And what about the latest fashion in quantum mysterian circles: the Wigners' friend thought-experiment of Daniela Frauchiger and Renato Renner! You have the quantum system, observed by Mr. Wigner and also by Mrs. Wigner; and you have the Wigners' friend (ie, the friend of both of the two Wigners) who can observe both of them.
https://arxiv.org/abs/1604.07422 "Quantum theory cannot consistently describe the use of itself"
Abstract: Quantum theory provides an extremely accurate description of fundamental processes in physics. It thus seems likely that the theory is applicable beyond the, mostly microscopic, domain in which it has been tested experimentally. Here we propose a Gedankenexperiment to investigate the question of whether quantum theory can, in principle, have universal validity. The idea is that, if the answer was yes, it must be possible to employ quantum theory to model complex systems that include agents who are themselves using quantum theory. Analyzing the experiment under this presumption, we find that one agent, upon observing a particular measurement outcome, must conclude that another agent has predicted the opposite outcome with certainty. The agents' conclusions, although all derived within quantum theory, are thus inconsistent. This indicates that quantum theory cannot be extrapolated to complex systems, at least not in a straightforward manner.
Nature Communications 9, 3711 (2018); DOI: 10.1038/s41467-018-05739-8
I understand that Frauchiger and Renner end up with only the many-worlds theory, and qubism, as interpretations of quantum theory which are compatible with the predictions of quantum theory. However this is circular reasoning as by quantum theory, they mean explicitly "quantum theory with no collapse". So I think their thought experiment proves that we need collapse theories, such as the "eventum mechanics" of Slava Belavkin [RIP], or Joy Christian's theory. This leads us to quarrel about whether or not either is "local". I think we need a new definition of "local", or we need a new word instead of "spooky". Something benevolent and *not* scary, though obviously, something which is hard to understand. Maybe just "shut up and calculate" was the best answer, all along.
"Many worlds" means there is just the wave function of the universe which is evolving deterministically. So the only real thing in the universe is its wave function.
Qubism means that it is all in the mind (or minds) of observers. So there is nothing real in the universe at all except, I suppose, the minds of observers.