Tag Archives: causality

Q+ Hangout: Rob Spekkens

Here are the details of the next Q+ hangout.

Date/time: Tuesday 20th November 2pm GMT/UTC

Speaker: Rob Spekkens (Perimeter Institute)

Title: Quantum correlations from the perspective of causal discovery algorithms

Abstract: If correlation does not imply causation, then what does? The beginning of a rigorous answer to this question has been provided by researchers in machine learning, who have developed causal discovery algorithms. These take as their input facts about correlations among a set of observed variables and return as their output a causal structure relating these variables. We show that any attempt to provide a causal explanation of Bell-inequality-violating correlations must contradict a core principle of these algorithms, namely, that an observed statistical independence between variables should not be explained by fine-tuning of the causal parameters. In particular, we demonstrate the need for such fine-tuning for most of the causal mechanisms that have been proposed to underlie Bell correlations, including superluminal causal influences, superdeterminism (that is, a denial of freedom of choice of settings), and retrocausal influences which do not introduce causal cycles. This work suggests a novel perspective on the assumptions underlying Bell’s theorem: the nebulous assumption of “realism” is replaced with the principle that all correlations ought to be explained causally, and Bell’s notion of local causality is replaced with the assumption of no fine-tuning. Finally, we discuss the possibility of avoiding the fine-tuning by replacing conditional probabilities with a noncommutative generalization thereof.

Based on arXiv:1208.4119.

Joint work with Chris Wood.

To watch the talk live, go to http://gplus.to/qplus at the appointed hour.

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Rerun: Caslav Brukner’s Q+ hangout

We are rerunning Caslav Brukner’s Q+ hangout due to problems with the livestream and video recording in July. Note the earlier than usual starting time.

Date: 18th September 2012

Time: 12noon British Summer Time

Speaker: Caslav Brukner (University of Vienna)

Title: Quantum correlations with indefinite causal order


In quantum physics it is standardly assumed that the background time or definite causal structure exists such that every operation is either in the future, in the past or space-like separated from any other operation. Consequently, the correlations between operations respect definite causal order: they are either signalling correlations for the time-like or no-signalling correlations for the space-like separated operations. We develop a framework that assumes only that operations in local laboratories are described by quantum mechanics (i.e. are completely-positive maps), but relax the assumption that they are causally connected. Remarkably, we find situations where two operations are neither causally ordered nor in a probabilistic mixture of definite causal orders, i.e. one cannot say that one operations is before or after the other. The correlations between the operations are shown to enable performing a communication task (“causal game”) that is impossible if the operations are ordered according to a fixed background time.

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