Measurement: still a problem in standard quantum theory

TL;DR

This paper critiques recent claims denying the measurement problem in standard quantum theory and emphasizes the longstanding issues with interpreting reduced density operators as epistemic mixtures.

Contribution

It refutes claims that quantum measurement problems are resolved and clarifies the conceptual issues with reduced density operators as epistemic states.

Findings

Recent claims dismissing the measurement problem are unfounded

Reduced density operators cannot be interpreted as ignorance-based mixtures

The measurement problem remains a fundamental issue in quantum theory

Abstract

It is argued that recent claims by A. Hobson that standard quantum theory has no measurement problem cannot be sustained. Moreover, it is pointed out that taking the reduced density operator of a component system as an epistemic (ignorance-based) mixture has long been known to be untenable based on direct proof.