Spacetimes via Continuous posets: Old foundations and new developments
We introduce the reader to the continuous posets approach to spacetime geometry, a topic pioneered by Martin and Panangaden. We provide all relevant proofs from standard domain-theory sources to ease the transition for readers from Lorentzian geometry and general relativity. We then present new results for the spacetime interpretation. Under the identification of the way-below relation with the chronological relation $I$, we determine the unique choice for $\le$ and causality condition to obtain specific poset properties. Continuous posets require $\le=D_p$ and correspond to "past-distinction and future-reflectivity"; bicontinuous posets are causally continuous spacetimes with $\le=D$; globally hyperbolic posets are precisely globally hyperbolic spacetimes with $\le=J$ proving a necessity result beyond Martin and Panangaden's sufficiency. The theory, being phrased entirely in terms of a poset, is of low-regularity. We introduce a notion of topological Kronheimer-Penrose causal space that is sufficiently general to encompass the Lorentzian length spaces in the literature, and give weak conditions making it a (bi)continuous poset. Once a spacetime is a continuous poset, domain-theoretic constructions apply directly, e.g., completion schemes yield spacetime boundaries. We recall a few; the most natural for preserving continuity, Lawson's round-ideal completion, is proved to equal the future GKP completion. The directed completion recently studied by Gigli et al. was also proved to be equivalent to the future GKP completion; however with additional SC condition and forward approximation which we are able to remove. Finally, physical considerations on the recently established key role of past-reflectivity in black hole evaporation lead us to suggest that the spacetime is, at the fundamental level, a co-continuous poset.