Recent no-go theorems for absolute facts in single-world interpretations are widely considered the strongest arguments in favour of ‘quantum relativism’: interpretations according to which measurement results are observer-relative. In this article, however, I show that relativist interpretations are themselves vulnerable to mathematically identical ‘revenge’ theorems, unless they assume a particularly radical form. To this end, a novel distinction between tame and feral varieties of quantum relativism is introduced, where a relativist interpretation counts as tame if and only if it postulates absolute facts about relative facts (absolute facts of second order) and the possibility of communication across perspectives. While at first sight preferable to feral views, tame relativism turns out to be subject to revenge problems: arguments against absolute measurement outcomes can be modified and turned against absolute facts about which outcome a measurement has relative to a particular observer. This observation is developed in detail for the case of the ‘local friendliness’ extended Wigner’s friend theorem. Moreover, it is shown to generalize to absolute facts about the facts that obtain relative to sequences of specified reference systems of arbitrary length. This suggests that only the most radical types of relativist interpretations have a hope of being motivated with recourse to extended Wigner’s friend scenarios.
Recent no-go theorems for absolute facts in single-world interpretations are widely considered the strongest arguments in favour of ‘quantum relativism’: interpretations according to which measurement results are observer-relative. In this article, however, I show that relativist interpretations are themselves vulnerable to mathematically identical ‘revenge’ theorems, unless they assume a particularly radical form. To this end, a novel distinction between tame and feral varieties of quantum relativism is introduced, where a relativist interpretation counts as tame if and only if it postulates absolute facts about relative facts (absolute facts of second order) and the possibility of communication across perspectives. While at first sight preferable to feral views, tame relativism turns out to be subject to revenge problems: arguments against absolute measurement outcomes can be modified and turned against absolute facts about which outcome a measurement has relative to a particular observer. This observation is developed in detail for the case of the ‘local friendliness’ extended Wigner’s friend theorem. Moreover, it is shown to generalize to absolute facts about the facts that obtain relative to sequences of specified reference systems of arbitrary length. This suggests that only the most radical types of relativist interpretations have a hope of being motivated with recourse to extended Wigner’s friend scenarios.
Recent no-go theorems challenging the absoluteness of events have highlighted the possibility that dynamical facts in quantum theory are observer-relative. This thesis examines the metaphysical and conceptual foundations of quantum relativism, focusing on Relational Quantum Mechanics (RQM). It analyzes arguments for relativism through Wigner’s Friend and Extended Wigner’s Friend thought experiments, explores the link between relative physical facts and experiences, and offers the first metaphysical definition of "relative facts." The thesis argues: RQM supports a perspectival retro-determination of past events based on later acquisition of information; Quantum relativists must also relativize facts about observers’ experiences; the strongest arguments for relativism lead to unbounded higher-order relativity, precluding any absolute 'view from nowhere'; Only a metaphysical framework that relativized the composition of reality itself can accommodate such an unrestricted iteration. These conclusions deepen our grasp of a thoroughly perspectival quantum world.
I take a pragmatist perspective on quantum theory. This is not a view of the world described by quantum theory. In this view quantum theory itself does not describe the physical world, nor our observatons, experiences or opinions of it. Instead, the theory offers reliable advice on when to expect an event of one kind or another, and on how strongly to expect each possible outcome of that event. The actual outcome is a perspectival fact: a fact relative to a physical context of assessment. Measurement outcomes and quantum states are both perspectival. By noticing that each must be relativized to an appropriate physical context one can resolve the measurement problem and the problem of nonlocal action. But if the outcome of a quantum measurement is not an absolute fact, then why shoud the statistics of such outcomes give us any objective reason to accept quantum theory? One can describe extensions of the scenario of Wigner's friend in which a statement expressing the outcome of a quantum measurement would be true relative to one such context but not relative to another. However, physical conditions in our world prevent us from realizing such scenarios. Since the outcome of every actual quantum measurement is certified at what is essentially a single context of assessment, the outcome relative to that context is an objective fact in the only sense that matters for science. We should accept quantum theory because the statistics these outcomes display are just those it leads us to expect.
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.
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.
In his famous thought experiment, Wigner assigns an entangled state to the composite quantum system made up of Wigner's friend and her observed system. While the two of them have different accounts of the process, each Wigner and his friend can in principle verify his/her respective state assignments by performing an appropriate measurement. As manifested through a click in a detector or a specific position of the pointer, the outcomes of these measurements can be regarded as reflecting directly observable "facts". Reviewing arXiv:1507.05255, I will derive a no-go theorem for observer-independent facts, which would be common both for Wigner and the friend. I will then analyze this result in the context of a newly-derived theorem arXiv:1604.07422, where Frauchiger and Renner prove that "single-world interpretations of quantum theory cannot be self-consistent". It is argued that "self-consistency" has the same implications as the assumption that observational statements of different observers can be compared in a single (and hence an observer-independent) theoretical framework. The latter, however, may not be possible, if the statements are to be understood as relational in the sense that their determinacy is relative to an observer.
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.