The ontology doesn’t feel muddled to me, although it does feel… not very quantum? Like a thing that seems to be happening with collapse postulates is that it takes seriously the “everything should be quantized” approach, and so insists on ending up with one world (or discrete numbers of worlds). MWI instead seems to think that wavefunctions, while having quantized bases, are themselves complex-valued objects, and so there doesn’t need to be a discrete and transitive sense of whether two things are ‘in the same branch’, and instead it seems fine to have a continuous level of coherence between things (which, at the macro-scale, ends up looking like being in a ‘definite branch’).
[I don’t think I’ve ever seen collapse described as “motivated by everything being quantum” instead of “motivated by thinking that only what you can see exists”, and so quite plausibly this will fall apart or I’ll end up thinking it’s silly or it’s already been dismissed for whatever reason. But somehow this does seem like a lens where collapse is doing the right sort of extrapolating principles where MWI is just blindly doing what made sense elsewhere. On net, I still think wavefunctions are continuous, and so it makes sense for worlds to be continuous too.]
Like, I think it makes more sense to think of MWI as “first many, then even more many,” at which point questions of “when does the split happen?” feel less interesting, because the original state is no longer as special. When I think of the MWI story of radioactive decay, for example, at every timestep you get two worlds, one where the particle decayed at that moment and one where it held together, and as far as we can tell if time is quantized, it must have very short steps, and so this is very quickly a very large number of worlds. If time isn’t quantized, then this has to be spread across continuous space, and so thinking of there being a countable number of worlds is right out.
I think it makes more sense to think of MWI as “first many, then even more many,” at which point questions of “when does the split happen?” feel less interesting, because the original state is no longer as special. [...] If time isn’t quantized, then this has to be spread across continuous space, and so thinking of there being a countable number of worlds is right out.
What I called the “nice ontology” isn’t so much about the number of worlds or even countability but about whether the worlds are well-defined. The MWI gives up a unique reality for things. The desirable feature of the “nice ontology” is that the theory tells us what a “version” of a thing is. As we all seem to agree, the MWI doesn’t do this.
If it doesn’t do this, what’s the justification for speaking of different versions in the first place? I think pure MWI makes only sense as “first one, then one”. After all, there’s just the universal wave function evolving and pure MWI doesn’t give us any reason to take a part of this wavefunction and say there are many versions of this.
The ontology doesn’t feel muddled to me, although it does feel… not very quantum? Like a thing that seems to be happening with collapse postulates is that it takes seriously the “everything should be quantized” approach, and so insists on ending up with one world (or discrete numbers of worlds). MWI instead seems to think that wavefunctions, while having quantized bases, are themselves complex-valued objects, and so there doesn’t need to be a discrete and transitive sense of whether two things are ‘in the same branch’, and instead it seems fine to have a continuous level of coherence between things (which, at the macro-scale, ends up looking like being in a ‘definite branch’).
[I don’t think I’ve ever seen collapse described as “motivated by everything being quantum” instead of “motivated by thinking that only what you can see exists”, and so quite plausibly this will fall apart or I’ll end up thinking it’s silly or it’s already been dismissed for whatever reason. But somehow this does seem like a lens where collapse is doing the right sort of extrapolating principles where MWI is just blindly doing what made sense elsewhere. On net, I still think wavefunctions are continuous, and so it makes sense for worlds to be continuous too.]
Like, I think it makes more sense to think of MWI as “first many, then even more many,” at which point questions of “when does the split happen?” feel less interesting, because the original state is no longer as special. When I think of the MWI story of radioactive decay, for example, at every timestep you get two worlds, one where the particle decayed at that moment and one where it held together, and as far as we can tell if time is quantized, it must have very short steps, and so this is very quickly a very large number of worlds. If time isn’t quantized, then this has to be spread across continuous space, and so thinking of there being a countable number of worlds is right out.
What I called the “nice ontology” isn’t so much about the number of worlds or even countability but about whether the worlds are well-defined. The MWI gives up a unique reality for things. The desirable feature of the “nice ontology” is that the theory tells us what a “version” of a thing is. As we all seem to agree, the MWI doesn’t do this.
If it doesn’t do this, what’s the justification for speaking of different versions in the first place? I think pure MWI makes only sense as “first one, then one”. After all, there’s just the universal wave function evolving and pure MWI doesn’t give us any reason to take a part of this wavefunction and say there are many versions of this.