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Note that the Hamiltonian that appears in the final line above is the Heisenberg Hamiltonian (), which may differ from the Schrödinger Hamiltonian (). An important special case of the equation above is obtained if the Hamiltonian H S {\displaystyle H_{\rm {S}}} does not vary with time.
The concept of time evolution may be applicable to other stateful systems as well. For instance, the operation of a Turing machine can be regarded as the time evolution of the machine's control state together with the state of the tape (or possibly multiple tapes) including the position of the machine's read-write head (or heads). In this case ...
In quantum mechanics, the Schrödinger equation describes how a system changes with time. It does this by relating changes in the state of the system to the energy in the system (given by an operator called the Hamiltonian). Therefore, once the Hamiltonian is known, the time dynamics are in principle known.
Time evolution described by a time-independent Hamiltonian is represented by a one-parameter family of unitary operators, for which the Hamiltonian is a generator: () = ^ /. In the Schrödinger picture , the unitary operators are taken to act upon the system's quantum state, whereas in the Heisenberg picture , the time dependence is ...
The time evolution of Hamilton's equations is a symplectomorphism, meaning that it conserves the symplectic 2-form. A numerical scheme is a symplectic integrator if it also conserves this 2-form. Symplectic integrators possess, as a conserved quantity, a Hamiltonian which is slightly perturbed from the original one. [1]
In physics and thermodynamics, the ergodic hypothesis [1] says that, over long periods of time, the time spent by a system in some region of the phase space of microstates with the same energy is proportional to the volume of this region, i.e., that all accessible microstates are equiprobable over a long period of time.
Thus, the time evolution of a function on a symplectic manifold can be given as a one-parameter family of symplectomorphisms (i.e., canonical transformations, area-preserving diffeomorphisms), with the time being the parameter: Hamiltonian motion is a canonical transformation generated by the Hamiltonian.
where the interaction-picture perturbation Hamiltonian becomes a time-dependent Hamiltonian, unless [H 1,S, H 0,S] = 0. It is possible to obtain the interaction picture for a time-dependent Hamiltonian H 0,S ( t ) as well, but the exponentials need to be replaced by the unitary propagator for the evolution generated by H 0,S ( t ), or more ...