I'm not a subscriber to the many-worlds interpretation so I'm not the right person to defend it, partly for this very reason: the question of when one universe "splits off" doesn't actually have an answer, for the same reason that the question of "when does a measurement actually happen?" doesn't actually have an answer. Many-worlds and Copenhagen both have this rhetorical problem: they want to draw a sharp line when something "happens" (collapse, universe-split) and there is no such sharp line. The transition from quantum to classical is gradual, not abrupt. It typically happens very fast (picosecods or femtoseconds to get to the point where the state of the system is no longer distinguishable from a classical state) which is why it appears to be an abrupt transition, but it's not. This is one of the many reasons that I find the QIT/zero-worlds interpretation to be the most satisfactory. It and decoherence are the only ones that don't have this problem. But decoherence is too mathy :-)
IANAP, but I think part of the problem with MWI is that popularizers of it wanted to sound cool by calling it the Many-Worlds Interpretation in the first place.
The idea was originally known as the Theory of the Universal Wave Function, which makes a lot more sense as long as you're unafraid of mathematics.
There is no "splitting off" of universes; I agree with you that that wouldn't make much sense. Instead, the observer simply becomes entangled with the observed physical system during the measurement process. This entanglement is a gradual process, though it certainly happens very quickly.
This basically pushes all the weirdness out of quantum mechanics and into the fact that we just don't understand consciousness very well. Why don't we perceive the full linear combination of quantum states? And can we somehow map our subjective experience of probability and statistics to what happens in the Universal Wave Function? It's much easier to ignore that weirdness, since we're used to ignoring it in our daily lives anyway. It's also a more parsimonious interpretation than the others, because it doesn't postulate anything special about us (the "observers").
(This does not mean that consciousness is a quantum or even meta-physical phenomenon. Quite the opposite, actually: I find that the mystery lies more in why consciousness is unable to perceive quantum states despite existing in a universe that has quantum physics.)
> Why don't we perceive the full linear combination of quantum states?
My personal belief is that it is because consciousness is a classical information-processing phenomenon. In other words, we can only directly perceive things that can be described as real numbers rather than complex numbers because we are Turing machines, and Turing machines are classical.
That's an interesting thought, especially with the addendum of the no-cloning theorem.
This fits with a Hofstadter-like perspective that consciousness is about "strange loops", where we somehow repeatedly evaluate simplified models of the world including yourself. Doing such repeated evaluations requires (at least partial) "cloning" of the state of the world outside for the purpose of evaluation, and cloning quantum states is impossible, hence consciousness must be a classical phenomenon.
I like that line of argument.
Note: I wouldn't state it as being unable to perceive things that can be described as complex numbers, but rather complex linear combinations.
No, because records are necessarily classical. This is because of the no-cloning theorem. You cannot copy quantum information, only classical information.
Wait a minute, since when is there any difference between Many Worlds and Decoherence? I always assumed they referred to the same interpretation, do they not?
There are actually three different ideas in play here: decoherence as described by Zurek, the relative-state interpretation as described by Everett, and the popular account of the universe "splitting" when a measurement is made. All three of these are different, at least rhetorically, even though at a high enough conceptual level they all amount to the same thing.
