Reply To: Retrocausal Bohm Model

Ruth Kastner

Thanks Rod,

I certainly would not dismiss a model simply because it implied a block universe. I have had extensive discussions with the Relational Block World folks (Mark and Michael), and while that’s not my favored approach, it’s not because it’s a block world.

My concern about the model you’re proposing is that while it describes dynamical entities (guiding waves etc.) it seems to me that these kinds of dynamics end up being superfluous if there is a future BC in addition to a past BC. It seems to me you get a ‘superdetermined’ situation. You talk above about someone choosing to measure a different observable. But if there are future BC in place with the constraints implied by the dependence of particle trajectories on the quantum states, then it seems to me that all those particle trajectories must be determinate in the block world. This seems to mean that what measurements we make are ‘already’ there in the future, so in what sense could we make different measurements than those dictated by the past and future BC and the states involved–the ones already there in spacetime? Thus the only form of ‘free will’ that seems to be available in this picture is a compatibilist sort–an account that must define free will as consistent with the physical determination of all our choices “before” we get to them.

Regardless of whether or not one is concerned about free will, the other issue is that the dynamics seems superfluous: events/trajectories are what they are, and nothing is being ‘guided’ in any dynamical sense to where it will end up–because it’s already there.

In contrast, PTI allows for emergence of spacetime from a quantum substratum in a growing universe picture, so there is no future BC to constrain future events in this way. What I prefer about PTI is that the quantum dynamics is actually doing something-giving rise to spacetime events. Also, it has an open future that provides for a robust form of free will.

In any case, though I should emphasize that I am not prima facie opposed to a block world. My main concern is consistency–and it seems to me that imposing past and future BC gives us a block world in which talk of causation, either forward or backward, is superfluous or even inconsistent, Everything is just there in the block world so it does not need any causation. It’s a static picture. The only sense in which causation can be applied is in terms of static relations between objects or events–thus rendering the idea of ‘retrocausation’ empty.

The same concern applies to other ‘retrocausal’ models such as Huw Price’s time-symmetric hidden variables and Ken Wharton’s version of that idea. Everything is just there in the block world and the given dynamics don’t seem to have anything dynamically to do. The quantum states just become epistemic measures of our ignorance of where all the hidden variables actually are. Again, a static block world picture.


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