Causality in Time-Neutral Cosmologies

TL;DR

This paper analyzes time-neutral cosmological models allowing superluminal signalling, showing they can be internally consistent and challenge traditional views of causality in relativistic quantum theory.

Contribution

It demonstrates that Gell-Mann and Hartle's time-neutral models can permit superluminal signals without paradoxes, questioning the necessity of Minkowski causality in quantum theory.

Findings

Superluminal signalling is possible in these models.

Signal transmission probability can approach one with redundancy.

Standard causality paradoxes do not apply due to quantum measurement context dependence.

Abstract

Gell-Mann and Hartle (GMH) have recently considered time-neutral cosmological models in which the initial and final conditions are independently specified, and several authors have investigated experimental tests of such models. We point out here that GMH time-neutral models can allow superluminal signalling, in the sense that it can be possible for observers in those cosmologies, by detecting and exploiting regularities in the final state, to construct devices which send and receive signals between space-like separated points. In suitable cosmologies, any single superluminal message can be transmitted with probability arbitrarily close to one by the use of redundant signals. However, the outcome probabilities of quantum measurements generally depend on precisely which past {\it and future} measurements take place. As the transmission of any signal relies on quantum measurements, its transmission probability is similarly context-dependent. As a result, the standard superluminal signalling paradoxes do not apply. Despite their unusual features, the models are internally consistent. These results illustrate an interesting conceptual point. The standard view of Minkowski causality is not an absolutely indispensable part of the mathematical formalism of relativistic quantum theory. It is contingent on the empirical observation that naturally occurring ensembles can be naturally pre-selected but not post-selected.