MWI is more than one theory, because everything is more than one thing[*].
If you defined MWI as just the evolution of the SWE (as required by the simplicity theory), then calculating a bunch of non-interacting states is getting it wrong.
If you start with the idea that MWI is a bunch of non-interacting observers observing different things, then the MTM might get it right. The problem is that no-one knows how
to get the second kind of MWI out of the maths. That is where things like the basis problem come in.
[*]
There is an approach based on coherent superposiitions, and and an version based on decoherence. These are incompatible opposites.
Worlds are superpositions, so they in exist at small scales, they can continue to interact with each other, after, “splitting” , and and they can be erased. These coherent superposed states are the kind of “world” we have direct evidence for, although they seem to lack many of the properties requited for a fully fledged many worlds theory, hence the scare quotes. Call these Small worlds.
Worlds are large, in fact universe-like. They are causally and informationally isolated from each other. This approach is often based on quantum decoherence. Call these Big Worlds.
MWI is more than one theory, because everything is more than one thing[*].
If you defined MWI as just the evolution of the SWE (as required by the simplicity theory), then calculating a bunch of non-interacting states is getting it wrong.
If you start with the idea that MWI is a bunch of non-interacting observers observing different things, then the MTM might get it right. The problem is that no-one knows how to get the second kind of MWI out of the maths. That is where things like the basis problem come in.
[*]
There is an approach based on coherent superposiitions, and and an version based on decoherence. These are incompatible opposites.
Worlds are superpositions, so they in exist at small scales, they can continue to interact with each other, after, “splitting” , and and they can be erased. These coherent superposed states are the kind of “world” we have direct evidence for, although they seem to lack many of the properties requited for a fully fledged many worlds theory, hence the scare quotes. Call these Small worlds.
Worlds are large, in fact universe-like. They are causally and informationally isolated from each other. This approach is often based on quantum decoherence. Call these Big Worlds.