Not sure, but I am a physicist and am definitely bringing out the popcorn for this thread.

The deep reason is that he read a popsci book he liked

I mean, his arguments for Many Worlds aren’t great. But you can certainly find some physicists who have very similar opinions. David Deutsch obviously, Stephen Hawking also once said MWI was “trivially true”. You can also count in Roger Penrose who says quantum mechanics implies MWI as true and that’s one of the reasons why he thinks quantum mechanics has to be broken somehow: https://web.archive.org/web/20071023063403/http://www.sciencewatch.com/interviews/roger_penrose2.htm

My perspective from the sidelines is that of the physicists who think it is worth thinking about a lot of them like many worlds or at least agree that it makes some amount of sense - but with the larger mainstream concluding that this is ultimately not really something to worry about since it doesn’t lead to falsifiable predictions so it’s pointless guessing.

The rationalsphere exalts and relies on popsci explanations because they refuse to delve deeper into the actual science.

Why many world when few world do trick?

^^sorry

Sneeringly, it’s obviously true because otherwise his entire body of work would be invalidated. Can’t adjust your priors like that, it’s Not Done in rationality.

MWI and modal realism are strictly different concepts, with the one being a scientific explanation for quantum phenomena and the latter being a metaphysical explanation for phenomena in logic

I don’t know if you’re conflating the two but your bringing up philosophy and metaphysics suggests to me that you’re doing that

MWI enthusiasts think that it preserves a common sense understanding of reality by adding to quantum weirdness a myriad of universes which split off from each other whenever a quantum event is observed; it isn’t the metaphysics of modal realism because modal realists (most famously David Lewis) aim for an account of logical necessity

Yudkowsky’s account of MWI is fairly standard, even if both grandiose and shallow - a pop-science understanding of the relevant physics, which I’ll admit is sort of where I’m coming from too, although I have some background knowledge at a higher level - in that he wants to do the same thing: preserve realism about physical objects without throwing our hands up in the air and saying “shut up and calculate”

Speaking in my capacity as a philosopher of science, the basic idea is as stated above: MWI fans want to say “reality is as real as we already found it, not a mere mathematical construct”; MWI fans like Yudkowsky add the auxiliary hypothesis that there are multiple universes which exist side-by-side (so to speak) which are all as real as each other

The distinction between the metaphysics and the physics is that the latter are at least internally supposed to be provable by observation

Yudkowsky’s own arguments for this are vague and unimpressively so, and drawn from pop-science and his insistence - ironically given what I’ve already said - on a naturalistic metaphysics which demands amongst other things such as utilitarianism and realism about personal identity, that quantum fluctuations be as real as you, your body, your cat etc.

Stripping away a lot of verbiage and what seem like ex-post-facto justifications, I think the underlying psychology is that he found the* MWI the “simplest” interpretation and rolled with it, getting that thrill that comes with proclaiming belief in something that you can present as counterintuitive. It’s a harsh truth, or an uncomfortable truth, or a strange truth — you know, the kind of truth that makes you a more impressive person for espousing.

*Obligatory footnote that, in years of following arguments in quantum foundations, I don’t think I’ve met two MWI’ers who have agreed on how to wring probabilities out of it and make it an actual predictive scientific theory. The formal literature will say that the attempts to do so have been “many and generally incompatible”, but if anything that undersells the disagreements you’ll get at conferences or in e-mail threads. The Rationalist take seems to be that the scientific community is broken because the scientists can’t agree on the simple answer, indicating that the scientists are not sufficiently Rational. Though if the answer were really so simple, it perhaps wouldn’t come in so many flavors, each defended in articles that run to dozens of pages….

I wouldn’t overthink it. It’s a cool sounding idea that’s reasonably easy for a casual person to feel like they understand, and lets him come up with all kinds of sci-fi-ish stuff on top of that. Any other explanation he’s given is just a rationalization of why what he wants to be true must be correct.

Because it allows him and his followers to propose all kinds of batshit crazy multiverse stuff without any kind of evidence. IIRC Roko once said on twitter that the basilisk actually came from some MWI bullshit.

Well, it is the only way to quantum immortality. (I have no clue btw, I just like the … I think Neil Gaiman … short story about quantum immortality).

So I don’t have any reason to assume that any of these concepts are being used correctly, but if I had to reconstruct his claim the way he says it:

Yudkowsky’s understanding of Reductionism and Probability Anti-Realism, two positions that he defends, is that it should be possible to set up an experiment so that every quantum element in the experiment space is exactly identical every time you run the experiment, and that every single time you run the experiment it should produce an identical result. But non MWI positions in quantum physics suggest this might not be true: that you might run the same quantum state experiment, and 75% of the time get one result and 25% of the time get another. The very possibility of this result is a threat to the simultaneous truth of Reduction and Anti-Realism, so these other models must be rejected, leaving MWI as the only surviving choice.