But we're not in one of them. We're in all of them. Yours is a misunderstanding of how it feels to be in a quantum superposition. There's no component of the wave function in which Schrodinger's cat experiences anything other than being alive or being dead. There's no component in which it feels like it's in some ghostly combination of both outcomes. Both components of the wave function exist in superposition, and to the extent that they are decoherent, they do not interact and do not even notice that they have company. I don't mean this only as an academic statement; it's also true of you and me, at this very moment.
> There's no component of the wave function in which Schrodinger's cat experiences anything other than being alive or being dead.
The problem is that this is not what Schrodinger predicts, though. If |alive> and |dead> are valid solutions to the Schrodinger equation of some system, then so is any linear combination of them whose norm is one, such as 1/sqrt(2) |alive> + 1/sqrt(2) |dead>. So the math of QM predicts that there should be infinitely many cats that should feel partly alive and partly dead, many times more than the one live cat and the one dead cat. And yet you don't find this in any classical experiment.
1/sqrt(2) |alive> + 1/sqrt(2) |dead> is a valid solution to the Schrodinger equation. That cat is half alive and half dead.
That is different than |A cat that feels half alive and half dead>, which is its own completely different state. For a cat to feel like it is half alive and dead, or to notice that it is half alive and half dead, there are distinct things that happening in its brain that don't happen in either the |alive> or |dead> basis vectors, let alone any linear combination of the two.
We could even put all those states in a superposition!
But you can pick those other values as basis vectors, and they are just as good a choice as alive/dead. The only reason that you can say "the cat's brain doesn't experience those other states" is that you've presupposed a classical cat, that has some preferred basis. But this is just begging the question. We only get classical behavior when the wavefunction gets entangled with a classical object, but the math predicts you should also have non-classical "half-here" objects that would experience non-classical "half states" - and those are actually infinitely many.
So how did the classical world arise? Or how come we happen to be in the classical world, when there are infinitely many other non-classical observers? And note that the anthropic principle doesn't help here - all of the real laws of physics are the same for those other observers, you get the same chemistry etc, it's just the classical approximations that fail.
Not a perfect analogy, of course, but people have competing intuitions of what to do when the map fails to match the territory. MWI abandons the latter, most prefer the former.
- That humans can't be put into quantum superpositions, or
- That a human being in a quantum superposition of two states A and B would feel at all to that human like being in a quantum superposition of two states A and B, when neither of those states is "a human feeling like they're in a superposition of two states A and B".
Follow this to its conclusion, and you end up with a superposition of A and B meaning: "the universe is in a superposition, containing a component where a human believes that they're in state A, and a component where a human believes that they're in state B. Within both branches, the human thinks their wavefunction collapsed in that state". This is exactly the way that all interpretations of QM already understand things to work with particles in superpositions.
Every state is a superposition. Even eigenstates (of some operator, e.g. X) are superpositions (of eigenstates of some other operator, e.g. P). So everything is a superposition and being superposition or not is relative.
