# Operational causality in spacetime

## TL;DR

This paper introduces the 'dynamical no-signalling principle' which constrains superluminal communication in physical theories and demonstrates its implications using quantum dynamics, highlighting its universal applicability across classical, quantum, and post-quantum models.

## Contribution

It unveils a new principle that limits superluminal signaling via evolving physical objects, applicable to any theory with well-defined detection statistics in spacetime.

## Key findings

- The principle constrains admissible models of dynamics.
- It can be used to identify limits of quantum and post-quantum models.
- An example shows how Schrödinger dynamics could enable superluminal protocols.

## Abstract

The no-signalling principle preventing superluminal communication is a limiting paradigm for physical theories. Within the information-theoretic framework it is commonly understood in terms of admissible correlations in composite systems. Here we unveil its complementary incarnation --- the 'dynamical no-signalling principle' ---, which forbids superluminal signalling via measurements on simple physical objects (e.g. particles) evolving in time. We show that it imposes strong constraints on admissible models of dynamics. The posited principle is universal --- it can be applied to any theory (classical, quantum or post-quantum) with well-defined rules of calculating detection statistics in spacetime. As an immediate application we show how one could exploit the Schr\"odinger equation to establish a fully operational superluminal protocol in the Minkowski spacetime. This example illustrates how the principle can be used to identify the limits of applicability of a given model of quantum or post-quantum dynamics.