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The notorious Wigner's friend thought experiment (and modifications thereof)has in recent years received renewed interest especially due to new argumentsthat force us to question some of the fundamental assumptions of quantumtheory. In this paper, we formulate a no-go theorem for the persistent realityof Wigner's friend's perception, which allows us to conclude that theperceptions that the friend has of her own measurement outcomes at differenttimes cannot "share the same reality", if seemingly natural quantum mechanicalassumptions are met. More formally, this means that, in a Wigner's friendscenario, there is no joint probability distribution for the friend's perceivedmeasurement outcomes at two different times, that depends linearly on theinitial state of the measured system and whose marginals reproduce thepredictions of unitary quantum theory. This theorem entails that one musteither (1) propose a nonlinear modification of the Born rule for two-timepredictions, (2) sometimes prohibit the use of present information to predictthe future -- thereby reducing the predictive power of quantum theory -- or (3)deny that unitary quantum mechanics makes valid single-time predictions for allobservers. We briefly discuss which of the theorem's assumptions are morelikely to be dropped within various popular interpretations of quantummechanics.

A no-go theorem for the persistent reality of Wigner's friend's perception