Philosophy of Physics Seminar (Thursday - Week 2, TT25)

Abstract: Werner Heisenberg’s 1927 paper introducing his uncertainty principle contains an unusual postscript. It flags apparent shortcomings in the work, particularly concerning a thought experiment with a gamma-ray microscope, that were pointed out to Heisenberg by his supervisor and mentor, Niels Bohr. Does this postscript truly reflect a mistake in Heisenberg’s reasoning, or does it mask a philosophical disagreement with Bohr? Historians such as Max Jammer and Mara Beller have tended to endorse Bohr’s claim that Heisenberg’s reasoning about the gamma-ray microscope contains a mistake: a failure to account for the finite aperture of the lens of the supposed microscope. Conversely, we argue that the postscript reflects a disagreement about whether the gamma-ray microscope should emphasize wave-particle duality—i.e., roughly, complementarity—or particles with properties created through observation.

Our argument is twofold. First, we reconstruct Heisenberg’s original version of his gamma-ray microscope from letters he sent to Pauli and Dirac, showing that it contains no lens or imaging screen at all; rather, it is designed to roughly operationalize the paths of light in the Geiger-Bothe and Compton-Simon cloud chamber experiments from just a few years earlier. Second, we argue that Heisenberg was influenced to frame the difference as an error via psychological abuse from Bohr. The sort of abuse in play is what feminist philosopher Lauren Laydon-Hardy calls epistemic infringement, an abuser’s systematic subversion of the healthy social and epistemic norms governing their relationship with their target. We demonstrate how a model of this systematic subversion maps neatly onto Bohr and Heisenberg’s relationship from their first meeting in 1922 to the submission of the postscript in 1927. This case provides a striking example of how paying attention to the psychology of theory building can help us better understand past physics.