[Joker_vD]

Or. Or you can go with poorly-named "many worlds" by applying the Occam's razor to the ridiculous concepts of "classical observer" and "wave-function collapse".

There is no such special thing as a "classic observer": all instruments and people are in the end made out of quanta, and are themselves huge quantum systems.

There is no such special thing as a "wave function collapse from observation": what happens is that the observer (a quantum system) becomes entangled with the observed system. That's it. Initial state of "[ignorant observer] * ([cat is dead] + [cat is alive]) / sqrt(2)" which is equal to "([ignorant observer, cat is dead] + [ignorant observer, cat is alive]) / sqrt(2)" evolves into "([observer thinking of a dead cat, cat is dead] + [observer thinking of a living cat, cat is alive]) / sqrt(2)" without breaking the unitarity.

Yes, for "the" observer, her being part of this state, it looks like "the collapse has happened". That's how you get "many worlds": the state of the universe is a huge sum of independent states that continue to evolve independently, each of them evolving further into a sum of independent states.