Causality in a Quantum World

Časlav Brukner

Jacques Pienaar

Jürgen Audretsch

Alastair Wilson

Abstract Scientific explanations should have a character that matches their subject matter: if a fact is not apt for causal explanation, it should not be explained causally. In this chapter, the author first reviews some cases where causal explanation has been extended beyond its proper limits, identifying a pattern of error termed the problem of causal overreach. The discussion then relates these considerations to the debate over causal locality in Everettian (many-worlds) quantum mechanics (EQM), in the context of three metaphysical distinctions central to recent discussions of locality: event causation vs. worldshaping causation, local branching vs. global branching, and overlapping worlds vs. diverging worlds. The author argues that worlds in EQM are not properly regarded as themselves subject to causal explanation; instead, causation should be understood as a wholly in-world matter. This approach avoids any nonlocal causal explanation; quantum entanglement instead supports a new category of noncausal worldshaping explanations. The chapter concludes by considering the implications of worldbound causation for principles of causal inference, in particular for the Causal Markov condition, and recommends the use of quantum causal models as a foundation for in-world causal inference.

Kelvin J. McQueen, Lev Vaidman A

The many worlds interpretation of quantum mechanics (MWI) states that the world we live in is just one among many parallel worlds. It is widely believed that because of this commitment to parallel worlds, the MWI violates common sense. Some go so far as to reject the MWI on this basis. This is despite its myriad of advantages to physics (e.g. consistency with relativity theory, mathematical simplicity, realism, determinism, etc.). Here, we make the case that common sense in fact favors the MWI. We argue that causal explanations are commonsensical only when they are local causal explanations. We present several quantum mechanical experiments that seem to exhibit nonlocal "action at a distance". Under the assumption that only one world exists, these experiments seem immune to local causal explanation. However, we show that the MWI, by taking all worlds together, can provide local causal explanations of the experiments. The MWI therefore restores common sense to physical explanation.

Peter J. Riggs