A new study demonstrates that the no-superluminal-signalling (NSS) principle rules out operationally detectable causal loops in a broad class of conical spacetimes, including Minkowski spacetime with more than one spatial dimension. This finding contrasts with previous work suggesting the possibility of such loops in (1+1)-Minkowski spacetime without violating NSS, indicating that the relationship between NSS and the absence of causal loops inherently depends on spacetime geometry.

The no-superluminal-signalling (NSS) principle is a cornerstone of relativistic physics, stating that information cannot travel faster than light. Causal loops, on the other hand, imply that an event can influence its own past, posing fundamental paradoxes. The question of whether these two principles are intrinsically compatible or if one implies the other has been an active area of research, especially in contexts where causality might be ambiguous, such as in quantum gravity or certain spacetime configurations.

The current research resolves an open question by showing that, in conical spacetimes with more than one spatial dimension, NSS is sufficient to rule out all operationally detectable causal loops. This applies to classical, quantum, and post-quantum theories alike. The method used to reach this conclusion involves a rigorous analysis of the causal properties of these spacetimes, demonstrating how the geometric structure imposes fundamental restrictions on information propagation and, therefore, on the possibility of causal loops.

This result highlights the importance of spacetime geometry in the formulation of fundamental principles of physics. It suggests that causality and relativity cannot be considered in isolation from the metric structure of the universe. The implications of this work could extend to understanding causality in quantum gravity theories and in designing experiments to test the limits of relativity and quantum information in different spatial configurations.