A recent experiment seems to confirm that different observers see conflicting realities: https://www.technologyreview.com/s/613092/a-quantum-experime...
I'm not a physicist and only dug deeply enough into the topic to understand the basic quantum algorithms for factoring and discrete logarithm, but it seems to me that there is very obviously no conflict. It's just that the "observer of the observer" observes a meta-reality that contains all possible realities observed by the "observer". There doesn't seem anything strange about this if you just don't insist on "measurement" having a special physical meaning and accept the universal wave function. But maybe there's a twist I'm missing.
The latter. QM is weird, but it's not paradoxical.
If a set of atoms can be in a superposition, so can be you. Because you are a set of atoms, duh.
It does not follow that "if a set of atoms can be in a superposition, so can you." Here's some ways it might fail:
1. In the objective collapse theories, quantum systems undergo collapse when they reach a certain size. I cannot be in a superposition because I am physically too big.
2. In the ensemble interpretations, a quantum state applies to a set of many identically prepared systems. Since there is only one of me, we cannot speak meaningfully of my quantum properties.
Admittedly neither of these are satisfying interpretations, but they have their proponents.
The deeper objection is that "you can be in a superposition" risks detachment from experiments. If we cannot talk about the result of an experiment, that we cannot do science.
> From the friend’s perspective, the qubit has been measured and has collapsed to either |0〉 or |1〉. From Wigner’s perspective, no such thing has happened—there’s only been unitary evolution—and in principle, Wigner could even confirm that by measuring |ψ〉 in a basis that included |ψ〉 as one of the basis vectors. But how can they both be right?
> Many-Worlders will yawn at this question, since for them, of course “the collapse of the wavefunction” is just an illusion created by the branching worlds, and with sufficiently advanced technology, one observer might experience the illusion even while a nearby observer doesn’t. Ironically, the neo-Copenhagenists / Quantum Bayesians / whatever they now call themselves, though they consider themselves diametrically opposed to the Many-Worlders (and vice versa), will also yawn at the question, since their whole philosophy is about how physics is observer-relative and it’s sinful even to think about an objective, God-given “quantum state of the universe.” If, on the other hand, you believed both that
> 1. collapse is an objective physical event, and
> 2. human mental states can be superposed just like anything else in the physical universe,
> then Wigner’s thought experiment probably should rock your world.