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

TL;DR

This paper presents a no-go theorem showing that in Wigner's friend scenarios, perceptions of measurement outcomes at different times cannot share the same reality under standard quantum assumptions, challenging the notion of persistent measurement reality.

Contribution

The paper formulates a no-go theorem for the persistent reality of Wigner's friend's perception, highlighting fundamental limitations of quantum theory in describing observer experiences over time.

Findings

No joint probability distribution for the friend's outcomes at different times under quantum assumptions.

Implication that quantum theory may require nonlinear modifications or restrictions on predictions.

Challenges to the idea that unitary quantum mechanics provides valid single-time predictions for all observers.

Abstract

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