In previous work with Ken Wharton, it was proposed that Bell correlations are a special sort of selection artefact, explained by a combination of (i) collider bias and (ii) a boundary constraint on the collider variable. This requires no direct causal influence outside lightcones, and may hence offer a new way to reconcile Bell nonlocality and relativity. This piece outlines a new argument for the proposal. It explains how it is valid for a special class of ('W-shaped') Bell experiments involving delayed-choice entanglement swapping, and argues that it can be extended to the general ('V-shaped') case. A detailed version of the argument is now available in arXiv:2406.04571 [quant-ph].
In previous work with Ken Wharton, it was proposed that Bell correlations are a special sort of selection artefact, explained by a combination of (i) collider bias and (ii) a boundary constraint on the collider variable. This requires no direct causal influence outside lightcones, and may hence offer a new way to reconcile Bell nonlocality and relativity. This piece outlines a new argument for the proposal. It explains how it is valid for a special class of ('W-shaped') Bell experiments involving delayed-choice entanglement swapping, and argues that it can be extended to the general ('V-shaped') case. A detailed version of the argument is now available in arXiv:2406.04571 [quant-ph].
We propose that quantum entanglement is a special sort of selection artefact, explicable as a combination of (i) collider bias and (ii) a boundary constraint on the collider variable. We show that the proposal is valid for a special class of (`W-shaped') Bell experiments involving delayed-choice entanglement swapping, and argue that it can be extended to the ordinary (`V-shaped') case. The proposal requires no direct causal influence outside lightcones, and may hence offer a way to reconcile Bell nonlocality and relativity. The main argument is a detailed version of an approach previously outlined in arXiv:2404.13928 [quant-ph].
We show that Bell correlations may arise as a special sort of selection artefact, produced by ordinary control of the initial state of the experiments concerned. This accounts for nonlocality, without recourse to any direct spacelike causality or influence. The argument improves an earlier proposal in (arXiv:2101.05370v4 [quant-ph], arXiv:2212.06986 [quant-ph]) in two main respects: (i) in demonstrating its application in a real Bell experiment; and (ii) in avoiding the need for a postulate of retrocausality. This version includes an Appendix, discussing the relation of the proposal to the conclusions of Wood and Spekkens (arXiv:1208.4119 [quant-ph]).
In this piece, written for a general audience, we propose a mechanism for quantum entanglement. The key ingredient is collider bias. In the language of causal models, a collider is a variable causally influenced by two or more other variables. Conditioning on a collider typically produces non-causal correlations between its contributing causes. This phenomenon can produce associations analogous to Bell correlations, in suitable post-selected ensembles. Such collider artefacts may become real connections, resembling causality, if a collider is 'constrained' (e.g., by a future boundary condition). We consider the time-reversed analogues of these points in the context of retrocausal models of QM. Retrocausality yields a collider at the source of an EPR-Bell particle pair, and in this case constraint of the collider is possible by normal methods of experimental preparation. It follows that connections resembling causality may emerge across such colliders, from one branch of the experiment to the other. Our hypothesis is that this constrained retrocausal collider bias is the origin of entanglement. This piece is based on a suggestion first made in arXiv:2101.05370v4 [quant-ph], and is an ancestor of an essay now published online in Aeon magazine [Price & Wharton 2023a]. In an updated version of the argument in arXiv:2309.10969 [quant-ph] we (i) demonstrate its application in a real Bell experiment; and (ii) show that we can do without an explicit postulate of retrocausality
In this piece, written for a general audience, we propose a mechanism for quantum entanglement. The key ingredient is the familiar statistical phenomenon of collider bias, or Berkson's bias. In the language of causal models a collider is a variable causally influenced by two or more other variables. Conditioning on a collider typically produces non-causal associations between its contributing causes, even if they are actually independent. It is easy to show that this phenomenon can produce associations analogous to Bell correlations, in suitable post-selected ensembles. It is also straightforward that such collider artefacts may become real connections, resembling causality, if a collider is 'constrained' (e.g., by a future boundary condition). We consider the time-reversed analogues of these points in the context of retrocausal models of QM. Retrocausality yields a collider at the source of an EPR-Bell particle pair, and in this case constraint of the collider is possible by normal methods of experimental preparation. It follows that connections resembling causality may be expected to emerge across such colliders, from one branch of the experiment to the other. Our hypothesis is that this constrained retrocausal collider bias is the origin of entanglement. The piece is based on a suggestion first made in arXiv:2101.05370 [quant-ph].
Price and Wharton have recently suggested that "constrained retrocausal collider bias is the origin of entanglement." In this paper, we argue that their connection across a constrained collider (CCC) for the V-shaped case with the Bell states is not "a mechanism for entanglement," providing a negative answer to the title of arXiv:2406.04571. Rather, CCC should be viewed as a novel approach to the causal modeling of entanglement, providing a perspectival co-causal relationship between the two wings of the experiment that does not violate locality or statistical independence. The key is to accept that quantum mechanics is complete, so the Bell states provide the "mechanism for entanglement" and CCC provides a causal model of entanglement per causal perspectivalism in accord with the different subjective spacetime models of the experiment. When combined into an objective spacetime model of the experiment, the subjective co-causation disappears leaving an objective acausal view of entanglement in accord with the quantum reconstruction program. Essentially, the quantum reconstruction program has rendered quantum mechanics a principle theory based on the observer-independence of Planck's constant h as justified by the relativity principle ("no preferred reference frame" NPRF), exactly as special relativity is a principle theory based on the observer-independence of the speed of light c as justified by the relativity principle. Thus, NPRF + c is an adynamical global constraint on the configuration of worldtubes for the equipment in the experiment while NPRF + h is an adynamical global constraint on the distribution of quantum detection events in that context. Accordingly, CCC then provides a new way to understand causation subjectively per causal perspectivalism in this objectively acausal spacetime model of the experiment.
Methods of selecting samples from larger populations may produce bias, in either direction: inducing correlations between variables independent in the full population, or masking correlations between variables dependent in the full population. Here we propose a surprising application of these familiar ideas. We argue that they are relevant to puzzling correlations uncovered in quantum theory by John Stewart Bell (Bell 1964). In the light of Bell's work and subsequent experiments it is widely believed that the quantum world is 'nonlocal', in apparent tension with relativity. Many hold that the only alternative is to abandon 'realism', the view that there is an objective world independent of measurement. We propose instead that Bell's correlations are selection artefacts, in tension neither with relativity nor realism. In standard two-particle Bell experiments the relevant selection is a preselection, achieved by the preparation of the initial state. Again, selection bias via preselection is familiar elsewhere in science, but it doesn't seem to have been noticed that it is applicable in this case.