In quantum mechanics, the eigenvalue of an observable is said to be a good quantum number if the observable is a constant of motion. In other words, the quantum number is good if the corresponding observable commutes with the Hamiltonian. If the system starts from the eigenstate with an eigenvalue , it remains on that state as the system evolves in time, and the measurement of always yields the same eigenvalue .[1]
Good quantum numbers are often used to label initial and final states in experiments. For example, in particle colliders:[citation needed]