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Thomas comments on Open Thread: May 2010, Part 2 - Less Wrong
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Really? In which parallel universe? Every one? This one?
This one.
Don't we live in a multiverse? Doesn't our Universe splits in two after every quantum event?
How then Tegmark & Co. can predict something for the next 50 years? Almost certainly will happen - somewhere in the Multiverse. Just as almost everything opposite, only on the other side of the Multiverse.
According to Tegmark, at least.
Now he predicts a T shirt in 50 years time! Isn't it a little weird?
All predictions in a splitting multiverse setting have to understood as saying something like "in the majority of resulting branches, the following will be true." Otherwise predictions become meaningless. This fits in nicely with a probabilistic understanding. The correct probability of the even occurring is the fraction of multiverses descended from this current universe that satisfy the condition.
Edit: This isn't quite true. If I flip a coin, the probability of it coming up heads is in some sense 1/2 even though if I flip it right now, any quantum effects might be too small to have any effect on the flip. There's a distinction probability due to fundamentally probabilistic aspects of the universe and probability due to ignorance.
Let's remember that if we're talking about a multiverse in the MWI sense, then universes have to be weighted by the squared norm of their amplitude. Otherwise you get, well, the ridiculous consequences being talked about here... (as well as being able to solve problems in PP in polynomial time on a quantum computer).
Right ok. So in that case, even if we have more new universes being created by a given specific descendant universe, the total measure of that set of universes won't be any higher than that of the original descendant universe, yes? So that makes this problem go away.
Any credible reference to that?
Not off the top of my head. It follows from having the squared norm and from the transformations being unitary. Sniffnoy may have a direct source for the point.
How do you know that something will be included in the majority of branches. Suppose that a nuclear war starts in a branch. A lot of radioactivity will be around, a lot of quantum events, a lot of splittings and a lot of "postnuclear" parallel worlds. The majority? Maybe, I don't know. Tegmark knows? I don't think so.
The small amount of additional radioactivity shouldn't substantially alter how many branches there are. Keep in mind that in the standard multiverse model for quantum mechanics, a split occurs for a lot of events that have nothing to do with radioactivity. For example, a lot of behavior with electrons will also cause splitting. The additional radioactivity from a nuclear exchange simply won't matter much.
ANY increase, from whatever reason, in the number of splittings, would trigger an exponential surge of that particular branch.
The number of splitting is the dominant fitness factor. Those universes which split the most, inherit the Multiverse.
If you buy this Multiverse theory of course, I don't.
Hmm, that's a valid point. It doesn't increase linearly with the number of splitting. I still don't think it should matter. Every atom that isn't simple hydrogen atom is radioactive to some extent (the probability of decay is just really, really, tiny). I'm not at all sure that a radioactive planet (in the sense of having a lot of atoms with non-negligible chance of decay) will actually produce more branches than one which does not. Can someone who knows more about the relevant physics comment? I'm not sure I know enough to make a confident statement about this.