Which causal structures might support a quantum-classical gap?

A causal scenario is a graph that describes the cause and effect relationships between all relevant variables in an experiment. A scenario is deemed `not interesting' if there is no device-independent way to distinguish the predictions of classical physics from any generalised probabilistic theory (including quantum mechanics). Conversely, an interesting scenario is one in which there exists a gap between the predictions of different operational probabilistic theories, as occurs for example in Bell-type experiments. Henson, Lal and Pusey (HLP) recently proposed a sufficient condition for a causal scenario to not be interesting. In this paper we supplement their analysis with some new techniques and results. We first show that existing graphical techniques due to Evans can be used to confirm by inspection that many graphs are interesting without having to explicitly search for inequality violations. For three exceptional cases -- the graphs numbered 15,16,20 in HLP -- we show that there exist non-Shannon type entropic inequalities that imply these graphs are interesting. In doing so, we find that existing methods of entropic inequalities can be greatly enhanced by conditioning on the specific values of certain variables.Comment: 13 pages, 9 figures, 1 bicycle. Added an appendix showing that e-separation is strictly more general than the skeleton method. Added journal referenc

Quantum dynamics is linear because quantum states are epistemic

According to quantum theory, a scientist in a sealed laboratory cannot tell whether they are inside a superposition or not. Consequently, so long as they remain isolated, they can assume without inconsistency that their measurements result in definite outcomes. We elevate this to the status of a general principle, which we call Local Definiteness. We apply this principle in the context of modifications of quantum theory that allow the dynamics to be non-linear. We prove that any such theory satisfies Local Definiteness if and only if its dynamics is linear. We further note that any interpretation that takes quantum states to be epistemic necessarily satisfies the principle, whereas interpretations that take quantum states to be ontic do not satisfy it, unless they make additional assumptions that amount to presupposing linearity of the dynamics. Therefore the reason why experiments to date have not found evidence of non-linear dynamics might simply be that quantum states are epistemic.Comment: 13 pages, plus references and appendices. Arguments clarified and better organized; some minor corrections. The juicy bits are in section I

A single space-time is too small for all of Wigner's friends

Recent no-go theorems on interpretations of quantum theory featuring an assumption of `Absoluteness of Observed Events' (AOE) are shown to have an unexpectedly strong corollary: one cannot reject AOE and at the same time assume that the `observed events' in question can all be embedded within a single background space-time common to all observers. Consequently, all interpretations that reject AOE must follow QBism in rejecting a `block universe' view of space-time.Comment: 4 pages. Alternative title: `This space-time ain't big enough for the both of us