In The Conceptual Development of Quantum Physics, Max Jammer observes that “certain philosophical ideas of the late nineteenth century not only prepared the intellectual climate for, but contributed decisively to, the formation of the new conceptions of the modern quantum theory”—specifically, “contingentism, existentialism, pragmatism, and logical empiricism,” which, argues Jammer “united in rejecting causality though on different grounds” and “prepared, so to speak, the philosophical background for modern quantum mechanics.” Regarding this same rejection of causality in physics in Weimar Germany—a rejection central to the quantum interpretation—Paul Forman argues that “substantive problems in atomic physics played only a secondary role in the genesis of this acausal persuasion, [and] that the most important factor was the social-intellectual pressure exerted upon the physicists as members of the German academic community.” Along these same lines, in an interview with Einstein in 1932, James Gardner Murphy said, “scientists live in the world just like other people…. They cannot escape the influence of the milieu in which they live. And that milieu at the present time is characterized largely by a struggle to get rid of the causal chain in which the world has entangled itself.” In all of these cases, the development of quantum physics is attributed to the zeitgeist of the time, with the critics adopting a “hypodermic” model wherein intellectual currents have a direct influence on scientific discovery.
In The Conceptual Development of Quantum Physics, Max Jammer observes that “certain philosophical ideas of the late nineteenth century not only prepared the intellectual climate for, but contributed decisively to, the formation of the new conceptions of the modern quantum theory”—specifically, “contingentism, existentialism, pragmatism, and logical empiricism,” which, argues Jammer “united in rejecting causality though on different grounds” and “prepared, so to speak, the philosophical background for modern quantum mechanics.” Regarding this same rejection of causality in physics in Weimar Germany—a rejection central to the quantum interpretation—Paul Forman argues that “substantive problems in atomic physics played only a secondary role in the genesis of this acausal persuasion, [and] that the most important factor was the social-intellectual pressure exerted upon the physicists as members of the German academic community.” Along these same lines, in an interview with Einstein in 1932, James Gardner Murphy said, “scientists live in the world just like other people…. They cannot escape the influence of the milieu in which they live. And that milieu at the present time is characterized largely by a struggle to get rid of the causal chain in which the world has entangled itself.” In all of these cases, the development of quantum physics is attributed to the zeitgeist of the time, with the critics adopting a “hypodermic” model wherein intellectual currents have a direct influence on scientific discovery.
For over nine decades, quantum mechanics has profoundly shaped our understanding of nature, offering explanations for the stability of atoms, the formation of mole cules, the properties of materials, and phenomena such as light or radioactivity. It is essential to modern technologies—from microchips to lasers—and underpins the physics of elementary particles. Yet, despite its successes, it remains a deeply mysterious theory, confronting a central enigma: why does the quantum world behave so differently from everyday reality? Elements such as superposition and quantum entanglement—which allow particles to exist simultaneously in multiple states or remain interconnected regardless of distance—contradict classical world intuitions. This gap between microscopic and macroscopic behavior has given rise to what is known as the “measurement problem,” raising fundamental questions about the nature of reality. Diverging interpretations have led to two major schools of thought: anti-realism, promoted by Niels Bohr, which holds that reality depends on the observer and their interaction with measurement instruments; and realism, supported by Albert Einstein, which argues that quantum mechanics is incomplete and that eality exists independently of observation. Bohr’s complementary philosophy and Heisenberg’s uncertainty principle were central to the formulation of the “Copenhagen interpretation,” which became dominant in the 20th century. However, the persistent opposition of scientists like Einstein and Schrödinger inspired generations of physicists to seek a unification between quantum mechanics and relativity, culminating in new theories such as loop quantum gravity and string theory. Thus, the debate between realism and anti-realism remains alive, and philosophy continues to play a crucial role in exploring the new foundations of physics.
Prior to and into the first decade of Niels Bohr's 1913 atomic theory, physicists dealt with physical systems in which the usual space and time pictures of classical physics were assumed trustworthy and so could be extrapolated to any sort of matter in motion, for example, electrons move like billiard balls and light behaves analogously to water waves. In the German scientific milieu this visual imagery was accorded a reality status higher than viewing merely with the senses and was referred to as ‘ordinary intuition [ gewöhnliche Anschauung ]’. Ordinary intuition is the visual imagery abstracted from phenomena that we have actually witnessed in the world of sense perceptions. The concept of ordinary intuition was much debated during 1923–27 by physicists like Niels Bohr, Werner Heisenberg, Wolfgang Pauli and Erwin Schrödinger, all of whom used this term with proper Kantian overtones, and all of whom lamented its loss in the new quantum mechanics. Ordinary intuition is associated with the strong causality of classical mechanics. According to the law of causality, initial conditions (position and velocity or momentum and energy) can be ascertained with in-principle perfect accuracy. Consequently any system's continuous development in space and time can be traced with in-principle perfect accuracy using Newton's laws of motion and conservation of energy and momentum. Any limitations to the accuracy of measurements are assumed not to be intrinsic to the phenomena, that is they are interpreted to be systematic measurement errors that ideally can be made to vanish.
‘God’, said Albert Einstein, ‘does not play dice’. This famous remark by the author of the theory of relativity was not intended as an analysis of the recreational habits of a supreme being but expressed his reaction to the new scientific ideas, developed in the first quarter of the twentieth century, which are now known as quantum physics. Before we can fully appreciate why one of the greatest scientists of modern times should have been led to make such a comment, we must first try to understand the context of scientific and philosophical thought that had become established by the end of the nineteenth century and what it was about the ‘new physics’ that presented such a radical challenge to this consensus. What is often thought of as the modern scientific age began in the sixteenth century, when Nicholas Copernicus proposed that the motion of the stars and planets should be described on the assumption that it is the sun, rather than the earth, which is the centre of the solar system. The opposition, not to say persecution, that this idea encountered from much of the establishment of that time is well known, but this was unable to prevent a revolution in thinking whose influence has continued to the present day. From that time on, the accepted test of scientific truth has increasingly been observation and experiment rather than religious or philosophical dogma.
My thesis investigates a debate between Vienna and Berlin about the view that the basic laws of nature are genuinely indeterministic that started long before the advent of quantum mechanics. It involved two different readings of Ludwig Boltzmann's legacy statistical mechanics and two different answers to how causality and ontology ought to be combined. Having adopted Ernst Mach's weak notion of causality, the local Viennese tradition could more easily contemplate ontologies for irreducibly statistical laws that were different from classical physics, while the Berlin tradition departed from Planck's reading of the Kantian tradition, according to which strictly deterministic laws represented an indispensable basis even for probabilistic theories.\nVienna Indeterminism, as I shall call the first reading, became characteristic for a series of physicist-philosophers connected to the Institute of Physics at the University of Vienna, among them Franz Serafin Exner, Erwin Schrödinger, Philipp Frank and Richard von Mises. It involved the acceptance of the highly improbable events admitted by the second law of thermodynamics, an assent to Mach's definition of causality in terms of functional dependencies, an empiricist shift of the burden of proof on the determinist's shoulders, and the adoption of the relative frequency interpretation of probability. Planck and his student Moritz Schlick initially rejected all of these creeds.\nWhile the Viennese reading of Boltzmann's legacy made it possible to accept probabilistic laws as genuine laws years before the advent of quantum mechanics, Schlick had to take a strictly verificationist tack to reconcile his views on probability with the failure of a Kant-inspired notion of causality enforced by the changes in physics. Until his death however, Schlick remained committed to the Spielraum interpretation of Johannes von Kries, compensating his qualified acceptance of indeterminism with an emphasis on the limits of language that were enforced by Heisenberg's uncertainty relations.\nIn view of claims that scientists' abandonment of determinism and causality was triggered, or at least enhanced, by external factors, among them the anti-scientific Weimar milieu and Bohr's power politics, the reconstruction of the philosophical debates between Vienna and Berlin requires historical contextualization. To this end, I initially recapitulate the Forman thesis and the discussions ensuing from it. Most important among them are the recent books of Cushing and Beller that, rejecting Forman's strong claims, nevertheless deny any constructive influence of the scientific aspirations of German physicists on the philosophical debates about causality and indeterminism in the 1920s.\nAgainst this picture, I argue that the philosophical aspirations of physicist-philosophers were rather genuine and followed influential role models, among them Helmholtz and Mach. They represent a characteristic trait of the German-speaking scientific world and were expressed in academic addresses and in the publication of popular writings. After 1913, this philosophical discourse was conducted, to an increasing extent, on the pages of Die Naturwissenschaften. In a separate chapter, I study the role of philosophical and cultural matters in this journal, which can hardly be separated from the personality of its founding editor Arnold Berliner. At about 1930, this discourse among physicist-philosophers partly merged into scientific philosophy, as conducted by Logical Empiricists, and the newly founded Erkenntnis.\nOne of the characteristic features of the discourse among physicist-philosophers was that they were not strictly indebted to philosophical schools but rather willing to form strategic alliances. Such alliances emphasized one philosophical aspect that was considered pivotal at the time which other features, often expressing severe disagreements of content, were played down. This process can be witnessed at the end of the discussion between the Viennese and the Berliners in the 1930s, at a time when Schlick had already become the center of the Vienna Circle. Being confronted with a plethora of "metaphysical" misinterpretations and having taken the linguistic turn, Frank and Schlick formed a strategic alliance on the 1936 Copenhagen Congress for the Unity of Science. It brought them into opposition with Schrödinger's quest for a modification of quantum mechanics, although Schrödinger continued to be a staunch advocate of indeterminism. This year thus signifies the end of the coherence of Vienna Indeterminism, a tradition that reaches back to the publication of Boltzmann's Lectures of Gas Theory.
No scientific theory has caused more puzzlement and confusion than quantum theory. Beginning in 1900, the theory developed in fits and starts and found a consistent mathematical framing only when John von Neumann published his Mathematische Grundlagen der Quantenmechanik in 1932 [12]. But even today, we struggle to understand the world as pictured by quantum theory. Physics is supposed to help us to understand the world, and yet quantum theory makes it seem a very strange place. One might be tempted to push aside our puzzlement as the result of our clinging to a primitive worldview. But our puzzlement is not merely a psychological obstacle; it is also an obstacle to the development of physics itself. This obstacle is encountered primarily in our attempts to unify quantum theory and the general theory of relativity. As argued persuasively by Chris Isham (who is represented in this volume), Lee Smolin [10], and others, the primary obstacle between us and future physics is our own failure to understand the conceptual foundations of current physical theories. How, then, are we to make conceptual progress? What is the process by which we find a new perspective, a perspective in which previously puzzling phenomena find a place in an intelligible—and perhaps beautiful—structure? I do not wish to make prescriptions or to claim that conceptual progress can be achieved in only one way. But this book begins with the Ansatz that conceptual progress might be achieved through free creations of the human intellect.
The new physics just described was still based largely on Newtonian ideas. Indeed, in its theoretical aspects, it might not unfairly be described as a final attempt to explain the world in materialistic terms—as particles being pushed and pulled about in space and time. Nevertheless, the new physics had found it necessary to abolish most of the forces of pushing and pulling, replacing the gradual changes of motion of the particles under these forces by sudden and unpredictable jumps. These appeared to involve violations of the law of causality, both in the disintegration of radioactive atoms and also in the internal changes of ordinary atoms. We seemed to see Fate defying this law as she picked out certain atoms for disintegration or collapse and, by her apparently capricious acts, sent the universe along one path or another according to her whim. On such lines the new physics had explained many phenomena which had hitherto seemed inexplicable, but it had by no means met with complete success. For instance, while it gave a perfect interpretation of the simplest spectrum of all, namely that of the hydrogen atom, it failed with more complex spectra. This was not necessarily a fatal objection; a few emendations and possibly a few new ad hoc assumptions might have effected a complete reconciliation, although this seems improbable.