Any evidence or reason to expect a multiverse / Everett branches?

certain aspects of MWI theory (like how you actually get the Born probabilities) are unresolved

You can add the Born probabilities in with minimal additional Kolmogorov complexity, simply stipulate that worlds with a given amplitude have probabilities given by the Born rule(this does admittedly weaken the "randomness emerges from indexical uncertainty" aspect...)

The proof is last paragraph of his post.

Every non-deterministic interpretation has a virtually infinite Kolmogorov complexity because it has to hardcode the outcome of each random event.

Hidden-variables interpretations are uncomputable because they are incomplete.

Being uncertain of the implications of the hypothesis has no bearing on the Kolmogorv complexity of a hypothesis.

Hmm I think I can implement pilot wave in fewer lines of C than I can many-worlds. Maybe this is a matter of taste... or I am missing something?

Now simply delete the pilot wave part piloted part.

Bohmian mechanics adds hidden variables. why would it be simpler?

Disagree.

If you're talking about the code complexity of "interleaving": If the Turing machine simulates quantum mechanics at all, it already has to "interleave" the representations of states for tiny things like a electrons being in a superposition of spin states or whatever. This must be done in order to agree with experimental results. And then at that point not having to put in extra rules to "collapse the wavefunction" makes things simpler.

If you're talking about the complexity of locating yourself in the computation: Inferring which world you're in is equally complex to inferring which way all the Copenhagen coin tosses came up. It's the same number of bits. (In practice, we don't have to identify our location down to a single world, just as we don't care about the outcome of all the Copenhagen coin tosses.)

I found someone's thesis from 2020 (Hoi Wai Lai) that sums it up not too badly (from the perspective of someone who wants to make Bohmian mechanics work and was willing to write a thesis about it).

For special relativity (section 6), the problem is that the motion of each hidden particle depends instantaneously on the entire multi-particle wavefunction. According to Lai, there's nothing better than to bite the bullet and define a "real present" across the universe, and have the hyperparticles sometimes go faster than light. What hypersurface counts as the real present is unobservable to us, but the motion of the hidden particles cares about it.

For varying particle number (section 7.4), the problem is that in quantum mechanics you can have a superposition of states with different numbers of particles. If there's some hidden variable tracking which part of the superposition is "real," this hidden variable has to behave totally different than a particle! Lai says this leads to "Bell-type" theories, where there's a single hidden variable, a hidden trajectory in configuration space. Honestly this actually seems more satisfactory than how it deals with special relativity - you just had to sacrifice the notion of independent hidden variables behaving like particles, you didn't have to allow for superluminal communication in a way that highlights how pointless the hidden variables are.

Warning: I have exerted basically no effort to check if this random grad student was accurate.

I can help confirm that your blind assumption is false. Source: my undergrad research was with a couple of the people who have tried hardest, which led to me learning a lot about the problem. (Ward Struyve and Samuel Colin.) The problem goes back to Bell and has been the subject of a dedicated subfield of quantum foundations scholars ever since.

This many years distant, I can't give a fair summary of the actual state of things. But a possibly unfair summary based on vague recollections is: it seems like the kind of situation where specialists have something that kind of works, but people outside the field don't find it fully satisfying. (Even people in closely adjacent fields, i.e. other quantum foundations people.) For example, one route I recall abandons using position as the hidden variable, which makes one question what the point was in the first place, since we no longer recover a simple manifest image where there is a "real" notion of particles with positions. And I don't know whether the math fully worked out all the way up to the complexities of the standard model weakly coupled to gravity. (As opposed to, e.g., only working with spin-1/2 particles, or something.)

Now I want to go re-read some of Ward's papers...

How is this different from the situation in the late 19th century when only a few things left seemed to need a "consensus explanation"?

The question is not how big the universe under various theories is, but how complicated the equations describing that theory are.

Otherwise, we’d reject the so-called “galactic” theory of star formation, in favor of the 2d projection theory, which states that the night sky only appears to have far distant galaxies, but is instead the result of a relatively complicated (wrt to newtonian mechanics) cellular automata projected onto our 2d sky. You see, the galactic theory requires 6 parameters to describe each object, and posits an enormously large number of objects, while the 2d projection theory requires but 4 parameters, and assumes an exponentially smaller number of particles, making it a more efficient compression of our observations.

see also

The main reason for not favouring the Everett interpretation is that it doesn't predict classical observations , unless you make further assumptions about basis, the "preferred basis problem". There is therefore room for an even simpler interpretation.

There is an approach to MWI based on coherent superpositions, and a version based on decoherence. These are (for all practical purposes) incompatible opposites, but are treated as interchangeable in Yudkowsky's writings.

The original, Everettian, or coherence based approach , is minimal, but fails to predict classical observations. (At all. It fails to predict the appearance of a broadly classical universe). The later, decoherence based approach, is more emprically adequate, but seems to require additional structure, placing its simplicity in doubt

Coherent superpositions probably exist, but their components aren't worlds in any intuitive sense. Decoherent branches would be worlds in the intuitive sense, and while there is evidence of decoherence, there is no evidence of decoherent branching.There could be a theoretical justification for decoherent branching , but that is what much of the ongoing research is about -- it isn't a done deal, and therefore not a "slam dunk". And, inasmuch as there is no agreed mechanism for decoherent branching, there is no definite fact about the simplicity of decoherent MWI.

My intuition finds zero problem with many worlds interpretation.

Why do you care about the Born measure?

I don't understand what you're replying to. Is this about a possible experiment? What experiment could you do?

Gonna post a top-level post about it once it's made it through editing, but basically the wavefunction is a way to embed a quantum system in a deterministic system, very closely analogous to how a probability function allows you to embed a stochastic system into a deterministic system. So just like how taking the math literally for QM means believing that you live in a multiverse, taking the math literally for probability also means believing that you live in a multiverse. But it seems philosophically coherent for me to believe that we live in a truly stochastic universe rather than just a deterministic probability multiverse, so it also feels like it should be philosophically coherent that we live in a truly quantum universe.

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?

Well, slightly better than wild speculation. We observe broken symmetries in particle physics. This suggests that the corresponding unbroken symmetries existed in the past and could have been broken differently, which would correspond to different (apparent) laws of physics, meaning particles we call "fundamental" might have different properties in different regions of the Cosmos, although this is thought to be far outside our observable universe.

The currently accepted version of the Big Bang theory describes a universe undergoing phase shifts, particularly around the inflationary epoch. This wouldn't necessarily have happened everywhere at once. In the Eternal Inflation model, in a brief moment near the beginning of the observable universe, space used to be expanding far faster than it is now, but (due to chance) a nucleus of what we'd call "normal" spacetime with a lower energy level occurred and spread as the surrounding higher-energy state collapsed, ending the epoch.

However, the expansion of the inflating state is so rapid, that this collapse wave could never catch up to all of it, meaning the higher-energy state still exists and the wave of collapse to normal spacetime is ongoing far away. Due to chance, we can expect many other lower-energy nucleation events to have occurred (and to continue to occur) inside the inflating region, forming bubbles of different (apparent) physics, some probably corresponding to our own, but most probably not, due to the symmetries breaking in different directions.

Each of these bubbles is effectively an isolated universe, and the collection of all of them constitutes the Tegmark Level II Multiverse.

I don't know where you heard that, but the short answer is that no-one knows. There are models of space that curve back in on themselves, and thus have finite extent, even without any kind of hard boundary. But astronomical observations thus far indicate that spacetime is pretty flat, or we'd have seen distortions of scale in the distance. What we know to available observational precision, last I heard indicates that even if the universe does curve in on itself, it must be so slight that the total Universe is at least thousands of times larger (in volume) than the observable part, which is still nowhere near big enough for Tegmark Level I, but that's a lower bound and it may well be infinite. (There are more complicated models that have topological weirdness that might allow for a finite extent with no boundary and no curvature in observable dimensions that might be smaller.)

I don't know if it makes any meaningful difference if the Universe is infinite vs "sufficiently large". As soon as it's big enough to have all physically realizable initial conditions and histories, why does it matter if they happen once or a googol or infinity times? Maybe there are some counterintuitive anthropic arguments involving Boltzmann Brains. Those seem to pop up in cosmology sometimes.

QFT is relativistic quantum mechanics with fields i.e. a continuous limit of a lattice of harmonic oscillators, which you may have encountered in solid state theory. It is the framework for the standard model, our most rigorously tested theory by far. An interpretation of quantum mechanics that can't generalize to QFT is pretty much dead in the water. It would be like having an interpretation of physics that works for classical mechanics but can't generalize to special or general relativity.

(Edited to change "more rigorously" -> "most rigorously".)

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.

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.

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.

How would you formalize pilot wave theory without keeping "track of every possible position that any particle could have been" (which I assume refers to, not throwing away the wavefunction)?