What I am saying is that if one person says "all the worlds have always existed" and another says "the worlds spread out from one world," it's possible that both of them are being consistent, but then they are using two different definitions of "world." I am also saying that there is no basis that is "more real" than the others - only that some are more useful, and it's okay that people use different definitions as long as they're clear about it.

And yes, both pictures can describe the same thing. Have you worked with Bell states at all? Or am I misinterpreting your name and you actually haven't taken a class on quantum mechanics before?

The quantum world is like a diagonal line. One person comes up to it and says "Ah! Here is a diagonal line! It has just as much horizontal as it does vertical, therefore it is a mixture between horizontal and vertical." Another person comes up to it and says "Ah! Here is a diagonal line! It is a perfect rising diagonal, and is not even a little biased towards the falling diagonal." Will these two people argue over whether the line is made of two components or one?

Are all the worlds in the wavefunction from the beginning of time or do they somehow spring out from one world? This is called overlap vs non-overlap (first discussed by David Lewis).

So, by "world" in this post I'll mean "basis sate for the universe." The basis is arbitrary, so what "world" means will still depend on how I'm choosing what "worlds" are - there's the energy basis, for instance, where nothing ever changes if you look at just one of those "worlds." But you can have animals or computers in your basis states if you want - they aren't energy eigenstates, so they change with time.

Anyhow, currently the universe is spread out over a very wide variety of energy eigenstates, which is a fancy way of saying that lots of stuff changes. If we only allow quantum mechanics (that is, strictly follow MWI), this spread over "energy-worlds" is how the universe has been since the beginning of time. But if we look at the exact same state a different way, you could just call the initial state of the universe a basis state, and then, lo and behold, the universe would have sprung from one world, and the distribution of worlds then changed over time. This way of looking at things is probably pretty useful for cosmology. Or you could use worlds that change over time but don't include the original state of the universe, giving you overlap again. This is what we do unintentionally when we choose worlds that have humans in them, which is also pretty useful :)

For overlap vs. non-overlap to get more complicated than "both are valid pictures," you'd need some model where there weren't any static worlds to talk about - this would be a change to QM though. Also, this does raise the interesting question of how complicated that initial world (if we look at it that way) was. It doesn't have to be too complicated before we see interesting stuff.

Anyhow, it's pretty likely I was too hasty in my mistake-detection. But meh, I rarely regret putting off reading things. And I only occasionally regret putting my foot in my mouth :)

Stopped reading the linked paper when it made a mistake because of treating "worlds" as literal things being "split off." Gotta use quantum mechanics if you're going to talk about quantum mechanics. Maybe they corrected it later, but I didn't even want to wade through to find out.

I'm assuming this whole conversation is about non-relativistic quantum mechanics.

But MWI advocates (at least of the Oxford variety) claim that the properties of reality emerge from the wavefunction. No additional "beables" are required. I know you disagree, but I'm pretty sure that's the sort of view Aaronson is referring to when he says MWI is mathematically simpler. The fundamental ontology is the wavefunction itself, not worlds of matter/energy whose multiplication is described by the wavefunction.

BTW, it's important to note that by some polls an actual majority of theoretical physicists now believe in MWI, and this was true well before I wrote anything. My only contributions are in explaining the state of the issue to nonphysicists (I am a good explainer), formalizing the gross probability-theoretic errors of some critiques of MWI (I am a domain expert at that part), and stripping off a lot of soft understatement that many physicists have to do for fear of offending sillier colleagues (i.e., they know how incredibly stupid the Copenhagen interpretation appears nowadays, but will incur professional costs from saying it out loud with corresponding force, because there are many senior physicists who grew up believing it).

The idea that Eliezer Yudkowsky made up the MWI as his personal crackpot interpretation isn't just a straw version of LW, it's disrespectful to Everett, DeWitt, and the other inventors of MWI. It does seem to be a common straw version of LW for all that, presumably because it's spontaneously reinvented any time somebody hears that MWI is popular on LW and they have no idea that MWI is also believed by a plurality and possibly a majority of theoretical physicists and that the Quantum Physics Sequence is just trying to explain why to nonphysicists / formalize the arguments in probability-theoretic terms to show their nonambiguity.

There were plenty of physicists reading those posts when they first came out on OB (the most famous name being Scott Aaronson). Some later readers have indeed asserted that there's a problem involving a physically wrong factor of i in the first couple of posts (i.e. that's allegedly not what a half-silvered mirror does to the phase in real life), which I haven't yet corrected because I would need to verify with a trusted physicist that this was correct, and then possibly craft new illustrations instead of using the ones I found online, and this would take up too much time relative to the point that talking about a phase change of -1 instead of i so as to be faithful to real-world mirrors is an essentially trivial quibble which has no effect on any larger points. If anyone else wants to rejigger the illustration or the explanation so that it flows correctly, and get Scott Aaronson or another known trusted physicist to verify it, I'll be happy to accept the correction.

Aside from that, real physicists haven't objected to any of the math, which I'm actually pretty darned proud of considering that I am not a physicist.

Lack of coherence? where?

His monologue on color, for instance.

The states are still non-local.

This assumption is made by every other interpretation of quantum mechanics I know. On the other hand, I'm not a physicist; I'm clearly not up to date on things.

Local HV's do exist.

I meant the classical HV theories that were ruled out by actual experiments detecting violations of Bell's inequality.

MWI just calculates the wavefunction.

Copenhagen calculates the wavefunction but then has additional rules saying when some of the branches collapse.

Bohm calculates the wavefunction and then says that particles have single positions but are guided by the wavefunction.

Mitchell_Porter makes the case, but reading him makes my brain shut down for lack of coherence. I assume Yudkowsky doesn't favor Bohm because it requires non-local hidden variables. Non-local theories are unexpected in physics, and local hidden variables don't exist.