Scientific explanation often involves making inferences about what is unobservable from what is observable. An important example is the problem of making inferences about causal mechanisms from observed correlations. In the context of quantum theory, the problem of inferring causal mechanisms is particularly vexing. One of the central results in the foundations of quantum theory, Bell's theorem, can be understood as demonstrating that it is impossible to provide a causal explanation of the correlations that arise for entangled quantum systems without resorting to fine-tuning. Impossible, that is, using the standard framework of causal models. An intrinsically quantum notion of a causal model, however, holds promise for achieving such an explanation. Critically, this notion requires one to replace classical probability theory with a noncommutative generalization thereof. It also has practical applications, allowing one to infer causal relationships from observed correlations in scenarios where classically one could not. To be precise: correlation does not imply causation in a classical world, but in a quantum world, sometimes it does.
Dr. Robert Spekkens
