Re-evaluating Space-Time

Special relativity inspired a fundamental shift in our picture of reality, from a spatial state evolving in time to a static block universe. We will highlight some conceptual issues raised by the block universe viewpoint, particularly concerning its complexity, causality, and connection to quantum theory. In light of these issues, and inspired by recent results showing that relativity can emerge naturally in discrete space-time dynamics, we will explore whether the evolving state picture might be more natural after all

Connecting processes with indefinite causal order and multi-time quantum states

Recently, the possible existence of quantum processes with indefinite causal order has been extensively discussed, in particular using the formalism of process matrices. Here we give a new perspective on this question, by establishing a direct connection to the theory of multi-time quantum states. Specifically, we show that process matrices are equivalent to a particular class of pre- and post- selected quantum states. This offers a new conceptual point of view to the nature of process matrices. Our results also provide an explicit recipe to experimentally implement any process matrix in a probabilistic way, and allow us to generalize some of the previously known properties of process matrices. Furthermore we raise the issue of the difference between the notions of indefinite temporal order and indefinite causal order, and show that one can have indefinite causal order even with definite temporal order.Comment: 10 pages, 4 figure