# $\psi$-epistemic interpretations of quantum theory have a measurement problem

## TL;DR

This paper shows that all known $$-epistemic models of quantum theory fail to accurately represent state updates during measurement, challenging their core motivation and suggesting limitations of epistemic interpretations.

## Contribution

It proves that existing $$-epistemic models cannot correctly model measurement updates, highlighting fundamental issues with this class of interpretations.

## Key findings

- All known $$-epistemic models in dimension $$ or higher fail to represent state update.
- Wavefunction real interpretations avoid these restrictions but face other criticisms.
- The results suggest a potential no-go theorem for $$-epistemic models.

## Abstract

$\psi$-epistemic interpretations of quantum theory maintain that quantum states only represent incomplete information about the physical states of the world. A major motivation for this view is the promise to provide a reasonable account of state update under measurement by asserting that it is simply a natural feature of updating incomplete statistical information. Here we demonstrate that all known epistemic ontological models of quantum theory in dimension $d\geq3$, including those designed to evade the conclusion of the PBR theorem, cannot represent state update correctly. Conversely, interpretations for which the wavefunction is real evade such restrictions despite remaining subject to long-standing criticism regarding physical discontinuity, indeterminism and the ambiguity of the Heisenberg cut. This revives the possibility of a no-go theorem with no additional assumptions, and demonstrates that what is usually thought of as a strength of epistemic interpretations may in fact be a weakness.