I argue that the Extended Wigner's Friend theorems have a `loophole' that has previously not been appreciated, coming from the fact that we typically need a reference system to give meaning to a measurement outcome. I show how this loophole can be used in the context of the Local Friendliness theorem to preserve the predictions of quantum mechanics without denying Local Friendliness, and I respond to some objections. I explain how the loophole arises naturally from consideration of relevant symmetry transformations, and then I appeal to the partial/complete observables formalism to argue that the same loophole will work for any type of Wigner's Friend experiment, even if it is performed with a variable that does not obviously look like it requires relativization to a reference system. Finally I compare the loophole to other possible responses to the Local Friendliness theorem, and I show that the same loophole will work for other Extended Wigner's Friend theorems too.
Comments
Some of the material in this article is drawn from a previous paper, arxiv 2512.07101. However this new version is significantly reworked in response to discussions about the previous version. It also responds to a paper which responds to the original version, so for clarity I am publishing this as a separate preprint
I argue that the Extended Wigner's Friend theorems have a `loophole' that has previously not been appreciated, coming from the fact that we typically need a reference system to give meaning to a measurement outcome. I show how this loophole can be used in the context of the Local Friendliness theorem to preserve the predictions of quantum mechanics without denying Local Friendliness, and I respond to some objections. I explain how the loophole arises naturally from consideration of relevant symmetry transformations, and then I appeal to the partial/complete observables formalism to argue that the same loophole will work for any type of Wigner's Friend experiment, even if it is performed with a variable that does not obviously look like it requires relativization to a reference system. Finally I compare the loophole to other possible responses to the Local Friendliness theorem, and I show that the same loophole will work for other Extended Wigner's Friend theorems too.
This article suggests that thinking about the role of reference frames can provide new insight into Extended Wigner's Friend scenarios. This involves appealing to symmetries to make a principled distinction between properties of a system which are meaningful only relative to an external reference system and properties which are meaningful without further relativization. Thus we may propose that there are always well-defined facts about what observers have observed, but there are not necessarily well-defined facts about the relations between their reference frames, so there will not always exist a joint distribution over their outcomes which can meaningfully be compared to the predictions of quantum mechanics. In addition, this approach also offers a general argument against the idea that there should be a regress of relativization.
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.
The famous Wigner's friend experiment considers an observer-the friend-and a superobserver-Wigner-who treats the friend as a quantum system and her interaction with other quantum systems as unitary dynamics. This is at odds with the friend describing this interaction via collapse dynamics, if she interacts with the quantum system in a way that she would consider a measurement. These different descriptions constitute the Wigner's friend paradox. Extended Wigner's friend experiments combine the original thought experiment with non-locality setups. This allows for deriving local friendliness inequalities, similar to Bell's theorem, which can be violated for certain extended Wigner's friend scenarios. A Wigner's friend paradox and the violation of local friendliness inequalities require that no classical record exists, which reveals the result the friend observed during her measurement. Otherwise, Wigner agrees with his friend's description and no local friendliness inequality can be violated. In this article, I introduce classical communication between Wigner and his friend and discuss its effects on the simple as well as extended Wigner's friend experiments. By controlling the properties of a (quasi) classical communication channel between Wigner and the friend, one can regulate how much outcome information about the friend's measurement is revealed. This gives a smooth transition between the paradoxical description and the possibility of violating local friendliness inequalities, on the one hand, and the effectively collapsed case, on the other hand.
Two recent, prominent theorems-the "no-go theorem for observer-independent facts" and the "Local Friendliness no-go theorem"-employ so-called extended Wigner's friend scenarios to try to impose novel, non-trivial constraints on the possible nature of physical reality. While the former is argued to entail that there can be no theory in which the results of Wigner and his friend can both be considered objective, the latter is said to place on reality stronger constraints than the Bell and Kochen-Specker theorems. Here, I conduct a thorough analysis of these theorems and show that they suffer from a list of shortcomings that question their validity and limit their strength. I conclude that the "no-go theorem for observer-independent facts" and the "Local Friendliness no-go theorem" fail to impose significant constraints on the nature of physical reality.
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.
In their strongest usage, quantum reference frames have been described as referring to "the measurements performed by a superposed lab", "the perspective of a quantum particle", "the point of view of a superposed observer", etc. While exciting, these operational proposals have remained brief and ambiguous, leading to misinterpretations and criticism. Here, we provide a detailed specification and defense of the notion of a position-superposed lab or observer. We argue that this requires no exotic claims about quantum physics and raises no greater interpretive difficulty than ordinary quantum measurements. We then derive several consequences of taking this operational meaning seriously. We stress that the position-superposed observers that define quantum references frames are different from, and considerably less problematic than, the outcome-superposed observers considered in Wigners' friend scenarios. In particular, we show that outcomes obtained by a position-superposed observer may (without decohering the superposition) be broadcast to a well-localised one, in contrast with Wigner's friend scenarios, which require the outcomes to remain internal to the system at hand. Finally, we defend the possibility to roleplay a quantum reference frame from a classical reference frame.