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Live Options for Spacetime-Based Physics

Entanglement places severe restrictions on the class of quantum models where all beables reside in ordinary spacetime. Nevertheless, it is worth examining such models in detail, in part because the alternative – trying to recover spacetime in some emergent limit – has proven to be so problematic. Taking the block-universe perspective of general relativity and classical field theory, the best options appear to be those in which any given spacetime system is solved “all-at-once”, as a four-dimensional boundary problem. Such approaches can even save a key form of locality (in the no-action-at-a-distance sense), entanglement notwithstanding, via the so-called “retrocausal loophole”. Recent progress and future prospects for such all-at-once/retrocausal physics models will be discussed.