Here’s a very very good explanation of this:
Also, what you say about dollars is true because they have certain properties - effectively they have two different criteria of identity, one economic, the other physical. What is the criterion of identity for universes/branches?
But they are (presumed under this model to be) splitting. You posited a case where two later happen to be identical, but that doesn't mean they merge. You'd just have two of them.
>Also, what you say about dollars is true because they have certain properties - effectively they have two different criteria of identity, one economic, the other physical. What is the criterion of identity for universes/branches?
I'd guess that there is none, like electrons. But though those are even more indistinguishable than dollars, you can still have two of them.
Okay, but what's the actual model here? Let's talk about two different versions of many-worlds:
Version 1: Universes splitting
Version 2: No "splitting" per se ever actually occurs, universes are identical up to time t, different thereafter
Is there any difference between 1 and 2 from a physics perspective–any difference in the maths, or in conceivable results of observation or experiment? Or are they just different ways of wording it? A metaphysical/philosophical difference, but not an empirical/mathematical one?
Also, if the theory is symmetric with respect to time, you'd expect merger as well as splitting, since the former is just the time reversal of the latter – isn't QM supposed to have T-symmetry? Does MWI have T-symmetry?
I think it's not dissimilar to absolute determinism. Many people, even irrelegious, have a deep aversion to the idea that their choices may be deterministic. You might say why does it matter if they cannot be predicted, and of course the path is one that your brain decides to take, not something that is being forced on it. Still lots of people like to imagine some sort of weird quantum consciousness woo that lets brains make free thoughtful choices independent of physical laws.
That's ridiculous. It's like saying there's no point in playing a videogame because other people are playing the same game differently. If it doesn't affect you why do you allow it to make you care or not care? Personally I think it comes from this sort of arrogant thinking where people are aware they are an insignificant cosmic bug but also are somehow above caring about things at the level of a cosmic bug. It's an ass-backward way of thinking, this "I don't matter so I don't care". A better way of thinking is "I don't matter, so I can care about things that also don't matter, because they're no more insignificant than I am." If a dog turned to us and said "I can never write a symphony, therefore gnawing on bones is pointless" we would call him a foolish dog for refusing to engage with reality at his own level, for thinking that things so far beyond him should make any difference to him and how he lives his life.
To quote James Watson, "I don't think we're here for anything, we're just products of evolution. You can say 'Gee, your life must be pretty bleak if you don't think there's a purpose' but I'm anticipating a good lunch."