Why haven't other universes collided with ours?

[ThomasT]

You're right, of course. A disturbing thing about MWI, though, is that these improbable outcomes occur in some universe every time the experiment is done, i.e. for Classical physics these improbable events *could* occur where for MWI they *do* occur. I don't see how the equivalent of the Born rule can exist for MWI, or maybe what it would even mean. An outside observer would see that the pathological universes are of measure zero. The problem is that first of all, for Everett there is no such observer and second of all how can a measure be defined over the universes so that one could make statements about zero measure? Insights?

Well, I'm not expert on this. I think Hurkyl is expert. Maybe Ivan. Anyway, my understanding of relative states is not so alarming. Invoke Born rule ad hoc whenever. Why does Born rule work? This is just wave mechanics. You want to know outcome at particular place and particular time, then invoke Born rule. Intensity. Wave amplitude at particular place and particular time. Resulting probabilities generally hold. Schroedinger equation therefore must in some way correspond to deeper reality. But how, why? This is what I'm asking you, Hurkyl, Ivan, etc. Why? How?

Anyway, this is just some layman consideration of physics. No need to think of 'other worlds' etc. relative state interpretation is not so silly. It's about taking the wave equation seriously, and quite possibly corresponding to salient features of the underlying reality. After all, it does predict a rather wide range of phenomena. So, question is, what is it about the SE that is most important? What is it that corresponds to the deeper reality? It isn't a question about whether it does or doesn't. Obviously, 'something' about it does. So, what is it?

 

[ccrummer]

Only an "outside observer" would observe that. Inside observers are limited to "could".

No, inside observers whose worlds behave generally according to the laws of physics would observe these improbable events and in universes where there were strings of such events, the people would see them as miracles, things that defy the laws.

That mixed (relative) states can be described as a statistical distribution over pure states is basic QM.

Of course, you are right but that doesn't answer the problem of defining a "measure" over the set of universes. Such a measure would allow derivation of the Born rule and calculation of the Born probabilities.