“The experiment, designed by Daniela Frauchiger and Renato Renner, of the Swiss Federal Institute of Technology Zurich, involves a set of assumptions that on the face of it seem entirely reasonable. But the experiment leads to contradictions, suggesting that at least one of the assumptions is wrong. The choice of which assumption to give up has implications for our understanding of the quantum world and points to the possibility that quantum mechanics is not a universal theory, and so cannot be applied to complex systems such as humans.”
—Anil Ananthaswamy, “Frauchiger-Renner Paradox Clarifies Where Our Views of Reality Go Wrong.” Quanta Magazine. December 3, 2018.
Probably the clearest explainer you’ll find. The assumptions are that: quantum theory is universal, quantum theory is consistent, and opposite facts cannot both be true. This thought experiment suggests that at least one is false, and depending on which one either leads to positions that quantum theory collapses into classical physics at scale, observer perspective changes results, or the many worlds hypothesis.