entire story of alternative later approaches to interpreting QM, such as David Bohm's scheme and dynamical models of collapse, is omitted by Beller, because of the book's focus on the 1920s and 1930s. Seventy-five years on, foundational issues of QM are presently undergoing an extensive reappraisal. The role of "dialogical flux" in the current scenario needs its own critical study.

The 19th-century linguist Max Muller once observed: "I believe we both care far more for what is right than who is right ... Facts and correct deductions from facts are all we ought to care for." How hard the maintaining of this ethos is when personal prejudices and individual egos influence the making of a theory is the most important lesson conveyed by Quantum Dialogue .

Dipankar Home is professor of physics, Bose Institute, Calcutta, India.