Abstract
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.
Highlights
I met Paul Busch for the first time during the 1985 Symposium on the Foundations of Modern Physics in Joensuu, Finland
Let us suppose that both Alice’s and Bob’s particle find themselves in initial positions such that they will emerge “up” when they are the first to interact with their respective Stern–Gerlach devices
What we propose as an interpretative option is that the physical characteristics of what happens in Bob’s measurements depend on the local circumstances near Bob, and on the total state that determines the properties of the system that is being measured and on the hyperplane on which that state is defined
Summary
I met Paul Busch for the first time during the 1985 Symposium on the Foundations of Modern Physics in Joensuu, Finland. There is room for alternative views concerning this issue: as shown by non-collapse interpretations (e.g., relative-states/many-worlds interpretations [8,9], modal interpretations [10,11], including the Bohm interpretation [12]) it is possible to consistently interpret quantum mechanics in terms of physical properties of individual systems even without subscribing to the eigenvalue-eigenstate link. The key idea here is that it is a conceptual possibility that the quantum state specifies epistemic probabilities for the presence of a range of possible values of a physical quantity, even though only one value is realized in any individual case (in the many-worlds interpretation this description applies to the epistemic situation as seen from one single branch) In spite of such a range of non-vanishing epistemic probabilities, a perfect measurement may still unambiguously and faithfully reveal the present property with (conditional) probability 1. We shall in the following explore an alternate route, namely acknowledging the universality of unitary evolution and rejecting the eigenvalue-eigenstate link as a necessary condition for the attribution of (sharp) properties. As we shall see, attributing properties to systems in the context of unitary quantum mechanics leads to an unexpected, intuitively peculiar non-classical picture
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