Wednesday, October 5, 2016

Phenomenally accessible hidden variables

Consider Jeffrey Barrett's traveling minds interpretation of Quantum Mechanics (see also here). On this interpretation, minds traverse a branching Everett-style multiverse in accordance with the probabilities given by the Born rule. But unlike on the Albert-Loewer many-minds interpretation, the minds are constrained to travel together: they are always found in the same branch of the universe.

Here is something interesting about the position in logical space of this interpretation. It is a hidden-variables interpretation in the sense that it supposes that there are realities that cannot be reduced to the wavefunction. The hidden variables on this story correspond to facts about brain states. For instance, the wavefunction may place my brain in a superposition of a brain state in which I feel I am sitting with a state in which I feel I am standing, but the minds (jointly) pick out a branch of the wavefunction--the one in which I feel I am sitting (and writing a post on quantum mechanics). Notice, however, that the hidden variables in this story are hidden from the wavefunction but not hidden from us: they are phenomenally accessible to us.

Interestingly, the Bohm interpretation can be seen also to be a hidden-variables interpretation where the variables are not entirely hidden from us. For presumably it is the "hidden" positions of the particles that determine the brain state that gives rise to my phenomenal state. So from my phenomenal state, I can tell something about the hidden variables--for instance, that they comprise a brain. Bohm is a paradigmatic hidden variable interpretation, and yet it does not actually hide the variables from us. So we need to be cautious about the phrase "hidden variables".

I think the Albert-Loewer many-minds interpretation is also a hidden variable theory. The variables are the states of the many minds. But there is a difference between the Barrett and Albert-Lower interpretations, on the one hand, and the Bohm interpretation, on the other. In the Bohm interpretation, the hidden variables are a part of physical reality. On the mind-based interpretations, the hidden variables are a part of mental reality. In all cases, we have at least partial access to the hidden variables.


Michael Gonzalez said...

Pruss: Do you think that parsimony (Occam's Razor, etc) would favor a Bohmian interpretation over any of these other "hidden variables" theories?

Why should we suppose that brains exist at all or that their states have anything to do with our experience, if we are postulating a theory on which our experience is a fabrication anyway? We only believe in brains, that brain states are connected to mental states, and that QM predictions have been widely correct (warranting all this work on explaining how it works) because we take our experience to be indicative of the actual world! On these views, though, we experience all these things because the wavefunction collapses in such a way as to produce that experience; not because the world is actually anything like how we experience it. Doesn't that heavily flirt with radical skepticism, in which case we should wonder whether the probabilistic outcomes of QM experiments, the work of scientists from Einstein onward, the existence of scientists or humans at all, etc etc becomes tenuous?

Alexander R Pruss said...

On these other views, our experience is indicative of the actual world, both of a certain aspect of the global wavefunction and of the place of minds in it.

Michael Gonzalez said...

But can we rationally believe, based on our experience, that brains, brain-to-mind relations, laboratories with statistically regular results, etc, all exist and have behaved in a way that lead us to formulate these various interpretations of QM... WHILE also holding that our experience is not the result of those things actually existing, but is imposed upon us by a wavefunction collapse specifying our mental state? I mean, how is this not a souped-up Cartesian demon?