Instantaneous measurements of nonlocal variables in relativistic quantum theory (a review)

TL;DR

This review discusses the historical and recent developments in measuring nonlocal variables in relativistic quantum mechanics, highlighting how such measurements can be performed instantaneously without violating causality.

Contribution

It provides a comprehensive overview of six key papers, illustrating the evolution of understanding regarding nonlocal variable measurement in relativistic quantum theory.

Findings

Nonlocal variables can be measured instantaneously without causality violation.

Measurement methods have evolved from impossibility to feasible protocols.

Recent techniques do not rely on standard projective measurements.

Abstract

This article reviews six historically important papers in the development of the theory of measurement for nonlocal variables in quantum mechanics, with special emphasis the non violation of relativistic causality. Spanning more than seventy years, we chart the major developments in the field from the declaration, by Landau and Peierls in 1931, that measurement of nonlocal variables was impossible in the relativistic regime to the demonstration, by Vaidman in 2003, that all such variables \emph{can} be measured instantaneously without violation of causality through an appropriate act of "measurement", albeit not of a standard projective (Von Neumann) type.