It’s the simplest explanation (in terms of Kolmogorov complexity).
It’s also the interpretation which by far has the most elegant explanation for the apparent randomness of reality. Most interpretations provide no mechanism for the selection of a specific outcome, which is absurd. Under the MWI, randomness emerges from determinism through indexical uncertainty, i.e., not knowing which branch you’re in. Some people, such as Sabine Hossenfelder for example, get confused by this and ask, “then why am I this version of me?”, which implicitly assumes dualism, as if there is a free-floating consciousness which could in principle inhabit any branch; this is patently untrue because you are by definition this “version” of you. If you were someone else (including someone in a different branch where one of your atoms is moved by one Planck distance) then you wouldn’t be you; you would be literally someone else.
Note that the Copenhagen interpretation is also a many-worlds explanation, but with the added assumption that all but one randomly chosen world disappears when an “observation” is made, i.e., when entanglement with your branch takes place.
Right, so we both agree that the randomness used to determine the result of a measurement in Copenhagen, and the information required to locate yourself in MWI is the same number of bits. But the argument for MWI was never that it had an advantage on this front, but rather that Copenhagen used up some extra bits in the machine that generates the output tape in order to implement the wavefunction collapse procedure. (Not to decide the outcome of the collapse, those random bits are already spoken for. Just the source code of the procedure that collapses the wavefunction and such.) Such code has to answer questions like: Under what circumstances does the wavefunction collapse? What determines the basis the measurement is made in? There needs to be code for actually projecting the wavefunction and then re-normalizing it. This extra complexity is what people mean when they say that collapse theories are less parsimonious/have extra assumptions.
My understanding is that pilot wave theory (ie Bohmian mechanics) explains all the quantum physics
This is only true if you don't count relativistic field theory. Bohmian mechanics has mathematical troubles extending to special relativity or particle creation/annihilation operators.
Is there any reason at all to expect some kind of multiverse?
Depending on how big you expect the unobservable universe to be, there can also be a spacelike multiverse.
I would like to ask a followup question: since we don't have a unified theory of physics yet, why isn't adopting strongly any one of these nonpredictive interpretations premature? It seems like trying to "interpret" gravity without knowing about general relativity.
I've usually heard the justification for favoring Everett over pilot wave theory is on simplicity terms. We can explain everything we need in terms of just wave functions interacting with other wave functions, why also add in particles to the mix too? You get more complicated equations (so I'm told), with a greater number of types of objects, and a less elegant theory, for what? More intuitive metaphysics? Bah!
Though the real test is experimental, as you know. I don't think there's any experiments which separate out the two hypotheses, so it really is still up in the air which actually is a better description of our universe.
The multiverse interpretation takes the wavefunction literally and says that since the math describes a multiverse, there is a multiverse.
YMMV about how literally you take the math. I've come to have a technical objection to it such that I'd be inclined to say that the multiverse theory is wrong, but also it is very technical and I think a substantial fraction of multiverse theorists would say "yeah that's what I meant" or "I suppose that's plausible too".
But "take the math literally" sure seems like good reason/evidence.
And when it comes to pilot wave theory, its math also postulates a wavefunction, so if you take the math literally for pilot wave theory, you get the Everettian multiverse; you just additionally declare one of the branches Real in a vague sense.
I'm confused about what you're saying. In particular while I know what "decoherence" means, it sounds like you are talking about some special formal thing when you say "decoherent branches".
Let's consider the case of Schrodinger's cat. Surely the math itself says that when you open the box, you end up in a superposition of |see the cat alive> + |see the cat dead>.
Or from a comp sci PoV, I imagine having some initial bit sequence, |0101010001100010>, and then applying a Hadamard gate to end up with a superposition (sqrt(1/2) |0> + sqrt(1/2) |1>) (x) |101010001100010>. Next I imagine a bunch of CNOTs that mix together this bit in superposition with the other bits, making the superpositions very distant from each other and therefore unlikely to interact.
What are you saying goes wrong in these pictures?
every particle interaction creates n parallel universes which never physically interfere with each other”
Although a fairly standard way of explaining MWI, this is an example of conflating coherence and decoherence. To get branches that never interact with each other again, you need decoherence, but decoherence is a complex dynamical process..it takes some time...so it is not going to occur once per elementary interaction. It's reasonable to suppose that elementary interactions produce coherent superpositions, on the other hand, but these are not mutuall...
Getting rid of Many Worlds doesn't get rid of the Multiverse. They pop up in many different ways in cosmology. Max Tegmark elaborated four levels., the simplest of which (Level I) ends up looking like a multiverse if the ordinary universe is sufficiently large.
In the field of astronomy, there's the concept of a cosmological horizon. There are several kinds of these depending on exactly how they're defined. That's why they use the term "observable universe". Whatever process kicked off the Big Bang obviously created a lot more of it than we can see, and the...
IIRC pilot wave theory doesn't work for QFTs which is a big failure.
EDIT: I stand corrected. See:
QFT as pilot-wave theory of particle creation and destruction
Bohmian Mechanics and Quantum Field Theory
Relativistically invariant extension of the de Broglie-Bohm theory of quantum mechanics
Making nonlocal reality compatible with relativity.
Time in relativistic and non relativistic quantum mechanics.
So apparently there are de Broglie-Bohm variants of QFTs. I'm unsure if these are full QFTs i.e. they can reproduce the standard model. I am unsur...
Pilot wave theory keeps all of standard wave mechanics unchanged, but then adds on top rules for how the wave "pushes around" classical particles. But it never zeroes out parts of the wave—the entirety of wave mechanics, including the full superposition and all the multiple worlds that implies, are still necessary to compute the exact structure of that wave to understand how it will push around the classical particles. Pilot wave theory then just declares that everything other than the classical particles "don't exist", which doesn't really make sense because the multiple worlds still have to exist in some sense because you have to compute them to understand how the wave will push around the classical particles.
Pilot Wave is just Many Worlds in disguise:
Uncharitable punchline is "if you take pilot wave but keep track of every possible position that any particle could have been (and ignore where they actually were in the actual experiment) then you get many worlds." Seems like a dumb thing to do to me.
Except I don't know how you explain quantum computers without tracking that? If you stop tracking, isn't that just Copenhagen? The "branches" have to exist and interfere with each other and then be "unobserved" to merge them back together.
What does the Elitzur–Vaidman bomb tester look like in Pilot Wave? It makes sense in Many Worlds: you just have to blow up some of them.
My understanding is that pilot wave theory (ie Bohmian mechanics) explains all the quantum physics with no weirdness like "superposition collapse" or "every particle interaction creates n parallel universes which never physically interfere with each other". It is not fully "local" but who cares?
Is there any reason at all to expect some kind of multiverse? Why is the multiverse idea still heavily referenced (eg in acausal trade posts)?
Edit April 11: I challenge the properly physics brained people here (I am myself just a Q poster) to prove my guess wrong: Can you get the Born rule with clean hands this way?
Edit April 9: Well pilot wave vs many worlds is a holy war topic. People have pointed out excellent non-holy-war material: