observer

An observer is a person or instrument that perceives, measures, or records events or phenomena. In everyday use, an observer notices and reports what is seen or heard. In science, observers are central to data collection and experimental design, and the distinction between observed systems and observers is a recurring theme in interpretation and methodology.

In physics, an observer often refers to a reference frame or a measurement standpoint. In relativity, quantities such as time and simultaneity can depend on the observer’s frame of reference. In quantum mechanics, measurement is described as an interaction that yields a definite result, though interpretations differ on the role of a conscious observer versus any measuring device or interaction with the environment.

The observer effect describes how the act of observation can influence a system. This is explicit in experimental concerns such as the Heisenberg uncertainty principle, and in social sciences where the presence of an observer can change participants’ behavior, as in the Hawthorne effect. Researchers address such biases through study design and controls.

In computing and software engineering, the observer pattern is a design pattern in which an object (the subject) maintains a list of observers and notifies them of state changes, enabling decoupled event handling.

In philosophy and cognitive science, the concept of the observer raises questions about perception, knowledge, and how experience shapes the understanding of phenomena, with various schools examining the conditions under which observers interpret reality.