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arXiv:2510.26562 · Physical Review Research

Limits of absoluteness of observed events in timelike scenarios: A no-go theorem

Sumit Mukherjee, Jonte R. Hance

Published Mar 30, 2026• 0 citations
arXivHTMLPDF

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 , we introduce the , a time-ordered analog of it. In this framework, we replace the usual locality assumption with and show that, when combined with the assumptions of (AOE), , and , we obtain a Causal Time-Symmetric Friendliness 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.

DOI
10.1103/1c6t-45g8

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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 , we introduce the , a time-ordered analog of it. In this framework, we replace the usual locality assumption with and show that, when combined with the assumptions of (AOE), , and , we obtain a Causal Time-Symmetric Friendliness 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.
Does quantum theory apply at all scales, including that of observers? New light on this fundamental question has recently been shed through a resurgence of interest in the long-standing Wigner’s friend paradox. This is a thought experiment addressing the quantum measurement problem—the difficulty of reconciling the (unitary, deterministic) evolution of isolated systems and the (non-unitary, probabilistic) state update after a measurement. Here, by building on a scenario with two separated but entangled friends introduced by Brukner, we prove that if quantum evolution is controllable on the scale of an observer, then one of ‘No-Superdeterminism’, ‘Locality’ or ‘Absoluteness of Observed Events’—that every observed event exists absolutely, not relatively—must be false. We show that although the violation of Bell-type inequalities in such scenarios is not in general sufficient to demonstrate the contradiction between those three assumptions, new inequalities can be derived, in a theory-independent manner, that are violated by quantum correlations. This is demonstrated in a proof-of-principle experiment where a photon’s path is deemed an observer. We discuss how this new theorem places strictly stronger constraints on physical reality than Bell’s theorem. For a scenario of two separated but entangled observers, inequalities are derived from three fundamental assumptions. An experiment shows that these inequalities can be violated if quantum evolution is controllable on the scale of an observer.
Abstract The famous ‘Wigner’s friend’ paradox highlights the difficulty of modelling the evolution of quantum systems under measurement in situations where observers themselves are considered to be subject to the laws of quantum mechanics. In recent years, variations of the original Wigner’s friend paradox have been recognized as fruitful arenas for probing the foundations of quantum theory. In particular (Bong et al 2020 Nat. Phys. 16 1199) demonstrated a contradiction between a set of intuitive assumptions called ‘Local Friendliness’ (LF) and certain quantum phenomena on an extended version of the Wigner’s friend paradox. The LF assumptions can be understood as the conjunction of two independent assumptions: Absoluteness of Observed Events requires that any event observed by any observer has an absolute, rather than relative, value; Local Agency is the assumption that an intervention cannot be correlated with relevant events outside its future light cone. These assumptions are weaker than the assumptions that lead to Bell’s theorem, and thus while the LF result may be considered to be conceptually comparable to Bell’s result, its implications are even deeper. The proof of the LF no-go theorem, however, relies on probability theory, and a fundamental question remained whether or not LF is an inherently statistical concept. Here we present a probability-free version of the LF theorem, building upon Hardy’s no-go theorem for local hidden variables. The argument is phrased in the language of possibilities, which we make formal by using a modal logical approach. It relies on a weaker version of Local Agency, which we call ‘Possibilistic Local Agency’: the assumption that an intervention cannot affect the possibilities of events outside its future light cone.
The renowned Local Friendliness no-go theorem demonstrates the incompatibility of quantum theory with the combined assumptions of Absoluteness of Observed Events - the idea that observed outcomes are singular and objective - and Local Agency - the requirement that the only events correlated with a setting choice are in its future light cone. Granted that the Local Friendliness scenario can be realized, this result is stronger than Bell's theorem because the assumptions of Local Friendliness are weaker than those of Bell's theorem: Local Agency is less restrictive than local causality, and Absoluteness of Observed Events is encompassed within the notion of realism assumed in Bell's theorem. Drawing inspiration from the correspondence between nonlocality proofs in Bell scenarios and generalized contextuality proofs in prepare-and-measure scenarios, we present the Noncontextual Friendliness no-go theorem. This theorem demonstrates the inconsistency of quantum theory with the joint assumptions of Absoluteness of Observed Events and Noncontextual Agency, the latter being a weaker version of noncontextuality, in the same way that Local Agency is a weaker version of local causality. Our result generalizes the Local Friendliness no-go theorem and, granted that the scenario can be realized, is stronger than no-go theorems based on generalized noncontextuality.
The notion of measurements is central to many debates in quantum mechanics. One critical point is whether a measurement can be regarded as an absolute event, giving the same result for any observer in an irreversible manner. Using ideas from the gedanken experiment of Wigner's friend, it has been argued that, when combined with the assumptions of locality and no superdeterminism, regarding a measurement as an absolute event is incompatible with the universal validity of quantum mechanics. We consider a weaker assumption: Is the measurement event realized relatively to the observer when he only partially observed the outcome? We proposed a protocol to show that this assumption in conjunction with the natural assumptions of no superdeterminism and locality is also not compatible with the universal validity of quantum mechanics.
The Wigner’s friend thought experiment has gained a resurgence of interest in recent years thanks to no-go theorems that extend it to Bell-like scenarios. One of these, by us and co-workers, showcased the contradiction that arises between quantum theory and a set of assumptions, weaker than those in Bell’s theorem, which we named ‘Local Friendliness’. Using these assumptions, it is possible to arrive at a set of inequalities for a given scenario, and, in general, some of these inequalities will be harder to violate than the Bell inequalities for the same scenario. A crucial feature of the extended Wigner’s friend scenario in our aforementioned work was the ability of a superobserver to reverse the unitary evolution that gives rise to their friend’s measurement. Here, we present a new scenario where the superobserver can interact with the friend repeatedly in a single experimental instance, either by asking them directly for their result, thus ending that instance, or by reversing their measurement and instructing them to perform a new one. We show that, in these scenarios, the Local Friendliness inequalities will always be the same as Bell inequalities.
Recent no-go theorems based on extended Wigner's Friend scenarios (EWFS) reveal a deep tension between the universal validity of quantum theory and local friendliness (LF), the conjunction of three natural assumptions: the absoluteness of observed events, locality, and no-superdeterminism. In a recent work, Adlam argues that this tension can be dissolved by appealing to quantum reference frames (QRFs). We present and critically assess her proposal. We show that her proposed resolution does not arise from QRFs, but instead relies on three independent modifications: certain degrees of freedom are always definite, observers can flip upon measuring spins, and the outcomes the superobservers record and use to test the inequalities are not their friends' observed results. The proposal and the arguments for the plausibility of these modifications rest on several misconceptions about QRFs and EWFS, which we rectify. We conclude that, within standard quantum theory, quantum reference frames do not evade the EWFS no-go theorems.