Date:  2023-11-12

Article ID:  22871

In quantum theory, time evolution (the way a physical system changes in time) is usually expressed in terms of an operator called the hamiltonian. In a model with time translation symmetry, the hamiltonian can also be regarded as an observable representing the system's total energy. This article introduces some basic principles and tools related to time evolution in quantum theory, with emphasis on the role of the hamiltonian and its conceptual relationship to the action principle.