The Solvay Conferences are the preeminent series of lectures, colloquia, and meetings on Physics and Physical Chemistry held by the International Solvay Institute. The initial conference in 1911 was the first international congregation of minds in the history of science. Specifically, the first and also the fifth conferences in 1927 were seminal in the development of quantum theory. The discussions that took place at each of these events influenced the work of luminaries responsible for the quantum revolution in the 1920s and have had a lasting effect on physics today. The conferences allowed physicists who otherwise may have been working in isolation to network and début theories that may have gone unnoticed. The ideas and debates stimulated from these discussions facilitated the building of the framework which forever changed the course of physics and our understanding of Nature.

Belgian industrialist and philanthropist Ernest Solvay sponsored the endeavor and it was chaired by Dutch physicist Hendrik Lorentz, famous for discoveries such as the Lorentz transformations fundamental to the development of special relativity. The impetus for the first conference was the joint opinion of both Wilhelm Nernst and Max Planck who “considered that the current problems in the theory of radiation… had become so serious that an international meeting…should be convened in order to attempt to resolve the situation” (Bacciagaluppi & Valenti 4). Solvay and Lorentz then made it their goal to bring together in Brussels an international congress of the leading physicists of the time for a week-long discussion on such problems. This was the first conference of its kind in history.

The process of radiation was just recently hypothesized in 1900 by Planck, father of quantum theory. Based on his observations of radiating black-bodies, he postulated that electromagnetic energy can only be emitted discreetly in quanta. It is difficult to understate the importance of his theory which revolutionized the understanding of atomic process. Discreteness implies that nature is in fact discontinuous, a fact which startled physicists. This was the nascence of quantum theory which would mature in the hands of nearly every attendee of the conferences to be discussed. Planck’s theory of quanta had wide ranging effects on his colleagues. It immediately influenced Albert Einstein in his 1905 discovery of the law of the Photoelectric Effect, settling a classic debate on the nature of light. Einstein declared that light exhibits properties of both a particle and a wave, thus becoming known as the wave-particle duality, a major stepping stone towards quantum theory. It then became apparent to Planck, Nernst, Lorentz and Solvay in 1910 that the theory of radiation needed to be discussed. This led to the invitation of twenty-four eminent physicists to the first Solvay conference taking place from 29 October to 4 November 1911. Many had found it to be an informative event and as Bohr commented, “the reports and discussions…were most illuminating” (Bohr). However, the conference reflected the difficulties arising from a burgeoning revolution. Lindemann afterwards wrote to his father that “the discussions were most interesting but the result is that we seem to be getting deeper into the mire than ever. On every side there seem to be contradictions” (Beenakker).

The fifth conference on Electrons and Photons convened 24-29 October 1927. Among the twenty-nine invitees, seventeen either had received, or would go on to receive a Nobel Prize. It was the first conference to discuss the quantum theory of matter as well as three newly developed lines of research into quantum theory: de Broglie’s pilot-wave theory, Bohr and Heisenberg’s quantum mechanics, and Schrödinger’s wave mechanics. The new theories were directly influenced by Einstein’s wave-particle duality of light, and its recent extension to electrons in the quantum theory of matter developed by Louis de Broglie in his 1924 Ph.D. thesis. Now the duality included not only light, but matter as well; another central component of quantum theory. This is the era where physics departed from classical mechanics. Einstein explains this in his *Evolution of Physics*: “Quantum physics abandons individual laws of elementary particles and states directly the *statistical* laws governing aggregations. It is impossible, on the basis of quantum physics, to describe positions and velocities of an elementary particle or to predict its future path as in classical physics” (Einstein & Infeld 301-302). Various interpretations on this statistical character had been brewing, the culmination of which took place at this conference. These consisted of different mathematical formalisms and ontologies. Schrödinger mathematically described how a quantum state (wave function) evolves through time through the Schrödinger Equation. The other two theories utilized his equation in their interpretations of what mechanism leads to the probabilities of locating the potential positions of particles, and what causes their paths of arrival. The most widely accepted then and today is the Copenhagen Interpretation devised by Niels Bohr, Werner Heisenberg, and Max Born around 1925. Contending this was de Broglie’s pilot-wave theory published a few months prior and débuting at the conference. There are many technicalities, but the condensed version of the debate is that both agreed on certain aspects and even matched empirically, but diverged in their descriptions of how the probabilities arise. Bohr, Heisenberg and Born, inspired by instrumentalism, interpreted only the mathematics modeling phenomena, pure statistics, and denied the ability to describe its actual behavior. The future position of a particle, and therefore the path it takes, is *inherently indeterminate *until observed. Nature was not determined like the classical physicists and even Einstein had hoped for. The role the wave function played to them was out of mathematical utility, an instrument. Contrarily, de Broglie held on to realism; he viewed the wave function as an entity. To him there was only a mere *appearance**of randomness* in finding the particle and its path. Despite the fact that both interpretations result in the same statistical predictions, de Broglie’s was so unpopular that he was persuaded to abandon it. It wasn’t until its 1952 resurrection by David Bohm that it started to gain traction in the community. Had de Broglie not presented and defended his theory at the conference, it may have been forgotten. As if these disagreements weren’t enough, the very definition of the wave function was called into question. The sparring that took place on all of these topics birthed invaluable arguments, perhaps the most notable of which are the famed Bohr-Einstein debates which would continue on for years to come. All of these interpretational issues are yet to be resolved, but the initial discourse began at this meeting.

The conference was fondly looked upon by its attendees with Born even describing it as “the most stimulating scientific meeting [he had] ever taken part in.” However, many left with differing perspectives on what had actually been achieved. At the end of the conference, Paul Ehrenfest humorously compared the complications the physicists faced in understanding each other’s jargon to the biblical tale of the Tower of Babel in Genesis. This analogy implies that there were difficulties in communication; clarifying the varying languages used to describe new, abstract concepts was no easy task. Langevin shared a similar sentiment later writing that “the confusion of ideas [had] reached its peak.” It is clear that some felt there was still much yet to be reconciled. However, Heisenberg, whose formulation was in the mainstream, found the conferences to have been conclusive and contrastingly wrote:

“In…the development of the quantum theory, one must in particular not forget the discussions at the Solvay conference in Brussels in 1927, chaired by Lorentz. Through the possibility of exchange…between the representatives of different lines of research, this conference has contributed extraordinarily to the clarification of the physical foundations of the quantum theory; it forms so to speak the outward completion of the quantum theory.” (Bacciagaluppi & Valenti 4, 24)

Regardless of the confusion and differences felt in the midst of such upheaval, the impact of the 1927 conference is apparent through the relevance and continuation of its discussions today. Many important ideas were presented and perhaps most astonishingly the pilot-wave theory may have gone unnoticed had de Broglie not attended.

The conferences still continue today. Heisenberg mused that “the Solvay meetings have stood as an example of how much well planned and well organized conferences can contribute to the progress of science” (“International Solvay Institutes”). Before his death, Bohr spoke at the 1961 conference stating that they were “unique occasions for physicists to discuss the fundamental problems which were at the center of interest at the different periods, and have thereby in many ways stimulated modern development of physical science” (Bohr). In the development of quantum theory the first and the fifth conferences were of the most importance. They hosted an arena for discussing rival interpretations of the time and cultivated ideas forming the framework of modern physics.

These history of these conferences represent the astounding beginning of humanity’s modern conception of nature. It is important to discuss the lesser known pilot-wave theory as the Copenhagen Interpretation is almost exclusively taught today. Interestingly, there has been a gradual resistance toward the latter. In his 1969 Nobel Prize lecture, Murray Gell-Mann declared that Bohr had “brainwashed an entire generation of physicists into believing that the problem [of quantum theory] had been solved” (Hardesty). The flaunted epistemic limits of Bohr’s interpretation pride some physicists and dissuade others. Is Nature inherently statistical and undetermined? Einstein was troubled by this and famously quipped that “God does not play dice.” We do not get to decide how Nature operates. Science is no place for the metaphysical. Perhaps it is wise to stay in the mainstream and to heed the advice of Bohr, who supposedly retorted to Einstein, “Stop telling God what to do!”

Thanks for this! I’ve also been fascinated by these conferences. To have been there! Alas, born too late.

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You’re welcome! I hope you enjoyed it

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