I think that EY has played a cruel joke (or maybe it was a rationality test for the readers), where he misrepresented an active area of physics research as an open-and-shut case of the MWI being the One True Teaching. (The alternative is an unthinkable weirdtopia: EY failed at rationality?!?!)
Were it not for the Quantum Physics sequence, the LWers would not bring the issue up as often, given the many many other active areas of (Physics) research that are just as deceptively simple to an uninitiated.
Consider, for example, an alternate universe where the great rationalist Zainab Al-Arabi runs a forum she named Not As Misguided, where she advocated, among other things, that the Universe is obviously shaped like the Poincaré dodecahedral space, even though it has never been tested, and many other shapes fit the data just as well. The forum participants, NAMers, few of whom have the necessary background in the area, nevertheless engage in an occasional heated debate about the right shape of the Universe, frequently referring to ZAA's other teachings for justification.
I think the reality is that Eliezer Yudowsky, while a very bright mind and great man in terms of rationality, has overstepped his limits when it comes to physics.
He do admit that there is currently no satisfactory solution to the Born Rule issue, yet he has written several posts talking about MWI as it is "obviously true". That is quite irrational. Quantum mechanics is, after all, ALL about the probabilities predicted by Born Rule, that is the essence of QM, if a model gets these probabilities wrong, it is obviously in deep trouble.
I am quite dis...
The subject has already been raised in this thread, but in a clumsy fashion. So here is a fresh new thread, where we can discuss, calmly and objectively, the pros and cons of the "Oxford" version of the Many Worlds interpretation of quantum mechanics.
This version of MWI is distinguished by two propositions. First, there is no definite number of "worlds" or "branches". They have a fuzzy, vague, approximate, definition-dependent existence. Second, the probability law of quantum mechanics (the Born rule) is to be obtained, not by counting the frequencies of events in the multiverse, but by an analysis of rational behavior in the multiverse. Normally, a prescription for rational behavior is obtained by maximizing expected utility, a quantity which is calculated by averaging "probability x utility" for each possible outcome of an action. In the Oxford school's "decision-theoretic" derivation of the Born rule, we somehow start with a ranking of actions that is deemed rational, then we "divide out" by the utilities, and obtain probabilities that were implicit in the original ranking.
I reject the two propositions. "Worlds" or "branches" can't be vague if they are to correspond to observed reality, because vagueness results from an object being dependent on observer definition, and the local portion of reality does not owe its existence to how we define anything; and the upside-down decision-theoretic derivation, if it ever works, must implicitly smuggle in the premises of probability theory in order to obtain its original rationality ranking.
Some references:
"Decoherence and Ontology: or, How I Learned to Stop Worrying and Love FAPP" by David Wallace. In this paper, Wallace says, for example, that the question "how many branches are there?" "does not... make sense", that the question "how many branches are there in which it is sunny?" is "a question which has no answer", "it is a non-question to ask how many [worlds]", etc.
"Quantum Probability from Decision Theory?" by Barnum et al. This is a rebuttal of the original argument (due to David Deutsch) that the Born rule can be justified by an analysis of multiverse rationality.