Mastodawn

On Tuesday, I have read "Bell's Theorem for Temporal Order" from 2017 in my #dailyPaperChallenge. This paper is concerned with one of the big open questions in #physics : What happens when quantum mechanics and general relativity are applied simultaneously? Most theoretical work in that direction aims towards a #quantumFieldTheory of #gravity, that is, it seeks to understand the behaviour of "elementary particles of gravity" along the same lines as any other elementary particle. The present paper asks another question: In quantum theories, the causal (or temporal) order of events is well defined: Something happens first, and something else afterwards. In general relativity, whether or not something was "before" something else depends on time dilatation, which can be caused by massive objects. Such objects should in principle be subject to quantum mechanics. Hence, the proposal is: Create a specific type of an entangled state, where the entanglement refers to the causal order of operations. It is very hard to realize this with current laboratory equipment, but in principle it should be possible, and one can then decide with a concrete measurement whether quantum mechanical laws apply to causality or not, similar to the #BellInequalities. Obviously, this is all quite philosophical for now, but interesting nonetheless. #paperOfTheDay https://arxiv.org/abs/1708.00248

Bell's Theorem for Temporal Order

Time has a fundamentally different character in quantum mechanics and in general relativity. In quantum theory events unfold in a fixed time order while in general relativity temporal order is influenced by the distribution of matter. When the distribution of matter requires a quantum description, temporal order is expected to become non-classical -- a scenario beyond the scope of current theories. Here we provide a direct description of such a scenario. We consider a massive body in a spatial superposition and show how it leads to "entanglement" of temporal orders between time-like events in the resulting space-time. This entanglement enables accomplishing a task, violation of a Bell inequality, that is impossible under classical temporal order. Violation of the inequality means that temporal order becomes non-classical -- it cannot be described by locally defined classical variables. Our approach provides a quantitative method for investigating quantum aspects of space-time and gravity.

arXiv.org