Experimental evidene of the last decades has made the status of "collapses of the wave function" even more shaky than it already was on conceptual grounds: interference effects turn out to be detectable even when collapses are typically expected to occur. Non-collapse interpretations should consequently be taken seriously. In this paper we argue that such interpretations suggest a perspectivalism according to which quantum objects are not characterized by monadic properties, but by relations to other systems. Accordingly, physical systems may possess different properties with respect to different "reference systems". We discuss some of the relevant arguments, and argue that perspectivalism both evades recent arguments that single-world interpretations are inconsistent and eliminates the need for a privileged rest frame in the relativistic case.
Experimental evidene of the last decades has made the status of "collapses of the wave function" even more shaky than it already was on conceptual grounds: interference effects turn out to be detectable even when collapses are typically expected to occur. Non-collapse interpretations should consequently be taken seriously. In this paper we argue that such interpretations suggest a perspectivalism according to which quantum objects are not characterized by monadic properties, but by relations to other systems. Accordingly, physical systems may possess different properties with respect to different "reference systems". We discuss some of the relevant arguments, and argue that perspectivalism both evades recent arguments that single-world interpretations are inconsistent and eliminates the need for a privileged rest frame in the relativistic case.
Abstract The theories of pre-quantum physics are standardly seen as representing physical systems and their properties. Quantum mechanics in its standard form is a more problematic case: here, interpretational problems have led to doubts about the tenability of realist views. Thus, QBists and Quantum Pragmatists maintain that quantum mechanics should not be thought of as representing physical systems, but rather as an agent-centered tool for updating beliefs about such systems. It is part and parcel of such views that different agents may have different beliefs and may assign different quantum states. What results is a collection of agent-centered perspectives rather than a unique representation of the physical world. In this paper we argue that the problems identified by QBism and Quantum Pragmatism do not necessitate abandoning the ideal of representing the physical world. We can avail ourselves of the same puzzle-solving strategies as employed by QBists and pragmatists by adopting a perspectival quantum realism. According to this perspectivalism (close to the relational interpretation of quantum mechanics) objects may possess different, but equally objective properties with respect to different physically defined perspectives. We discuss two options for such a perspectivalism, a local and a nonlocal one, and apply them to Wigner’s friend and EPR scenarios. Finally, we connect quantum perspectivalism to the recently proposed philosophical position of fragmentalism.
It has often been suggested that similar quantum particles may be a counterexample to the principle of the Identity of Indiscernibles. In this article, I discuss the status of indiscernibility in a perspectival approach to quantum mechanics. I argue that adopting an internal physical perspective can have the effect of making entities discernible even if they are not discernible relative to external reference frames, and thus perspectivalism offers a new way to understand the physical meaning of weak discernibility. I discuss whether this approach is circular, and I consider whether the reference frames involved are physically meaningful. I conclude that the Identify of Indiscernibles can probably be maintained in this context, though there are outstanding questions pertaining to the meaning of the quantum state and the way in which a system should be represented in its own reference frame.
The present study investigates the tension between objectivism and perspectivism in light of the conceptual and philosophical implications of quantum theory. The guiding dictum of objectivism posits that science reveals the underlying structure of reality independently of human cognitive activity, that is, in terms of mind-independent, discourse-independent properties and relations occurring at any spatiotemporal location. It is shown that objectivism’s stance fits with the applicability of the bivalence principle as the semantic foundation of scientific discourse. It becomes then apparent that the non-Boolean logical structure of quantum theory systematically disrupts the objectivist framework. Along this vein, we provide succinct argumentation demonstrating quantum theory’s affinity to perspectivist reasoning. It is contended that the ubiquitous phenomenon of quantum contextuality, as established by Kochen-Specker’s celebrated theorem, strongly suggests that measurement values of quantum observables can, in general, be regarded as pertaining to an object under investigation only within a local probing frame or perspective from which the object-system is considered. The last remark is reinforced when considered within the endo-theoretic perspectivist framework of quantum mechanics. The essential philosophical meaning of the latter implies the view that the quantum world can be consistently comprehended through a multilevel structure of overlapping Boolean probing frames, realized as locally variable perspectives on a quantum system, being capable of forming jointly a coherent picture of the whole in a nontrivial way. Finally, in the appendix, the endo-theoretic perspectivist framework of quantum theory is critically compared to the relational interpretation of quantum mechanics, particularly in its recent development involving the postulate of “cross-perspective links”, revealing thereby the profoundly different picture of the quantum world’s architecture between the two proposals.
Paul Busch has emphasized on various occasions the importance for physics of going beyond a merely instrumentalist view of quantum mechanics. Even if we cannot be sure that any particular realist interpretation describes the world as it actually is, the investigation of possible realist interpretations helps us to develop new physical ideas and better intuitions about the nature of physical objects at the micro level. In this spirit, Paul Busch himself pioneered the concept of “unsharp quantum reality”, according to which there is an objective non-classical indeterminacy—a lack of sharpness—in the properties of individual quantum systems. We concur with Busch’s motivation for investigating realist interpretations of quantum mechanics and with his willingness to move away from classical intuitions. In this article we try to take some further steps on this road. In particular, we pay attention to a number of prima facie implausible and counter-intuitive aspects of realist interpretations of unitary quantum mechanics. We shall argue that from a realist viewpoint, quantum contextuality naturally leads to “perspectivalism” with respect to properties of spatially extended quantum systems, and that this perspectivalism is important for making relativistic covariance possible.
Pekka Lahti is a prominent exponent of the renaissance of foundational studies in quantum mechanics that has taken place during the last few decades. Among other things, he and coworkers have drawn renewed attention to, and have analyzed with fresh mathematical rigor, the threat of inconsistency at the basis of quantum theory: ordinary measurement interactions, described within the mathematical formalism by Schrödinger-type equations of motion, seem to be unable to lead to the occurrence of definite measurement outcomes, whereas the same formalism is interpreted in terms of probabilities of precisely such definite outcomes. Of course, it is essential here to be explicit about how definite measurement results (or definite properties in general) should be represented in the formalism. To this end Lahti et al. have introduced their objectification requirement that says that a system can be taken to possess a definite property if it is certain (in the sense of probability 1) that this property will be found upon measurement. As they have gone on to demonstrate, this requirement entails that in general definite outcomes cannot arise in unitary measuring processes. In this paper we investigate whether it is possible to escape from this deadlock. As we shall argue, there is a way out in which the objectification requirement is fully maintained. The key idea is to adapt the notion of objectivity itself, by introducing relational or perspectival properties. It seems that such a “relational perspective” offers prospects of overcoming some of the long-standing problems in the interpretation of quantum mechanics.
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.