Limits of Absoluteness of Observed Events in Timelike Scenarios: A No-Go Theorem

TL;DR

This paper introduces a new paradox and causal inequality related to quantum events' absoluteness, showing quantum mechanics violates classical assumptions even under weaker conditions, thus deepening foundational insights.

Contribution

It develops the Causal Friendliness Paradox and a related causal inequality, extending no-go theorems to timelike scenarios and analyzing assumptions about event absoluteness.

Findings

Quantum mechanics violates the causal inequality under the proposed framework.

Weaker operational assumptions of AOE still lead to violations by quantum theory.

The results reinforce the non-classical nature of observed events in quantum foundations.

Abstract

Wigner's Friend-type paradoxes challenge the assumption that events are absolute -- that when we measure a system, we obtain a single result, which is not relative to anything or anyone else. These paradoxes highlight the tension between quantum theory and our intuitions about reality being observer-independent. Building on a recent result that developed these paradoxes into a no-go theorem, namely the Local Friendliness Theorem, we introduce the Causal Friendliness Paradox, a time-ordered analogue of it. In this framework, we replace the usual locality assumption with Axiological Time Symmetry (ATS), and show that, when combined with the assumptions of Absoluteness of Observed Events (AOE), No Retrocausality (NRC), and Screening via Pseudo Events (SPE), we obtain a causal inequality. We then show that quantum mechanics violates this inequality and is therefore incompatible with at least one of these assumptions. To probe which assumption might be incompatible, we then examine whether AOE in its entirety is essential for this no-go result. We propose a weaker, operational form of AOE that still leads to inequalities that quantum mechanics violates. This result shows that even under relaxed assumptions, quantum theory resists reconciliation with classical notions of absolute events, reinforcing the foundational significance of Wigner's Friend-type paradoxes in timelike scenarios.