Is the quantum state real? An extended review of $ψ$-ontology theorems

the agent updates their beliefs about the outcomes of future interactions in a manner analogous to Bayesian updating. The fact that this update process happens to be conveniently described by a mathematical operation we call "collapse" is neither here nor there to a QBist. I think your second objection is something like this: some beliefs are better than others. In many situations there appears to be one belief which is "the best". Therefore whichever belief is "the best" is essentially an objective description of the world. Therefore the quantum state of a system is real and objective. Is that a fair assessment? Whether or not this objection holds water, I must retain the claim that in QBism the wave function is not "really objectively out there". It is clear that in QBism a quantum state represents an agent's beliefs (or in weaker interpretations, an agent's information) regarding a system, not something objective about the system. Fuchs, Schack, Caves and others have said this again and again. For example: > Contrary to those desires, quantum theory does not describe physical reality. What it does is provide an algorithm for computing probabilities for the macroscopic events (“detector clicks”) that are the consequences of our experimental interventions. http://www.phy.pku.edu.cn/~qhcao/resources/class/QM/PTO00007... > In other words, Fuchs argued, the wave function does not describe the world—it describes the observer. “Quantum mechanics,” he says, “is a law of thought.” Quantum Bayesianism, or QBism as Fuchs now calls it, solves many of quantum theory’s deepest mysteries. Take, for instance, the infamous “collapse of the wave function,” wherein the quantum system inexplicably transitions from multiple simultaneous states to a single actuality. According to QBism, the wave function’s “collapse” is simply the observer updating his or her beliefs after making a measurement. https://www.wired.com/2015/06/private-view-quantum-reality/ > QUANTUM STATES DO NOT EXIST > The world may be full of stuff and things of all kinds, but among all the stuff and all the things, there is no unique, observer-independent, quantum-state kind of stuff > Specifically, there can be no such thing as a right and true quantum state, if such is thought of as defined by criteria external to the agent making the assignment: Quantum states must instead be like personalist, Bayesian probabilities https://arxiv.org/pdf/1003.5209.pdf BTW, have you heard of the PBR theorem? It made me do a lot of thinking about what it could possibly mean for the quantum state to be a physical fact vs simply information about an underlying state. If these sorts of ideas are interesting to you, you might enjoy a write-up of the theorem by Matt Leifer: https://arxiv.org/pdf/1409.1570.pdf