One of the common objections to the Many Worlds Interpretation (MWI) of quantum physics is to invoke Occam’s Razor (“Entities should not be multipled without necessity.”) and to say that MWI certainly violates this principle with its infinity of actual universes. Many physicists (including both MWI proponents and others) consider this to not be a strong objection. I’d boil their responses down to the following:
- In some important sense, MWI is actually simpler than the alternatives.
- We shouldn’t expect the universe to conform to our expectations and intuitions. It could be much more extravagant than we can imagine.
Point 1 is basically a claim that MWI is “close to the math,” i.e. it is essentially just Schrodinger’s equation. There is no collapse that needs to be explained. If you just take Schrodinger’s equation literally, MWI is what you get. In that light, you could use Occam’s Razor to argue for MWI. I would like to push back on this in a couple of ways. First, it’s not clear that “close to the math” is really what we should care about. Second, and relatedly, it seems there is a mistaken reification of the Schrodinger equation happening — in MWI, it is not just an incredibly successful equation for prediction, but a literal description of reality. This move does not seem justified. Finally, even if you care about how “close to the math” a theory is, you could opt for pilot-wave theory, which also does not require collapse. MWI is still mathematically simpler, since pilot-wave theory requires additional rules, but pilot-wave theory does not require an infinity of universes. I agree with Lee Smolin that, at the very least, pilot-wave theory is underappreciated and deserves more consideration as a viable interpretation of quantum physics.
Regarding Point 2, I agree that the universe could be much stranger than we imagine, and we shouldn’t expect it to line up with our intuitions. But there is a stronger reading of this claim that seems to reject Occam’s Razor altogether, and since we can have an infinity of theories purporting to describe a given phenomenon, we need Occam’s Razor to help sift through them.
Even if you think Occam’s Razor cannot be wielded against MWI, there are other major challenges facing MWI, such as how to make sense of probability when everything that can happen does happen. Again, Lee Smolin’s book has a very good discussion of this challenge, and Philip Ball has an article that discusses this as well. Ball also talks about the challenge of making sense of the self in the context of MWI. Near the end of Smolin’s book, he also talks about the moral implications of MWI: in a multiverse where everything that can happen does happen, how do we make sense of moral decision-making?