[Physics] Full time-derivative of a function and Schrodinger equation

commutatorpoisson-bracketsquantum mechanicsschroedinger equation

From Hamiltonian formalism there is well known equation,
$$
\frac{d F}{dt} = \frac{\partial F}{\partial t} + \{F, H\}_{PB},
$$
where $ \{H, F\}_{PB}$ is the Poisson bracket.

After using Hamiltonian formalism in quantum mechanics it transforms into
$$
\frac{d \hat {F}}{dt} = \frac{\partial \hat {F}}{\partial t} + \frac{i}{\hbar }[\hat {H}, \hat {F}]
$$
in the Heisenberg picture, where $[\hat {H}, \hat {F}]$ is the commutator.

How can I get Schroedinger equation from this expression?

Best Answer

The link you're looking for is hidden by the fact that Heisenberg kets $| \psi \rangle_H$ are not Schroedinger kets $|\psi \rangle _S$. The tranlation from the Heisenberg to the Schroedinger pictures is done via the time-evolution operator $U(t - t_0) = \exp\{-i H * (t - t_0)/\hbar\}$ (I do not explicity put hats on operators, since it should be pretty obrvious what's an operator and what's not).

In the Heisenberg picture, kets $|\psi \rangle_H \equiv |\psi (t_0) \rangle_H$ are not time-dependent. Heisenbergs idea was that the state of a system is not time-dependent, but the observables are. So an operator in the Heisenberg picture at a time $t$ is $$ F(t) = U^\dagger(t - t_0) F(t_0) U(t-t_0)$$ The Heiseberg equation follows from this: $$ \frac{d}{dt} F(t) = U^\dagger(t-t_0) (\partial_t F(t_0)) U(t-t_0) + (\partial_t U^\dagger (t-t_0)) F(t_0) U(t-t_0) + + U^\dagger(t-t_0) F(t_0) (\partial_t U(t-t_0))$$ The Leibniz terms give exacly the commutator from the Heisenberg equation.

In the Schroedinger picture, the operators are constant $F(t) \equiv F(t_0)$, but the state changes $| \psi \rangle_S = | \psi(t) \rangle_S $. As all physics needs to be equal in both pictures, we can look at expectation values: \begin{aligned}\langle \psi | F | \psi \rangle &= {}_H\langle \psi | F(t) | \psi \rangle_H &&= \langle \psi | U^\dagger(t-t_0) F(t_0) U(t-t_0) | \psi \rangle \\ & = {}_S \langle \psi(t) | F(t_0) | \psi(t) \rangle_S &&= \langle \psi(t_0) | U^\dagger(t-t_0) F(t_0) U(t-t_0) | \psi(t_0) \rangle \end{aligned} where we take the basis of our Hilbert space to conincide for both pictures at $t_0$.

Thus, assuming the Heisenberg euqation is correct, the equation for the time-evolution of a state in the Schroedinger picture is $$ | \psi(t) \rangle_S = U(t-t_0) | \psi(t_0) \rangle_S$$ The time derviative then gives you the Schroedinger equation $$ \frac{d}{dt} |\psi(t)\rangle = \Big(\frac{d}{dt} U(t-t_0)\Big) |\psi(t_0)\rangle = -\frac{i}{\hbar} H | \psi(t_0) \rangle $$

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