On the limits of quantum theory: Contextuality and the quantum–classical cut

This paper is based on four assumptions: 1. Physical reality is made of linearly behaving components combined in non-linear ways. 2. Higher level behaviour emerges from this lower level structure. 3. The way the lower level elements behaves depends on the context in which they are embedded. 4. Quantum theory applies to the lower level entities. An implication is that higher level effective laws, based on the outcomes of non-linear combinations of lower level linear interactions, will generically not be unitary; hence the applicability of quantum theory at higher levels is strictly limited. This leads to the view that both state vector preparation and the quantum measurement process are crucially based on top-down causal effects, and helps provide criteria for the Heisenberg cut that challenge some views on Schrödinger’s cat.

► Gives a framework for looking at emergence based on quantum theory. ► Considers how the linearity of quantum theory relates to complex systems. ► Emphasizes the interaction of bottom-up and top-down causation. ► Uses this to discuss the classical–quantum cut. ► Applies this to Schrödinger’s cat.