Username, you're having a small conversion experience here, going from "causality is local" to "wavefunction collapse is preposterous" to "I understand quantum suicide" to "I'd better sign up for cryonics once I graduate" in rapid succession. It's a shame we can't freeze you right now, and then do a trace-and-debug of your recent thoughts, as a case study.

This was a somewhat muddled comment from Eliezer. Local causality does not imply an upper speed limit on how fast causal influences can propagate. Then he equivocates between locality within a configuration and locality within configuration space. Then he says that if only everyone in physics thought like this, they wouldn't have wrong opinions about how QM works. I can only guess how you personally relate all that to decoherence. And from there, you get to increased confidence in cryonics. It could only happen on Less Wrong. :-)

ETA: Some more remarks:

Locality does not imply a maximum speed. Locality just means that causes don't jump across space to their effects, they have to cross it point by point. But that says nothing about how fast they cross it. You could have a nonrelativistic local quantum mechanics with no upper speed limit. Eliezer is conflating locality with relativistic locality, which is what he is trying to derive from the assumption of locality. (I concede that no speed limit implies a de-facto or practical nonlocality, in that the whole universe would then be potentially relevant for what happens here in the "next moment"; some influence moving at a googol light-years per second might come crashing in upon us.)

Equivocating between locality in a configuration and locality in a configuration space: A configuration is, let's say, an arrangement of particles in space. Locality in that context is defined by distance in space. But configuration space is a space in which the "points" themselves are whole configurations. "Locality" here refers to similarity between whole configurations. It means that the amplitude for a whole configuration is only immediately influenced by the amplitudes for infinitesimally different whole configurations.

Suppose we're talking about a configuration in which there are two atoms, A and B, separated by a light-year. The amplitude for that configuration (in an evolving wavefunction) will be affected by the amplitude for a configuration which differs slightly at atom A, and also by the amplitude for a configuration which differs slightly at atom B, a light-year away from A. This is where the indirect nonlocality of QM comes from - if you think of QM in terms of amplitude flows in configuration space: you are attaching single amplitudes to extended objects - arbitrarily large configurations - and amplitude changes can come from very different "directions" in configuration space.

Eliezer also talks about amplitudes for subconfigurations. He wants to be able to say that what happens at a point only depends on its immediate environment. But if you want to talk like this, you have to retreat from talking about specific configurations, and instead talk about regions of space, and the quantum state of a "region of space", which will associate an amplitude with every possible subconfiguration confined to that region.

This is an important consideration for MWI, evaluated from a relativistic perspective, because relativity implies that a "configuration" is not a fundamental element of reality. A configuration is based on a particular slicing of space-time into equal-time hypersurfaces, and in relativity, no such slicing is to be preferred as ontologically superior to all others. Ultimately that means that only space-time points, and the relations between them (spacelike, lightlike, timelike) are absolute; assembling sets of points into spacelike hypersurfaces is picking a particular reference frame.

This causes considerable problems if you want to reify quantum wavefunctions - treat them as reality, rather than as constructs akin to probability distributions - because (for any region of space bigger than a point) they are always based on a particular hypersurface, and therefore a particular notion of simultaneity; so to reify the wavefunction is to say that the reference frame in which it is defined is ontologically preferred. So then you could say, all right, we'll just talk about wavefunctions based at a point. But building up an extended wavefunction from just local information is not a simple matter. The extended wavefunction will contain entanglement but the local information says nothing about entanglement. So the entanglement has to come from how you "combine" the wavefunctions based at points. Potentially, for any n points that are spacelike with respect to each other, there will need to be "entanglement information" on how to assemble them as part of a wavefunction for configurations.

I don't know where that line of thought takes you. But in ordinary Copenhagen QM, applied to QFT, this just doesn't even come up, because you treat space-time, and particular events in space-time, as the reality, and wavefunctions, superpositions, sums over histories, etc, as just a method of obtaining probabilities about reality. Copenhagen is unsatisfactory as an ontological picture because it glosses over the question of why QM works and of what happens in between one "definite event" and the next. But the attempt to go to the opposite interpretive pole, and say "OK, the wavefunction IS reality" is not a simple answer to your philosophical problems either; instead, it's the beginning of a whole new set of problems, including, how do you reify wavefunctions without running foul of relativity?

Returning to Eliezer's argument, which purports to derive the existence of a causal speed-limit from a postulate of "locality": my critique is as informal and inexact as his argument, but perhaps I've at least shown that this is not as simple a matter as it may appear to the uninformed reader. There are formidable conceptual problems involved just in getting started with such an argument. Eliezer has the essentials needed to think about these topics rigorously, but he's passing over crucial details, and he may thereby be overlooking a hole in his intuitions. In mathematics, you may start out with a reasonable belief that certain objects always behave in a certain way, but then when you examine specifics, you discover a class of cases which work in a way you didn't anticipate.

What if you have a field theory with no speed limit, but in which significant and ultra-fast-moving influences are very rare; so that you have an effective "locality" (in Eliezer's sense), with a long tail of very rare disruptions? Would Eliezer consider that a disproof of his intuitive idea, or an exception which didn't sully the correctness of the individual insight? I have no idea. But I can say that the literature of physics is full of bogus derivations of special relativity, the Born rule, the three-dimensionality of space, etc. This derivation of "c" from Pearlian causal locality certainly has the ingredients necessary for such a bogus derivation. The way to make it non-bogus is to make it deductively valid, rather than just intuitive. This means that you have to identify and spell out all the assumptions required for the deduction.