When multiple agents use quantum mechanics to predict each other’s observations they will invariably end up with inconsistent results, according to a thought experiment reported in Nature Communications. The findings suggest that current interpretations of quantum theory cannot be extrapolated to consistently describe complex, macroscopic (large enough to be visible with the naked eye) systems.
Quantum mechanics can be successfully used to describe the microscopic world, where particles can exist ‘in superposition’ between different states at the same time. However, to be universally valid, the theory should in principle be able to model complex systems that include agents who are themselves using quantum theory. It has been unclear how to reconcile this with the fact that in the laboratory a scientist will experience a single, specific value each time they measure one of the superposed states. There are currently many interpretations of quantum mechanics to provide an answer to this question.
Renato Renner and Daniela Frauchiger show that there are situations involving multiple observers where many of these interpretations of quantum mechanics invariably fail to give a consistent description of reality. Their thought experiment involves four different agents, each one measuring a different quantity. If all the agents use quantum theory to model what they observe and to predict each other’s observation (and if we assume that each observer sees a single measurement outcome), then each observer’s outcome will be opposite to what the other would have expected.
This result suggests that the extension of quantum theory to include macroscopic systems is even less straightforward than was previously thought, and needs further development.