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Entropy

Noncommuting Common Causes, Law of Total Probability, and No-Conspiracy

Gábor Hofer‐Szabó

Published Oct 21, 2026• 0 citations
Publisher

Abstract

Noncommuting common causes posited as explanations of quantum correlations violate the law of total probability. On the operationalist interpretation of quantum mechanics, this flaw might be interpreted as a result of the noncommuting common causes being conspiratorial. In this paper, we will show that this is not the case. Noncommuting common causes violate the law of total probability but they are not conspiratorial.

DOI
10.3390/e28101158

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Noncommuting common causes posited as explanations of quantum correlations violate the law of total probability. On the operationalist interpretation of quantum mechanics, this flaw might be interpreted as a result of the noncommuting common causes being conspiratorial. In this paper, we will show that this is not the case. Noncommuting common causes violate the law of total probability but they are not conspiratorial.
In this paper, we revisit the concept of noncommuting common causes; refute two objections raised against them, the triviality objection and the lack of causal explanatory force; and explore how their existence modifies the EPR argument. More specifically, we show that 1) product states screening off all quantum correlations do not compromise noncommuting common causal explanations; 2) noncommuting common causes can satisfy the law of total probability; 3) perfect correlations can have indeterministic noncommuting common causes; and, as a combination of the above claims, 4) perfect correlations can have noncommuting common causes which are both nontrivial and satisfy the law of total probability.
In this paper I assess the adequacy of no-conspiracy conditions present in the usual derivations of the Bell inequality in the context of EPR correlations. First, I look at the EPR correlations from a purely phenomenological point of view and claim that common cause explanations of these can not be ruled out. I argue that an appropriate common cause explanation requires that no-conspiracy conditions are re-interpreted as mere common cause-measurement independence conditions. Violations of measurement independence thus need not entail any kind of conspiracy (nor backwards in time causation). This new reading of measurement dependence provides the grounds for an explicitly non-factorizable (in the sense of Bell’s factorizability) common cause model for EPR. 1
Standard common causal explanations of the EPR situation assume a so-called joint common cause system that is a common cause for all correlations. However, the assumption of a joint common cause system together with some other physically motivated assumptions concerning locality and no-conspiracy results in various Bell inequalities. Since Bell inequalities are violated for appropriate measurement settings, a local, non-conspiratorial joint common causal explanation of the EPR situation is ruled out. But why do we assume that a common causal explanation of a set of correlation consists in finding a joint common cause system for all correlations and not just in finding separate common cause systems for the different correlations? What are the \nperspectives of a local, non-conspiratorial separate common causal explanation for the EPR scenario? And finally, how do Bell inequalities relate to the weaker assumption of separate common cause systems? \n
It has long been recognized that a local hidden variable theory of quantum mechanics can in principle be constructed, provided one is willing to countenance pre-measurement correlations between the properties of measured systems and measuring devices. However, this ‘conspiratorial’ approach is typically dismissed out of hand. In this article I examine the justification for dismissing conspiracy theories of quantum mechanics. I consider the existing arguments against such theories, and find them to be less than conclusive. I suggest a more powerful argument against the leading strategy for constructing a conspiracy theory. Finally, I outline two alternative strategies for constructing conspiracy theories, both of which are immune to these arguments, but require one to either modify or reject the common cause principle. 1. Introduction 2. The incompleteness of quantum mechanics 3. Hidden variables 4. Hidden mechanism conspiracy theories 5. Existing arguments against hidden mechanisms 6. A new argument against hidden mechanisms 7. Backwards-causal conspiracy theories 8. Acausal conspiracy theories 9. Conclusion
In the paper, the EPR-Bohm scenario will be reproduced in an algebraic quantum field theoretical setting with locally finite degrees of freedom. It will be shown that for a set of spatially separated correlating events (projections) maximally violating the Clauser–Horne inequality there can be given a common causal explanation if commutativity is abandoned between the common cause and the correlating events. Moreover, the noncommuting common cause will be local and supported in the common past of the correlating events.