At at the fifth Solvay conference in October 1927, the French physicist Louis de Broglie presented a report on ‘The Dynamics of Quanta’, in which he outlined of an alternative approach to quantum mechanics based on the idea that electrons and photons move along continuous trajectories. As is well known, de Broglie’s 'pilot-wave theory' met with a hostile reception in Brussels, and soon after returning to Paris, he abandoned the theory. The neglect of de Broglie’s program of wave mechanics is of historical interest, not only because it was one of the very few attempts in the 1920s to develop an alternative conception of wave mechanics to the one that ultimately prevailed, but because it emerged in an institutional and cultural context very different from the one in which quantum theory took shape. In this paper, I argue that the fate of de Broglie’s program in the late 1920s was not merely a consequence of the hostile reception it met with at the fifth Solvay conference, but was also partly a result of de Broglie’s own programmatic aims, his personal and professional idiosyncrasies, and the social, cultural and institutional context of French physics at the time. The social and intellectual context in which de Broglie's program was revived in the 1950s provides an instructive contrast to the situation that had prevailed before the war. These historical contingencies cannot be ignored if we are to understand why events unfolded the way they did.

Perturbation theory, originally developed in celestial mechanics, played a fundamental role in the development of quantum mechanics. Adapted and refined in the early years, the associated techniques proved indispensable for tackling the complexities of quantum systems. Over time, perturbation theory even evolved significantly, particularly with the advent of quantum field theory. Among its most revolutionary results were the Feynman diagrams, which transformed the perturbation series representation of quantum mechanics into an intuitive, computable visual framework. These diagrams not only simplified complex calculations, but also became one of the most powerful and widely used mathematical tools in the history of modern physics.

Interestingly, in a historical twist, many of the techniques originally developed for quantum theory, including Feynman diagrams, have found new applications in recent decades in celestial mechanics—or more precisely, in contemporary general relativity. These methods have proved particularly useful for solving the two-body problem, a long-standing challenge in gravitational physics. By linking perturbative approaches from different fields, these developments have deepened our understanding of binary systems composed of black holes and neutron stars, and contributed to groundbreaking discoveries such as the detection of gravitational waves.

This talk explores this fruitful century-long methodological interplay between celestial mechanics, quantum mechanics and general relativity. It highlights how perturbation theory not only became one of the cornerstones of quantum mechanics, but also a unifying framework that stimulated innovation and fostered links between various fields of physics.

In the 1920s and 1930s, physicists sought to extend quantum theory to new domains, notably the electromagnetic field and new radiation and cosmic-ray phenomena. Throughout this work, physicists used perturbation theory. Perturbation theory is a family of mathematical techniques that provide approximate solutions to mathematical equations when, as is usual, the exact solution is unknown. In the historiography of physics, perturbation theory is often portrayed as the practical method physicists apply to connect theories to experiments. It gets the numbers out. But it has done much more. Perturbation theory structured quantum electrodynamics. Famously, Paul Dirac claimed that all of space was suffused with an infinite number of electrons in never-before-observed negative-energy states. But competitor theories agreed that empty space was filled with ``pairs,'' or ``virtual'' quanta. In the absence of solid empirical evidence, why did such different theories agree on a counter-intuitive claim? I argue that the application of perturbation methods created physicists' intersubjective agreement about vacuum phenomena. If such agreement defines objectivity: common tools constituted the objectivity of the quantum vacuum. In addition, I argue that perturbation theory led physicists to propose the existence of novel phenomena, independent of empirical evidence, taking the example of the scattering of light by light. Light-light scattering was not connected to experiment, but it lived on as an exemplar of theory for theory's sake. This line of argument contributes to our understanding of the importance of paper tools in physics, and of the paths through which theoretical physics established itself, for itself.