Contrary to some misunderstandings of it, it doesn't "add" extra worlds; it removes the concept of "wave function collapse", and leaves all the other known laws of quantum mechanics completely unchanged.
Yes, it gets rid of the collapse postulate, but no, it actually introduces many worlds. You can wiggle a bit around, claim that prior to the wave function collapse there are also many worlds in Copenhagen or whatnot, but in the end many worlds makes a metaphysical claim that two cats exist, one dead, one alive while Copenhagen claims only one cat exists in the end.
No, this is the misunderstanding that I'm talking about.
The extra "worlds" follow directly and exclusively from the existence of the various basis states in a wave function, and the laws of entanglement. No other postulates are needed.
Before the measurement/entanglement, the system and environment are independent, and can be written (|0> + |1>) ⊗ (|0> + |1>). After the entanglement, the wave function of the universe can no longer be factored that way, and the system and environment are in a joint state of |00> + |11>. The |00> and the |11> are the multiple "worlds", they show up— in both interpretations— whether you want them to be there or not.
Copenhagen doesn't want them to be there, so it says that one of the |00> or |11> goes away... at some point... because [waves hands and mumbles]. Many worlds merely declines to do this, and that is legitimately the only difference between the two.
The extra "worlds" follow directly and exclusively from the existence of the various basis states in a wave function, and the laws of entanglement. No other postulates are needed.
The many worlds are in the entangled state but then the collapse postulate reduces them to one world. If you remove the collapse postulate you put them back in. And sure, the collapse postulate is an awful solution breaking unitary evolution and you have every right to reject it, but that does not change the fact that many world introduces - or at least not removes - additional worlds that are not there in Copenhagen.
The distinction between "adding" and "removing" a postulate is an important, non-arbitrary one.
The "worlds" are there in both theories; Copenhagen adds a new phenomenon (non-unitary evolution) which makes some of them disappear at unspecified times. The "worlds" are direct consequences of suppositions shared with Copenhagen.
Many worlds has N postulates, Copenhagen has no fewer than N+1. One theory is a strict subset of the other's premises. It is not at all accurate to say that many worlds is the one that "introduces" suppositions.
I think it is not as simple as counting the number of postulates. The ultimate arbiter is physical reality and that decides whether removing or adding postulates is what brings you into agreement. The fact that the quantum state in some equation before wave function collapse looks like many worlds say nothing whether you should take this at face value or whether you are missing an additional postulate that gives you only one world.
If you argue that the collapse postulate is stupid because it is non-unitary and therefore in conflict with experimental evidence, then sure, I totally agree with this. But just because it is an additional postulate it does not per se make many worlds the better theory. Relativity without the constant speed of light is also a theory with one fewer postulate but it of course in much worse agreement with reality.
The problem with the interpretations is that we are currently unable to distinguish them experimentally which unfortunately adds much more personal preferences to the discussion then there should be.
Copenhagen sort of agrees with observation by introducing non-locality, which disagrees with observation of locality from other observations, which means Copenhagen disagrees with observation.
>But just because it is an additional postulate it does not per se make many worlds the better theory.
The postulate of collapse contradicts the postulate of Schrodinger equation and makes the system of postulates contradictory. By removing the contradiction MWI is strictly better, this in turn also removes non-locality and achieves a strictly better agreement with observation, like special theory of relativity.
> Yes, it gets rid of the collapse postulate, but no, it actually introduces many worlds. [...] but in the end many worlds makes a metaphysical claim that two cats exist, [...] while Copenhagen claims only one cat exists in the end.
If you are saying that many worlds makes a greater number of claims than Copenhagen, that's incorrect, as explained above. The claims made by many worlds are a strict subset of Copenhagen.
If you are claiming that many words "introduces" the worlds but Copenhagen does not, that's also incorrect, because the worlds (it seems we agree) are also there in Copenhagen. If they weren't, there would be nothing to "collapse" in the first place!
If you're not saying one of those things, then I'm not sure what that paragraph is trying to say.
> But just because it is an additional postulate it does not per se make many worlds the better theory
It absolutely does, given that both agree equally well with observation.
Each additional assumption in a theory is an opportunity to be wrong. Therefore, given two theories which are in agreement with observation, the theory with fewer unchecked assumptions has a higher probability of being right.
A theory which disagrees with observation (like your altered relativity example) has zero percent chance of being right, so those kinds of examples aren't applicable.
This is just a somewhat more rigorous way of explaining why Occam's Razor is so effective.
For example, take [the dragon in Sagan's garage][1]. We have two models of reality: A garage containing (a) a dragon, who is (b) invisible, (c) dodges touch, (d) floats in air, (e) gives off no heat (f) etc, etc. Or we have a world/garage where none of those things are true.
Both models agree with observation— any measurement we make will not contradict either "empty garage" or "undetectable dragon". And yet one of them is a better theory. How do we know which one is which? The Undetectable Dragon Theory has far more unchecked assumptions (a...f) than the Empty Garage theory (everything in Undetectable Dragon, minus (a...f)).
Same thing for (forgive the extreme example) conspiracy theories. Moon landing hoaxers' ideas agree with observation— they just pile on a mountain of unchecked suppositions in order to avoid contradictions. If we're holding ourselves to good standards of belief, we pick the world model without all those extra unchecked assumptions. This becomes crucially important when there's disagreement about which theory is the Invisible Dragon theory.
Of course Copenhagen isn't nearly as bad as either of those, but the point is to call attention to the epistemological weight of each assumption we add, and what strategy we use for picking between two theories that are not (yet) contradicted by data. Copenhagen is doing more epistemological lifting, and so if we want to be good skeptics and efficient world model-builders, we should require its unchecked assumptions to be checked before we prefer it over other, more parsimonious theories which also agree with the data.
> which unfortunately adds much more personal preferences to the discussion
I quite disagree. The question is of "which strategy to use for picking models of reality which are most likely to be right". There are objectively good and bad strategies for doing that, in the same way that there are good and bad strategies for designing an airplane, or winning at chess, or proving a theorem. Of course no strategy guarantees success, and a "good" strategy might occasionally perform worse than a "bad" one— But without advance access to the solution, we don't know what those exceptions are, so our best bet is to go with the strategy which performs best a priori. In this case, for the sake of avoiding accidental belief in Dragons, the number of unchecked assumptions is centrally important!
I've noticed that some people are simply allergic to the MWI view, likely because of a religious background. Strangely, my physics classes were about 80% fundamental, bible-carrying Christians, as were many of the "Fathers of Quantum Mechanics"!
Many mathematically and physically theories that are perfectly reasonable are rejected by such people out of hand because it doesn't mesh well with their preconceptions of "The Earth is Special", "I have a unique soul that is me", "Jesus came to us, here, specifically", etc...
I know I'll probably get voted down for this, but these are the literal arguments that I was given once I pressed some of my fellow students hard enough on why they reject MWI.
It's not because they investigated the logic of the situation, like you have. They just "feel" like MWI makes them less special and unique in Creation.
Yes, it gets rid of the collapse postulate, but no, it actually introduces many worlds. You can wiggle a bit around, claim that prior to the wave function collapse there are also many worlds in Copenhagen or whatnot, but in the end many worlds makes a metaphysical claim that two cats exist, one dead, one alive while Copenhagen claims only one cat exists in the end.