Quote Originally Posted by NichG View Post
It becomes an issue when elements are injected into a theory or model of reality, which have no consequence. The reason it is an issue is that if something can be injected without consequence, then it is essentially arbitrary and I can choose it to be whatever I want. So if someone tells me 'there is this thing which cannot possibly interact with us, and yet at the same time it is essential that you accept that it exists in order to explain this other thing (which we can interact with)', then I know a logical error has been committed somewhere.

If you look at QM and say 'if we accept QM, we have to accept that other universes exist' and 'QM predicts that there would be no way for those universes to interact with us' and 'we have to accept QM, because it predicts physical and material properties better than anything else', then a logical error has been committed. QM's predictions of physical and material properties cannot possibly hinge on the existence of other universes, because if it did, then that would imply that there was some kind of interaction between those other universes and those physical and material properties - yet the math of QM itself says there should be no such interaction. So any claim about other universes using QM as a basis is, essentially, an arbitrary choice reflecting some bias of the claimant, rather than a prediction.

Or to put it another way, by adding interpretation to the math, one over-specifies the theory. However, since the interpretation is basically inert, it also can't be proven or disproven - but that means that its essentially irrelevant, at least within the evidentiary framework used to justify the theory in the first place.
I fully agree here, since there is a vast difference between a philosophical could possibly exist (which I was considering) and making something an integral part of a physical theory. I was just arguing that on the most basic level we cannot disprove existence of things that do not interact with us and interaction is not a strict requirement for existence.