Theories with no superluminal signaling have greater information-processing power than theories with no superluminal causation

Abstract

A central goal in the foundations of physics is to understand the structure of physical theories, such as quantum theory, from physical principles. This is often explored by considering various information-theoretic principles. Here, we initiate a similar approach considering relativistic causality principles. No superluminal causation (NSC) and no superluminal signalling (NSS) are distinct relativistic principles, requiring, respectively, that causal influence/the ability of agents to signal are within the future lightcone. After formalizing their distinction, we investigate how well theories constrained by NSC and NSS perform in a task that involves generating non-classical correlations. We find a spacetime configuration in which this task cannot be achieved in any theory (classical, quantum, or post-quantum) satisfying NSC. However, we show that theories violating NSC but satisfying NSS can perfectly achieve the task. We give a protocol that would, in a world allowing superluminal causation, enable its operational certification without violating NSS, in general spacetimes. In the case of $(1+1)$D Minkowski spacetime, the task remains achievable in a configuration where measurement outcomes occur arbitrarily earlier in time than the settings, allowing a new form of certifiable retrocausality without violating NSS. We illustrate our results by linking two different types of non-classical post-quantum resources: PR-boxes and jamming. Our work offers insights into the role of different relativistic causality principles in fundamental physics and paves the way for characterising the information-theoretic structure of theories obeying such principles.

6.5 pages + appendix, 2 figures. This paper contains improved versions of initial results included in VV's PhD thesis arXiv:2102.02393. V2 is close to published version, main changes relative to V1: modified figure 1, expanded main text to clarify framework and NSS, NSC definitions