The Many-Worlds Interpretation (MWI) of quantum mechanics is probably roughly correct. There is no reason to think that the rules of atomic phenomena would stop applying at larger scales when an experimenter becomes entangled with their experiment (kooky interjections about consciousness notwithstanding…).
However, MWI has the problem that it does not easily explain why quantum randomness leads to the probabilities that we observe. The Born Rule says that if a system is in a state , upon ‘measurement’ (in which we entangle with one or the other outcome), we measure the eigenvalue associated with the state with probability
The Born Rule is normally included as an additional postulate in MWI, and this is somewhat unsatisfying. Or at least, it is apparently difficult to justify, given that I’ve read a bunch of attempts, each of which talks about how there haven’t been any other satisfactory attempts. I think it would be unobjectionable to say that there is not a consensus on how to motivate the Born Rule from MWI without any other assumptions.
Anyway here’s an argument that I find somewhat compelling? See what you think.