Observable

An observable is something that can be perceived or measured. In science, an observable quantity is a property that can be directly or indirectly measured under controlled conditions, such as length, mass, temperature, or brightness. In philosophy and physics, observables are contrasted with unobservable or hidden aspects of a system, and they provide the basis for empirical tests and theories.

In quantum mechanics, observables are associated with mathematical operators on a Hilbert space. The possible results of measuring an observable are eigenvalues of the corresponding operator, and the theory assigns probabilities to these results via the state vector. Measurement can influence the system, and not all properties have definite values prior to measurement, leading to interpretations of quantum randomness and wavefunction collapse.

In computer science, particularly in reactive programming, an Observable is a construct that represents a stream of data over time. It allows observers to subscribe and receive values as they are produced by a data source. Observables support composition through operators that transform, filter, or combine streams, and they handle asynchronous events, errors, and completion. Common implementations appear in libraries for JavaScript, Java, and other languages.

Observables in different fields share a focus on observability and measurement, adapting the concept to their tools and problems. The term is often used without ambiguity within a given domain but may refer to distinct ideas across disciplines, from physical quantities to data streams.