The Causal Structure of the World and the Difference between Past and Future

Sally Shrapnel, Fabio Costa

Realist interpretations of quantum mechanics presuppose the existence of elements of reality that are independent of the actions used to reveal them. Such a view is challenged by several no-go theorems that show quantum correlations cannot be explained by non-contextual ontological models, where physical properties are assumed to exist prior to and independently of the act of measurement. However, all such contextuality proofs assume a traditional notion of causal structure, where causal influence flows from past to future according to ordinary dynamical laws. This leaves open the question of whether the apparent contextuality of quantum mechanics is simply the signature of some exotic causal structure, where the future might affect the past or distant systems might get correlated due to non-local constraints. Here we show that quantum predictions require a deeper form of contextuality: even allowing for arbitrary causal structure, no model can explain quantum correlations from non-contextual ontological properties of the world, be they initial states, dynamical laws, or global constraints.

B. F. McGuinness, Friedrich Waismann

D. I. Blokhint︠s︡ev, Yu.A. Gol'fand, Владимир Георгиевич Кадышевский +2 more

Dieter Eigler

This paper examines the possibility that past and future events are fundamentally alike in their ontological openness, while differing in their causal accessibility. The conventional distinction between a fixed past and an open future becomes problematic in light of the relativity of simultaneity, which provides no universal present at which openness could turn into ontological definiteness. As an alternative to the fully determined history of a block universe, the paper considers a symmetric conception in which neither past nor future is necessarily ontologically invariant. The proposed distinction separates ontological openness from causal accessibility. While past events cannot be influenced from our present position, parts of the causal future remain accessible to intervention. This asymmetry is compatible with the invariant causal structure of relativistic spacetime and does not require a universal transition from openness to definiteness. Schrödinger's cat provides a further illustration. If the cat is found dead and already in rigor mortis, the observed traces indicate an earlier death. Nevertheless, this does not by itself establish when the corresponding history became ontologically definite. The paper explores the possibility that a later measurement may be associated with the ontological articulation of an earlier, causally ordered history, without requiring backward causation. The argument is situated in relation to relational quantum mechanics, Wheeler's delayed-choice experiments, and philosophical discussions of retrocausality. The paper does not claim to prove an ontologically open past. Rather, it develops this possibility as a serious philosophical alternative to an invariant block universe, distinguishing physical evidence from ontological interpretation and clarifying the conceptual limits of what can be established.

Mateus Araújo, Cyril Branciard, Fabio Costa +3 more

Our common understanding of the physical world deeply relies on the notion that events are ordered with respect to some time parameter, with past events serving as causes for future ones. Nonetheless, it was recently found that it is possible to formulate quantum mechanics without any reference to a global time or causal structure. The resulting framework includes new kinds of quantum resources that allow performing tasks—in particular, the violation of causal inequalities —which are impossible for events ordered according to a global causal order. However, no physical implementation of such resources is known. Here we show that a recently demonstrated resource for quantum computation—the quantum switch —is a genuine example of ‘indefinite causal order’. We do this by introducing a new tool—the causal witness —which can detect the causal nonseparability of any quantum resource that is incompatible with a definite causal order. We show however that the quantum switch does not violate any causal inequality.

Robert S. Cohen, John J. Stachel