On Tue, Jan 20, 1998 at 10:14:24AM -0800, Robin Hanson wrote: > I need to get back to even more basic basics. > > 1) There's a certain elegant simplicity to the claim "all possible universes > exist", at least if "possible" is interpreted as broadly as possible, i.e., > "not logically inconsistent". But if you start substituting other meanings > for "possible", I think the elegance quickly disappears. You'd then still > have to explain why other non-logically-inconsistent universes don't exist, > and you'd have a bunch more weird universes to explain why we don't easily > observe them.

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The problem is coming up with a definition of "possible" that actually leads to a useful theory. Otherwise we're left with "All possible universes exist, so what?" I think the set of all Turing machines is broad enough and simple enough to be considerably more elegant than assuming that only one specific universe exists. It's not a problem to explain why we don't observe other universes. This is simply because TMs are independent of each other. I agree that it would be nice to either have a broader useful definition of "possible" or a strong justification of why non-computable universes do not exist, but I don't think it is necessary. > I find the many worlds theory of quantum mechanics elegant, but not because > it can be thought of as equally the above claim with a certain odd quantum > definition of "possible". I find it elegant because it seems simpler than > the known alternatives which account for the empirical data. > > 2) I can see why you might want some sort of prior over universes, so you > can make inferences about what universe you are in. But why should your > difficulty in choosing such a prior be an argument against universes > existing? Just because you have trouble thinking about a universe doesn't > mean it doesn't exist. I think one of the main reasons for considering the idea that all possible universes exist is to establish a rigorious basis for induction, and for that we need a prior over universes. > 3) My basic problem with the "all possible universes exist" claim is that > I find it hard to figure out whether my actions have any consequences. > If all possible universes exist, then for any me in one universe choosing > one action, there is another me in another universe choosing another action. > In a global sense I can't choose actions anymore. All possible actions get > chosen. But isn't this also a problem with the many-worlds interpretation of quantum mechanics? I'm not sure what the solution is but I assume there is one, since the many-worlds interpretation seems to be fairly widely accepted.