Chapter 7. Causality in Some Quantum Experiments

Raphael Neelamkavil

Raphael Neelamkavil

Raphael Neelamkavil

David Thomas Pegg

Kelvin J. McQueen, Lev Vaidman A

This chapter argues that locality is a necessary condition for causal explanations to be commonsensical. It takes it as a requirement on an adequate physical theory that it be able to offer local causal explanations of all experimental outcomes. The chapter discusses how the demand for local causal explanation shaped classical physics. It introduces quantum physics in the context of a simple experiment, which although strange, can be explained locally. It specifies three refined common sense principles of locality. These principles are satisfied by classical physics and by the simple quantum experiment. The chapter also presents several quantum experiments that do seem to violate the principles and therefore defy common sense. It shows that common sense can be restored to quantum theory if we adopt the many worlds interpretation. The chapter also discusses the significance of our results to debates over the interpretation of quantum mechanics.

Manuel Bernardo Muñoz Bastias

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.