Observable
Definition and meaning of Observable in chemistry.
An observable is a measurable property or quantity of a chemical system that can be determined through experiment; in quantum mechanics, observables are formally represented as mathematical operators.
In more detail
In classical and general chemistry, observables include any measurable quantity such as temperature, pressure, concentration, pH, and mass. In quantum mechanics, observables must have corresponding Hermitian operators that act on wavefunctions; measuring an observable returns one of its eigenvalues. Not all classical properties have quantum mechanical analogs with well-defined operators, which reflects fundamental differences between classical and quantum systems.
Key facts
| Field | Physical Chemistry |
|---|---|
| Quantum representation | Hermitian operators |
| Common observables | Energy, momentum, position, angular momentum |
| Measurement result | Eigenvalue of the operator |
The total energy of a molecule is an observable represented by the Hamiltonian operator; measuring its energy yields one of the molecule's allowed energy eigenvalues.
Frequently asked questions
What is the difference between an observable and a measurement?
An observable is the physical quantity being measured; measurement is the act of determining its value in an experiment.
Why must observables be Hermitian operators in quantum mechanics?
Hermitian operators guarantee that all eigenvalues (measurement outcomes) are real numbers, which is necessary for physical observables to have real-valued results.