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.

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.

Do you really think that a talk held by Max Tegmark will not attract people who share his views?

Actually, this is not true. Having been in academia for some time, I can vouch that a celebrity talk like that would attract many faculty members regardless of their views on the matter.

You're making an assertion with zero evidence...

You said you suspect this is necessary, but that you hope we can recover a similar MWI, but isn't it more reasonable to expect that at the planck scale something else will explain the quantum weirdness?

When I talk about recovering MWI, I really just mean absorbing the lesson that our theory does not need to deliver determinate measurement results, and ad hoc tools for satisfying this constraint (such as collapse or hidden variables) are otiose. Of course, the foundations of our eventual theory of quantum gravity might be different enough from those of quantum theory that the interpretational options don't translate. How different the foundations will be depends on which program ends up working out, I suspect. If something like canonical quantum gravity or loop quantum gravity turns out to be the way to go, then I think a lot of the conceptual work done in interpreting NRQM and QFT will carry over. If string theory turns out to be on the right track, then maybe a more radical interpretational revision will be required. The foundations of string theory are now thought to lie in M-theory, and the nature of this theory is still pretty conceptually opaque. It's worth noting though that Bousso and Susskind have actually suggested that string theory provides a solid foundation for MWI, and that the worlds in the string theory landscape are the same thing as the worlds in MWI. See here for more on this. The paper has been on my "to read" list for a while, but I haven't gotten around to it yet. I'm skeptical but interested.

Have you given Gerard 't Hoofts idea of cellular automata which he claims salvage determinism, locality and realism any thought?

I know of 't Hooft's cellular automata stuff, but I don't know much about it. Speaking from a position of admitted ignorance, I'm skeptical. I suspect the only way to construct a genuinely deterministic local realist theory that reproduces quantum statistics is to embrace superdeterminism in some form, i.e. to place constraints on the boundary conditions of the universe that make the statistics work out by hand. This move doesn't seem like good physics practice to me. Do you know if 't Hooft's strategy relies on some similar move?

You're overstating the extent of the opposition.

No, it's because MWI has broad support among physicists as at least being a very plausible candidate interpretation. Support for cryonics among biologists and neuroscientists is much more limited.

This article (PDF) gives a nice (and fairly accessible) summary of some of the issues involved in extending MWI to QFT. See sections 4 and 8 in particular. Their focus in the paper is wavefunction realism, but given that MWI (at least the version advocated in the Sequences) is committed to wavefunction realism, their arguments apply. They offer a suggestion of the kind of theory that they think can replace MWI in the relativistic context, but the view is insufficiently developed (at least in that paper) for me to fully evaluate it.

A quick summary of the issues raised in the paper:

• In NRQM, the wave function lives in configuration space, but there is no well-defined particle configuration space in QFT since particle number is not conserved and particles are emergent entities without precisely defined physical properties.

• A move to field configuration space is unsatisfactory because quantum field theories admit of equivalent description using many different choices of field observable. Unlike NRQM, where there are solid dynamical reasons for choosing the position basis as fundamental, there seems to be no natural or dynamically preferred choice in QFT, so a choice of a particular field configuration space description would amount to ad hoc privileging.

• MWI in NRQM treats physical space as non-fundamental. This is hard to justify in QFT, because physical space-time is bound up with the fundamentals of the theory to a much greater degree. The dynamical variables in QFT are operators that are explicitly associated with space-time regions.

• This objection is particularly clever and interesting, I think. In MWI, the history of the universe is fully specified by giving the universal wavefunction at each time in some reference frame. In a relativistic context, one would expect that all one needs to do in order to describe how the universe looks in some other inertial reference frame is to perform a Lorentz transformation on this history. If the history really tells us everything about the physical state of the universe, then it gives us all the information required to determine how the universe looks under a Lorentz transformation. But in relativistic quantum mechanics, this is not true. Fully specifying the wavefunction (defined on an arbitrarily chosen field configuration space, say) at all times is not sufficient to determine what the universe will look like under a Lorentz transformation. See the example on p. 21 in the paper, or read David Albert's paper on narratability. This suggests that giving the wavefunction at all times is not a full specification of the physical properties of the universe.

On the other hand, my understanding is that QFT itself doesn't exist in a rigorous form yet, either.

I assume you're referring to the infinities that arise in QFT when we integrate over arbitrarily short length scales. I don't think this shows a lack of rigor in QFT. Thanks to the development of renormalization group theory in the 70s, we know how to do functional integrals in QFT with an imposed cutoff at some finite short length scale. QFT with a cutoff doesn't suffer from problems involving infinities. Of course, the necessity of the cutoff is an indication that QFT is not a completely accurate description of the universe. But we already know that we're going to need a theory of quantum gravity at the Planck scale. In the domain where it works, QFT is reasonably rigorously defined, I'd say.