9 Particle Physics without Particles? On Causal Realism in Quantum Field Theory

Raphael Neelamkavil

Matthias Egg

Particle physics studies highly complex processes which cannot be directly observed. Scientific realism claims that we are nevertheless warranted in believing that these processes really occur and that the objects involved in them really exist. This book defends a version of scientific realism, called causal realism, in the context of particle physics. The first part of the book introduces the central theses and arguments in the recent philosophical debate on scientific realism and discusses entity realism, which is the most important precursor of causal realism. It also argues against the view that the very debate on scientific realism is not worth pursuing at all. In the second part, causal realism is developed and the key distinction between two kinds of warrant for scientific claims is clarified. This distinction proves its usefulness in a case study analyzing the discovery of the neutrino. It is also shown to be effective against an influential kind of pessimism, according to which even our best present theories are likely to be replaced some day by radically distinct alternatives. The final part discusses some specific challenges posed to realism by quantum physics, such as non-locality, delayed choice and the absence of particles in relativistic quantum theories.

Werner Karl Heisenberg

Kaj Börge Hansen

Eliahu Cohen, Marina Cortês, Avshalom C. Elitzur +1 more

Of all basic principles of classical physics, realism should arguably be the last to be given up when seeking a better interpretation of quantum mechanics. We examine the de Broglie-Bohm pilot wave theory as a well-developed example of a realistic theory. We present three challenges to a naive reading of pilot wave theory, each based on a system of several entangled particles. With the help of a coarse graining of pilot wave theory into a discrete system, we show how these challenges can be answered. However, this comes with a cost. In the description of individual systems, particles appear to scatter off empty branches of the wave function as if they were particles and conversely travel through particles as if they were waves. More generally, the ``particles'' of pilot wave theory are led by the guidance equation to move in ways no classical particle would, involving apparent violations of the principles of inertia and momentum conservation. We next argue that the aforementioned cost can be avoided within a retrocausal model. In the proposed version of the pilot wave theory, the particle is guided by a combination of advanced and retarded waves. The resulting account for quantum physics seems to have greater heuristic power, demands less damage to intuition, and moreover provides some general hints regarding spacetime and causality. This is the first of two papers. In the second [E. Cohen, M. Cort\^es, A. C. Elitzur, and L. Smolin, Realism and causality. II. Retrocausality in energetic causal sets, Phys. Rev. D 102, 124028 (2020).], we show that, in the context of an explicit model, retrocausality, with respect to an effective, emergent spacetime metric, can coexist with a strict irreversibility of causal processes.

Nadji Belkheiri

The predictive success of modern physics from rigid body mechanics to quantum field theory stands among the greatest achievements of the human mind. Yet this success does not necessarily entail completeness of explanatory understanding. This paper investigates the epistemological gap between mathematical description and causal explanation, taking the behavior of rotating bodies (gyroscopes) as a preliminary example, then extending to two central problems: the measurement problem in quantum mechanics and the nature of mass in particle physics. The study shows that concepts such as angular momentum and precession, despite their predictive accuracy, redescribe phenomena in mathematical language more than they provide intuitive causal mechanisms. It also demonstrates that Noether’s theorem and the principle of least action, for all their depth, reformulate fundamental questions rather than resolving them. The paper discusses the framework of structural realism as a philosophical stance that accommodates this gap, examines criticisms of this framework (the problem of structural individuation and the problem of abandoned theories), and concludes that acknowledging this gap is not an attack on physics but a condition for its philosophical maturity.