The observer effect

TL;DR

This paper offers a detailed classification of observation types in physics, clarifying how different observational processes relate to quantum and classical measurements, emphasizing the role of control, invasiveness, and relational properties.

Contribution

It introduces a comprehensive framework for understanding observation in physics, integrating classical, quantum, and intermediate properties through a classification based on control, invasiveness, and relational aspects.

Findings

Observation can be non-invasive or invasive, involving creation or destruction.

Quantum measurements involve product tests, leading to probabilistic outcomes.

Observations can target intrinsic, relational, or intermediate properties.

Abstract

Founding our analysis on the Geneva-Brussels approach to the foundations of physics, we provide a clarification and classification of the key concept of observation. An entity can be observed with or without a scope. In the second case, the observation is a purely non-invasive discovery process; in the first case, it is a purely invasive process, which can involve either creation or destruction aspects. An entity can also be observed with or without a full control over the observational process. In the latter case, the observation can be described by a symmetry breaking mechanism, through which a specific deterministic observational process is selected among a number of potential ones, as explained in Aerts' hidden measurement approach. This is what is called a product test, or product observation, whose consequences are that outcomes can only be predicted in probabilistic terms, as it is the case in typical quantum measurements. We also show that observations can be about intrinsic (stable) properties of the observed entity, or about relational (ephemeral) properties between the observer and observed entities; also, they can be about intermediate properties, neither purely classical, nor purely quantum. Our analysis allows us to propose a general conceptual characterization of quantum measurements, as observational processes involving three aspects: (1) product observations, (2) pure creation aspects and (3) ephemeral relational properties. We also discuss the important concept of non-spatiality and emphasize some of the differences and similarities between quantum and classical/relativistic observations.