Forward Causation + Backward Causation = Zero Causation

Many researchers continue to discuss “retrocausal” approaches to quantum theory that involve a basic assumption that one can combine forward-directed causal flow and backward-directed causal flow “in spacetime.” (My own proposal, RTI, has basic time symmetry but that is broken at the spacetime level, so it does not fall into this category.) In this post, I review a result that I presented some years ago, which points out that one cannot in fact combine these two constraints and still retain any sort of dynamical causal flow. The basic point is that if one is laying down both initial and final boundary conditions (together with some physical law that coordinates the two), the inevitable result is a static spacetime. If the underlying physical law is indeterministic, then one either has to choose a particular solution (a single static spacetime) or allow for a set of solutions, but all such solutions are static spacetimes. The only ‘causation’ in a picture imposing both initial and final spacetime boundary consists in the initial setting up of the entire set of spacetime events. This point has never been refuted (to my knowledge), although it is routinely disregarded or deflected. Thus, I offer the discussion again here: