Thanks for your question, Peter. Causality and change can certainly be represented in 4D spacetime, so I didn’t mean to imply otherwise. The Geroch quote is, as you say, pointing out the fact that the 4D perspective *itself* doesn’t change. We use that “changeless” 4D perspective for two reasons. First, 4D spacetime lacks a Now in the sense that there is no “movie projector for creating preferred moments in M4” per Brian Greene’s analogy https://www.youtube.com/watch?v=GpgGJaQfrgE. [At the 29:43 mark, just after David Albert says, “Physics does radical violence to this everyday experience of time” at the 29:05 mark.] If you want to see how one might account formally (and robustly) for a Now, see Ruth Kastner’s PTI. Second, QM is not compatible with classical causality per Wood and Spekkens (perhaps because QM requires directionless links in its causal diagrams, see Evans’ paper in this forum). So, the notion of retrocausality in a block universe is, as an anonymous referee stated (footnote 3 of our paper in this forum), “superfluous at best, and inconsistent at worst.” If you want an example of non-deflationary retrocausality, again see Ruth’s PTI.
Since RBW’s fundamental ontological entities for modeling QM phenomena are 4D spacetimesource elements, I don’t see RBW as containing any robust sense of change and causation at the fundamental level. Perhaps Silberstein will have more to say.
I look forward to your further comments and questions!
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