> I have never understood what the classic Many World Interpretation actually is.
There's little to understand: at its core, Many Worlds is real close to "shut up and calculate". The only significant assumption is that the wave function as described by the math is real. And when the math says there are 2 non-interacting blobs, well, this translates to 2 universes.
That's about it.
As for why people seems to reject it instinctively, it's probably because our subjective experience is linear —or mono-threaded. However, linear histories aren't contradictory with trees. Imagine a Git History that never merges: each leaf is the product of a linear set of modifications, a perfectly clean history. From that final commit's perspective, other branches might as well not exist. You'd need merges to break that illusion, and our universe doesn't have macro-merges —we only observe them at a very small scale, for instance with photon interference.
So the wave function, a function say on configuration space, is real. And that's it?
But what does that mean? What is the mapping to my experience? Is it that if the wave function is non-zero at a configuration point, then the configuration is realized as an actual universe? One big problem with this is that wave functions can be non-zero at all configuration points and the dispersive nature of the Laplacian tends to make that happen.
So then it becomes one of size of |psi|^2? Is there a cutoff for considering that to be real? Having a probability of something is needed to deal with this, but it is not clear to me what the probability would correspond to here.
In BM, the probability is that of finding particles in some region. In collapse theories, it is the probability of collapsing to a specific state.
As for the Git History, my above concern could be phrased as a history in which all possible streams of texts are possible though maybe some have a large font-size. This is library of babel kind of stuff. How is the evolution of the text that I consider myself to be a part of handled? How are nearby configurations connected by the evolution of a wave function?
Also, macro-mergers are not theoretically ruled out; it is practically ruled out.
Let's say that I convinced you that 2 non-interacting blobs did not objectively evolve to occur, but rather a smearing over all. Would this break your interpretation?
> Let's say that I convinced you that 2 non-interacting blobs did not objectively evolve to occur, but rather a smearing over all. Would this break your interpretation?
If I understand correctly, what you speak of should actually be observable —you could make falsifiable predictions about it. That would break more than my interpretation, I think.
I didn't say that well. I mean, non-zero everywhere. There can be bumps with most of the support, but integrating |psi|^2 over any region gives a non-zero result, if mostly below the order of, say 10^-100 outside of your two bumps. That is, from a probabilistic point of view, it can be discounted without a thought, but from a "this is a real world" it does not seem to me to be so easy to discard.
That is to say, where is the magnitude of the wave function being used in this interpretation?
That is a really excellent analogy. It is also important to understand that there is a reason we don't have "macro merges", and that is because some irreversible process is required to create classical information, which is what we (the entities having this conversation) are actually made of. See:
There's little to understand: at its core, Many Worlds is real close to "shut up and calculate". The only significant assumption is that the wave function as described by the math is real. And when the math says there are 2 non-interacting blobs, well, this translates to 2 universes.
That's about it.
As for why people seems to reject it instinctively, it's probably because our subjective experience is linear —or mono-threaded. However, linear histories aren't contradictory with trees. Imagine a Git History that never merges: each leaf is the product of a linear set of modifications, a perfectly clean history. From that final commit's perspective, other branches might as well not exist. You'd need merges to break that illusion, and our universe doesn't have macro-merges —we only observe them at a very small scale, for instance with photon interference.