Physikalisch-Philosophisches Kolloqium: Experiments, thought experiments and rigorous proofs: Philosophical reflections on quantum physics

Prof. Dr. Michael Stöltzner
University of South Carolina
Experiments, thought experiments and rigorous proofs:
Philosophical reflections on quantum physics


There has been a long tradition among quantum physicists to separate their own research practice from questions of interpretation. The aim of my talk is not to deny the legitimacy of adopting what John Bell called FAPP, but to show at three case studies that the borderline between the physical-research mode and the foundational-philosophical mode has itself changed repeatedly over the past decades. This might help to foster collaboration and allow cohabitation between quantum physicists and their philosophers. My first example is, expec­tedly, the long path from a thought experiment about completeness, causality and the elements of reality to experiments that promise both stunning applications and defy our basic mereolo­gical intuitions: the basic elements of quantum reality are entangled states spanning macro­scopic distances. The second example concerns the mathematical foundations of quantum mechanics. How could mathematician John von Neumann dare a no-hidden variable theorem? He was wrong, but mathematization can both be exploratory and philosophically motivated. It made much philosophical sense to go for categoricity, that is, to prove that the theory had only one model up to equivalences which justified the known equivalence of the Schrödinger and Heisenberg representations. Surprisingly, we nevertheless can put so much structure of alterna­tive interpretations into the theory without changing its empirical content. My final example concerns the emergence of a new observable quantity that was not consi­dered as fundamental in the days before quantum mechanics, to wit, fluctuations. Generations observed Brownian motions under the microscope and tried to explain the fluctuations in their velocities in ever more sophisticated ways through statistical mechanics. They failed by five orders of magnitude until Einstein and von Smoluchowski explained the phenomenon through an ingenious thought experiment and long calculations respectively. Nobody had thought that a phenomenon solidly in the camp of city hygiene and physical chemistry decided a question as foundational as the existence of atom, instead of the second law of thermodynamics.


Michael Stöltzner is professor of philosophy at the University of South Carolina and a founding Co-PI of the interdisciplinary DFG-FWF Research Unit “Epistemology of the Large Hadron Collider”. Before he has held positions at the Universities of Salzburg, Bielefeld, and Wuppertal. His main areas of research are the philosophy of elementary particle physics, especially the dyna­mics of models and explanations; the philosophy of mathematical physics and applied mathema­tics; history of philosophy of science, especially the Vienna Circle and the debates about causality and probability in the early 20th century; and the principle of least action and formal teleology. He has won the 2021 Russell Research Award for Humanities and Social Science of the University of South Carolina. He has served as Chair of his University’s Committee for Tenure and Promotions and was a member of the Advisory Board of the German Academic International Network.