My question is about quantum theory and the afterlife. In the many worlds interpretation of quantum mechanics, even if I die in *this* branch of the multiverse then "I" will still exist in some parallel universes. If we subscribe to the theistic position that every individual has a soul, then what happens to my soul upon death? Will it go the afterlife? What about the parallel "me's"; do they each have their own soul? I'm confused.

The obvious response is that there isn't a single response, and for a simple reason: quantum theory doesn't have anything to say (or not obviously, anyway) about souls -- at least not if a soul is some non-material thing that doesn't fit into the equations we use to do physics.

There is a view that's rather like many worlds and that allows for something soul-like. It's called the Many Minds interpretation, and you can read a short account of it (and get further references) by following this link:

http://plato.stanford.edu/entries/qm-everett/#6

However, this won't address your worries about the afterlife. And since this is a topic that physics has even less to say about than it says about souls, it's even clearer that there's no good answer.

That said, a handful of extra thoughts. The first is that IF there is a non-physical soul (a very big "if), then we can start by asking what happens to it after death on our usual non-quantum picture. And then we could say that whatever the story may be within a single world it's the story we should tell within branches of the Everettian multiverse. Whether that would call for a separate Heavens for each branch, or, if not, what God does about all the near-duplicate souls is a question far above my pay grade. Or possibly the soul doesn't divide but follows some one path through the garden, though which path and why that one rather than another is liable to remain a Great Mystery.

But what I'd really like to suggest is this: the idea of a detachable, non-physical soul is so nebulous as to be a dubious notion from any points of view. The main reason, near as I can tell, that many people believe in it is that they believe in an afterlife and think that the two ideas go hand in hand. In fact, they needn't. There are possible theories of an afterlife that aren't based on souls, and there are possible views of souls on which they don't survive eternally anyway. If talk of the soul is nothing more than a blank space to write a belief in the afterlife onto, then one might as well reason directly about the afterlife and associated issues about what it would mean for some being around after your death to be you. Saying "it's me if it has the same soul" is a case of trying to explain the obscure by the obscurer. If we add many-worlds quantum mechanics to that already unholy brew, the safest thing to say is that we have a recipe for intellectual indigestion.

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