Johannes Fankhauser

Quantum theory has an epistemic horizon, i.e. exact values cannot be assigned simultaneously to incompatible physical quantities. As shown by Spekkens’ toy theory, positing an epistemic horizon akin to Heisenberg’s uncertainty principle in a classical mechanical setting also leads to a plethora of quantum phenomena. We introduce a deterministic theory—nomic toy theory—in which information gathering agents are explicitly modelled as physical systems. Our main result shows the presence of an epistemic horizon for such agents. They can only simultaneously learn the values of observables whose Poisson bracket vanishes. Therefore, nomic toy theory has incompatible measurements and the complete state of a physical system cannot be known. The best description of a system by an agent is via an epistemic state of Spekkens’ toy theory. Our result reconciles us to measurement uncertainty as an aspect of the inseparability of subjects and objects. Significantly, the claims follow even though nomic toy theory is essentially classical. This work invites further investigations of epistemic horizons, such as the one of (full) quantum theory.

I argue that the gravitational redshift effect cannot be explained purely by way of uniformly accelerated frames, as sometimes suggested in the literature. This is due to the fact that in terms of physical effects a uniformly accelerated frame is not exactly equivalent to a homogeneous gravitational field let alone to a gravitational field of a point mass. In other words, the equivalence principle only holds in the regime of certain approximations. The concepts in need of clarification are spacetime curvature, inertia, and the weak equivalence principle with respect to our understanding of gravitational redshift. Furthermore, I briefly discuss gravitational redshift effects due to charge.

In this paper I discuss the delayed choice quantum eraser experiment by giving a straightforward account in standard quantum mechanics. At first glance, the experiment suggests that measurements on one part of an entangled photon pair can be employed to control whether the measurement outcome of the other part of the photon pair produces interference fringes at a screen after being sent through a double slit. Significantly, the choice whether there is interference or not can be made long after the signal photon encounters the screen. The results of the experiment have been alleged to invoke some sort of 'backwards in time influences'. I argue that in the standard collapse interpretation the issue can be eliminated by taking into account the collapse of the overall entangled state due to the signal photon. Likewise, in the de Broglie-Bohm picture the particle's trajectories can be given a well-defined description at any instant of time during the experiment. Thus, there is no need to resort to any kind of 'backwards in time influence'. As a matter of fact, the delayed choice quantum eraser experiment turns out to resemble a Bell-type measurement, and so there really is no mystery.