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Although, at first glance, it might appear that the Ehrenfest theorem is saying that the quantum mechanical expectation values obey Newton's classical equations of motion, this is not actually the case. If the pair were to satisfy Newton's second law, the right-hand side of the second equation would have to be
which is typically not the same as
If for example, the potential is cubic, (i.e. proportional to ), then is quadratic (proportional to ). This means, in the case of Newton's second law, the right side would be in the form of , while in the Ehrenfest theorem it is in the form of . The difference between these two quantities is the square of the uncertainty in and is therefore nonzero.
An exception occurs in case when the classical equations of motion are linear, that is, when is quadratic and is linear. In that special case, and do agree. Thus, for the case of a quantum harmonic oscillator, the expected position and expected momentum do exactly follow the classical trajectories.
For general systems, if the wave function is highly concentrated around a point , then and will be almost the same, since both will be approximately equal to . In that case, the expected position and expected momentum will approximately follow the classical trajectories, at least for as long as the wave function remains localized in position.
where A is some quantum mechanical operator and ?A? is its expectation value. This more general theorem was not actually derived by Ehrenfest (it is due to Werner Heisenberg).
It is most apparent in the Heisenberg picture of quantum mechanics, where it is just the expectation value of the Heisenberg equation of motion. It provides mathematical support to the correspondence principle.
The reason is that Ehrenfest's theorem is closely related to Liouville's theorem of Hamiltonian mechanics, which involves the Poisson bracket instead of a commutator. Dirac's rule of thumb suggests that statements in quantum mechanics which contain a commutator correspond to statements in classical mechanics where the commutator is supplanted by a Poisson bracket multiplied by i?. This makes the operator expectation values obey corresponding classical equations of motion, provided the Hamiltonian is at most quadratic in the coordinates and momenta. Otherwise, the evolution equations still may hold approximately, provided fluctuations are small.
Derivation in the Schrödinger picture
Suppose some system is presently in a quantum state?. If we want to know the instantaneous time derivative of the expectation value of A, that is, by definition
Often (but not always) the operator A is time-independent so that its derivative is zero and we can ignore the last term.
Derivation in the Heisenberg picture
In the Heisenberg picture, the derivation is trivial. The Heisenberg picture moves the time dependence of the system to operators instead of state vectors. Starting with the Heisenberg equation of motion
we can derive Ehrenfest's theorem simply by projecting the Heisenberg equation onto from the right and from the left, or taking the expectation value, so
We can pull the out of the first term since the state vectors are no longer time dependent in the Heisenberg Picture. Therefore,
The expectation values of the theorem, however, are the very same in the Schrödinger picture as well. For the very general example of a massive particle moving in a potential, the Hamiltonian is simply
where x is the position of the particle.
Suppose we wanted to know the instantaneous change in the expectation of the momentum p. Using Ehrenfest's theorem, we have
since the operator p commutes with itself and has no time dependence. By expanding the right-hand-side, replacing p by -i??, we get
As explained in the introduction, this result does not say that the pair satisfies Newton's second law, because the right-hand side of the formula is rather than . Nevertheless, as explained in the introduction, for states that are highly localized in space, the expected position and momentum will approximately follow classical trajectories, which may be understood as an instance of the correspondence principle.
Similarly, we can obtain the instantaneous change in the position expectation value.
This result is actually in exact accord with the classical equation.
Derivation of the Schrödinger equation from the Ehrenfest theorems
It was established above that the Ehrenfest theorems are consequences of the Schrödinger equation. However, the converse is also true: the Schrödinger equation can be inferred from the Ehrenfest theorems. We begin from
Here, apply Stone's theorem, using ? to denote the quantum generator of time translation. The next step is to show that this is the same as the Hamiltonian operator used in quantum mechanics. Stone's theorem implies
where ? was introduced as a normalization constant to the balance dimensionality. Since these identities must be valid for any initial state, the averaging can be dropped and the system of commutator equations for ? are derived:
Assuming that observables of the coordinate and momentum obey the canonical commutation relation[x?, p?] = i?. Setting , the commutator equations can be converted into the differential equations
^Smith, Henrik (1991). Introduction to Quantum Mechanics. World Scientific Pub Co Inc. pp. 108-109. ISBN978-9810204754.
^In bra-ket notation,
where is the Hamiltonian operator, and H is the Hamiltonian represented in coordinate space (as is the case in the derivation above). In other words, we applied the adjoint operation to the entire Schrödinger equation, which flipped the order of operations for H and ?.
^Although the expectation value of the momentum ?p?, which is a real-number-valued function of time, will have time dependence, the momentum operator itself, p does not, in this picture: Rather, the momentum operator is a constant linear operator on the Hilbert space of the system. The time dependence of the expectation value, in this picture, is due to the time evolution of the wavefunction for which the expectation value is calculated. An Ad hoc example of an operator which does have time dependence is ?xt2?, where x is the ordinary position operator and t is just the (non-operator) time, a parameter.