CHAPTER 4 Physical Science. Quantum Mechanics

This chapter completes the physical groundwork of the thesis by examining quantum mechanics, the second of the two theories that the following chapters interpret philosophically. Rather than offering an exhaustive mathematical treatment, it selects the concepts required to follow the two transversal principles identified in Chapter 3: causality and the principle of stationary action. It presents the emergence of the quantum of action, the formal structure of the theory (states, observables, Born’s rule, Schrödinger dynamics, superposition and entanglement), the double-slit experiment, the uncertainty principle, the measurement problem, and the sequence formed by the EPR argument, Bell’s theorem and its experimental tests. Using the three senses of causality distinguished in Chapter 3, the chapter shows that Heisenberg’s 1927 attempt to refute the law of causality did not succeed: unitary evolution preserves dynamical determination at the level of the quantum state, while the no-signalling condition and relativistic microcausality preserve causal precedence and causal propagation, even though Bell-local models fail. It then shows that action, present at the very birth of the theory as Planck’s quantum of action, governs quantum dynamics through Feynman’s path integral and Schwinger’s quantum action principle, and that the classical principle of stationary action re-emerges in the classical limit. The principal result, restricted to physics, is that causality and stationary action are transversal to classical, relativistic and quantum physics, although their mathematical formulation changes. Ontological conclusions are reserved for the following chapters.