I had recently posted a question on the Philosophy stack exchange about "true" randomness, and a lot of the discussion centered around Quantum Mechanics. One of the responders claimed:
MWI has the same problem as Bohm, proliferation of entities that are unobservable in principle (decohered branches). And like all interpretations of QM that single out time, both deterministic and indeterministic like Copenhagen, it does not naturally extend to QFT, again despite long efforts. Because in QFT time is relative, and running branching or collapses in different frames turns out to be inconsistent.
Now I happen to be partial to the MWI interpretation (and others that maintain determinism), but I have never once heard that it's incompatible with Quantum Field Theory. Bohmian Mechanics has problems incorporating relativity, but again it's striking to me that high profile physicists like Sean Carroll, David Deutsch, and others would be proponents of an idea that can't be naturally extended to QFT. In fact, when I search for information on this I find the complete opposite, that it can be trivially extended to QFT with absolutely no problems.
The responder and I have differed before when it comes to determinism/randomness, and I do not know whether some of it may be ideological (a belief that a deterministic universe is too "mechanistic" and lacking in Romantic notions of mystery/ineffability). Regardless, I would greatly appreciate anyone here spelling out whether this has any veracity, as I can't stand misinformed opinion.
Best Answer
You can trivially apply MWI to QFT, so just do it and ignore nay sayers.
Or if you want to talk to them, then the burden is on them to argue why you can't do something that you are literally doing.
After all, it is just too hard to predict what imaginary concerns they have.
Now, in your case a specific concern was listed:
But the point of MWI is that branches fork and that at some point it becomes practical for each forked branch to lapse into a kind of solipsism and regard itself as like an entire self contained world. But it also doesn't have to do so, so it doesn't matter when or if that thermodynamic limit exerts itself as justifying the solipsism. Plus, any opinions about frames would be about a branch examining itself.
This is because information that you can use probability about is information about a branch and the properties of a branch.
It's just about linearity. A linear theory can allow different worlds as branches because they can act as if they were the only world.
Your quote objected to deterministic and indeterministic theories, so that's not it. This is about an imagined problem between branches and some imagined global absolute time. And a confusion about the entire point of MWI which is that everything is in that branch, it's called a world because it can act as if it were everything.
And really time symmetry should be the objection. Retrodictions about the past for a branch would be wrong if it actually thought it was the whole world.