There has recently been a renewed focus on ‘perspectival’ quantum theories. which simultaneously maintain the existence of single measurement outcomes and the universality of unitary evolution. At the same time, these theories have come under attack with results by Frauchiger and Renner, Baumann and Wolf, and others. This dissertation aims to respond to a number of these attacks by providing a groundwork for these types of theories. To lay this groundwork I focus on encapsulated measurements, which involve an isolated observer and a superobserver (who measures the observer). I first distinguish between invasive and non-invasive measurements. Each leads to a possible inconsistency: In non-invasive measurements, the observer is certain of the superobserver’s measurement outcome while the superobserver’s physics predicts multiple possible outcomes. In invasive measurements the superobserver can be certain of his measurement outcome while the observer predicts non-zero probabilities for all possible outcomes. I argue that in the case of non-invasive measurements, the perspectivalist avoids diffculty by denying that the observer’s result has any impact on the physics experienced by the superobserver. Consistency is then maintained between them by looking to the unitary evolution of the superobserver’s measurement. This response leads to a detailed discussion about the metaphysical commitments of the perspectival approach. Here I argue the perspectivalist must accept one surprising result – there is a significant divorce of fundamental ontological states from physical dynamics. Turning to invasive measurements, I argue that the concern here is entirely misplaced. Arguments that raise worries about invasive measurements assume the observer should describe herself to be in a quantum state of having observed her measurement outcome when predicting the superobserver’s measurement results. I argue that this is incorrect. Rather, I explain that it is impossible for any observer to know her quantum state and so she should never describe herself as being in any quantum state at all, let alone use such a description to make predictions about a superobserver’s measurement. To conclude, I explain how the perspectivalist responds to concerns raised about entanglement and the possibility of action at a distance. Combining this with the results above brings into focus how the perspectivalist may develop a consistent, single-world picture of quantum mechanics.
There has recently been a renewed focus on ‘perspectival’ quantum theories. which simultaneously maintain the existence of single measurement outcomes and the universality of unitary evolution. At the same time, these theories have come under attack with results by Frauchiger and Renner, Baumann and Wolf, and others. This dissertation aims to respond to a number of these attacks by providing a groundwork for these types of theories. To lay this groundwork I focus on encapsulated measurements, which involve an isolated observer and a superobserver (who measures the observer). I first distinguish between invasive and non-invasive measurements. Each leads to a possible inconsistency: In non-invasive measurements, the observer is certain of the superobserver’s measurement outcome while the superobserver’s physics predicts multiple possible outcomes. In invasive measurements the superobserver can be certain of his measurement outcome while the observer predicts non-zero probabilities for all possible outcomes. I argue that in the case of non-invasive measurements, the perspectivalist avoids diffculty by denying that the observer’s result has any impact on the physics experienced by the superobserver. Consistency is then maintained between them by looking to the unitary evolution of the superobserver’s measurement. This response leads to a detailed discussion about the metaphysical commitments of the perspectival approach. Here I argue the perspectivalist must accept one surprising result – there is a significant divorce of fundamental ontological states from physical dynamics. Turning to invasive measurements, I argue that the concern here is entirely misplaced. Arguments that raise worries about invasive measurements assume the observer should describe herself to be in a quantum state of having observed her measurement outcome when predicting the superobserver’s measurement results. I argue that this is incorrect. Rather, I explain that it is impossible for any observer to know her quantum state and so she should never describe herself as being in any quantum state at all, let alone use such a description to make predictions about a superobserver’s measurement. To conclude, I explain how the perspectivalist responds to concerns raised about entanglement and the possibility of action at a distance. Combining this with the results above brings into focus how the perspectivalist may develop a consistent, single-world picture of quantum mechanics.
The production of events in quantum theory, that is, of outcomes to which a probability can be associated, is inconsistent with a unitary evolution for systems in arbitrary initial states. In particular, within unitary quantum mechanics, there exist global protocols that allow verifying that no definite event occurs. Instead, states that start in a coherent superposition over possible outcomes always remain as a superposition. We show that when taking into account fundamental errors in measuring length and time intervals, which have been put forward as a consequence of a conjunction of quantum mechanical and general relativity arguments, there are instances in which such global protocols no longer allow one to distinguish whether or not the state is in a superposition. All predictions become identical as if one of the outcomes occurs, with probability determined by the state. We use this as a criteria to define events, as put forward in the Montevideo interpretation of quantum mechanics. We analyze in detail the occurrence of events in the paradigmatic case of a particle in a superposition of two different locations. We argue that our approach provides a consistent (C) single-world (S) picture of the universe, thus allowing an economical way out of the limitations imposed by a recent theorem by Frauchiger and Renner showing that having a self-consistent single-world description of the universe is incompatible with quantum theory. In fact, the main observation of this paper may be stated as follows: If quantum mechanics is extended to include gravitational effects to a quantum-gravitational (QG) theory, then QG, S, and C are satisfied.
This paper offers a conceptual and formal analysis of non-absoluteness in perspectival interpretations of quantum mechanics. Extended Wigner-type experiments reveal a fundamental tension between the universality of quantum mechanics, the absoluteness of observed events, and the possibility of intersubjective agreement. I argue that denying observer-independent events cannot remain a mere interpretive slogan: it requires a precise account of how perspectival statements, tracking assumptions, meta-level descriptions, and intersubjective communication are to be formulated. Many existing approaches either implicitly reintroduce observer-independent structure or leave key notions underdetermined. Within this framework, Convivial Solipsism provides a coherent account of non-absolute events, preserving the universality of quantum mechanics while rejecting the need for a global viewpoint. To make this claim precise, the paper introduces the basic elements of a formal perspectival logic designed to distinguish well-formed perspectival statements from illegitimate unindexed or absolutizing statements. The paper concludes that scientific objectivity need not rest on observer-independent facts, but can instead be grounded in the internal coherence of perspectival structures.
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.
Few, if any, issues in physics have engendered as much discussion as the measurement problem in quantum mechanics. It is generally agreed that the `normal' dynamical evolution of the state vector in quantum mechanics is given by a unitary map. The linearity of this map implies that the state vector will, in general, be found in a superposition of eigenstates of a given observable (or, similarly, that the density matrix describing a subsystem will not correspond to a definite value of this observable). However, when we make a measurement of an observable, we always obtain a define value—although it is impossible to predict with certainty which value will be obtained. The traditional response to this issue is to postulate that when a measurement is made, the wavefunction `collapses' to an eigenstate of the observable being measured, perhaps due to the inherent classicality of the measuring apparatus (Bohr), or to the consciousness of the observer (Wigner), or possibly to some modification of quantum dynamics that occurs even when observations are not being made. The main motivation for the Everett (`many worlds') interpretation is to avoid introducing any such collapse postulate. This volume commemorates the 50th anniversary of the publication of Everett's paper in 1957 and contains 20 original articles as well as the transcripts of several discussions that took place at meetings devoted to the Everett interpretation at Oxford University and the Perimeter Institute. The attractiveness of the Everett interpretation is very succinctly summarized by a sentence from Vaidman's contribution (p 582): `The collapse, with its randomness, non-locality and the lack of a well-defined moment of occurrence, is such an ugly scar on quantum theory, that I, along with many others, am ready to follow Everett and deny its existence.' But the main drawback of the interpretation is then equally succinctly stated in the next sentence: `The price is the many worlds interpretation, i.e., the existence of numerous parallel worlds.' In other words, if the state vector evolves to have a non-zero amplitude for two different values of an observable that I `measure' (i.e., interact with in an appropriate way), each of these alternatives remains equally `real', i.e., there is a `branch' of the wavefunction in which the observable takes the first value and I believe that it has this first value and a branch of the wavefunction where it takes the second value and I believe that it has this second value. Each of these `me's' sees a definite value for the observable, thus nicely accounting for what I believe I experience without invoking collapse. But then, it seems that I have to believe that there are many other `me's' around—even if they are very difficult for this particular me to access—and no way I can predict which `me' I will be in the future. Indeed, Saunders writes on p 192, `If Alice is a person (of course she is) then we must say, even prior to branching at tj that there are many Alices present, atom for atom duplicates...Alice should be uncertain after all— each Alice should be uncertain—for each as of tj does not (and as a matter of principle, cannot) know which of these branching persons she is.' Thus, if taken seriously, the Everett viewpoint leads directly to questions like `What determines which ``me'' I am now and/or will feel that I am in the future?' The response `You are all of them' somehow doesn't seem satisfying, and further pursuit of these issues would seem to lead to questions as fruitless as questions like `Why am I ``me'' and not ``you''?' I believe that this accounts for why the Everett interpretation does not have universal appeal. But enough of `me' for now (more later); let's discuss the book. Unlike many edited volumes and conference proceedings in physics—which often consist of hastily slapped-together re-hashes of previous papers—this volume meets very high standards in the choice of topics and authors and in the quality of the writing. Although each of the articles is closely tied in with the Everett interpretation, there are many articles of considerable value and interest in their own right, such as the discussion of quasiclassical realms by Hartle and and the discussion of the de Broglie–Bohm interpretation by Valentini. There is also an excellent introduction by Saunders to the quantum measurement problem and the main issues that arise in the Everett interpretation. A central issue in the Everett interpretation is the status of the `Born rule', which asserts that, for state ψ, the probability of obtaining a particular outcome of a measurement is ||Pψ||2, where P is the projection operator onto the eigensubspace associated with the measurement outcome. In traditional interpretations, the Born rule is simply postulated as part of the collapse hypothesis. In the Everett interpretation, it is far from obvious that the Born rule even has any meaning—if all outcomes occur, how can one talk about the probability of a particular outcome? Given the importance of this issue, it is highly appropriate that four chapters of the book (by Saunders, Papineau, Wallace, and Greaves and Myrvold) are devoted to addressing probability and the Born rule from the Everett viewpoint, and three chapters (by Kent, Albert, and Price) are devoted to criticising these views. As indicated by the above quote from Saunders' article, uncertainty and probability can enter the Everett interpretation via each Alice being uncertain as to which Alice she is or will be. However, in view of the difficulties to which this kind of argumentation will lead, it is perhaps not surprising that in the 123 pages of combined discussion by Saunders, Papineau, Wallace, and Greaves and Myrvold, there are only fleeting references to the use of probability along these lines. Rather, the approach taken is reminiscent of that of adept politicians: if you can't answer a question, then answer a different question, and answer it with conviction and detail. Instead of considering probabilities of outcomes—a highly problematical notion when all of the possible outcomes actually occur—consider only decision theory issues. In particular, Wallace gives a precise and detailed mathematical proof that, under certain hypotheses, the only rational strategy that a quantum observer can follow in making decisions is to weight the possible outcomes by Born rule probabilities. To illustrate this more graphically, suppose tomorrow's weather were determined by some quantum mechanical, Stern–Gerlach-like process. I would like to know whether it is likely to rain tomorrow. However, when I ask an Everett follower (who knows the state vector and has done the Born rule calculation), all she will tell me in direct response to my question is that `it will rain and it won't rain.' Nevertheless, she is willing tell me whether or not it is rational for me to take an umbrella. I suppose that if she tells me that it would be highly irrational for me to take an umbrella, I can take the hint and deduce that it is very unlikely to rain. However, there does seem something wrong with the fact that she is not allowed to say this directly. In any case, if the conclusion of a mathematically correct argument is that rational decision strategies require the Born rule, then there must be quite a bit lying in the assumptions. The articles by Kent, Albert, and Price do a good job of fleshing out these assumptions and pointing out the weaknesses and flaws in the probability and decision theory discussions within the Everett framework. I hope it is clear from what I have written above that this is an outstanding volume, which should be of great interest to anyone interested in the foundations of quantum theory. Indeed, I feel quite fortunate to be the particular `me' that happens to exist in the same branch of the wavefunction of the universe in which this book was published.
It is argued that quantum measurements do pose a problem, within the context created by the fundamental aim of science, which is identified as the construction of a cohesive, comprehensive, and rationally coherent idea of the nature of the world in which we live. Models of nature are divided into two classes: (1), those in which there is a selection process that, for any possible measurement, would, if that measurement were to be performed, pick out one single outcome, and, (2), all others. It is proved that any model of class that reproduces the predictions of quantum theory must violate the condition that there be no faster-than-light influences of any kind. This result is used to motivate the study of models in which unitary evolution is maintained and there is no selection of unique outcomes. A consideration of ontic probabilities, historical records, and the form of the mind-brain connection leads to an elaboration of the Everett many-worlds interpretation that appears to provide the basis of satisfactory solution of the measurement problem. 18 refs.
Abstract The measurement problem in quantum mechanics is commonly framed as a conflict between unitary dynamics and the appearance of definite outcomes. This paper argues that the conflict arises from a misplaced assumption about what outcomes must be. In experimental practice, outcomes are not arbitrarily precise microscopic states or instantaneous events, but stable physical records produced in measurement interactions. Treating outcomes as such records allows them to be formalized as coarse-grained equivalence classes of physical states that persist in time and support consistent interaction across systems. On this basis, decoherence explains the stability of outcomes without selecting a unique global result or modifying quantum dynamics. Objectivity is recovered through the robustness of records rather than through absolute, observer-independent outcome facts. The measurement problem is thereby dissolved without invoking collapse mechanisms, branching universes, or observer-dependent postulates, while preserving all standard empirical predictions of quantum mechanics.