# How does a state in quantum mechanics evolve?

I have a question about the time evolution of a state in quantum mechanics. The time-dependent Schrodinger equation is given as

$$i\hbar\frac{d}{dt}|\psi(t)\rangle = H|\psi(t)\rangle$$

I am self-learning the quantum mechanics so there are few conceptual questions confused me. I am reading a book, in which the author emphasizes that any state could be expanded in terms of the eigenstates and eigenvalues of Hamiltonian. So does it mean in above equation, it is not requiring $\psi(t)$ to be an eigenstate (i.e. it could be anything)?

I am wondering it is only true when we are trying to expanding a steady state in terms of the eigenstates, that is

$$c_n(0) = \langle E_n|\psi(0)\rangle$$

But if the state is time-dependent, the expanding coefficient $c_n$ is not longer stationary but evolving in time, the book gives

$$c_n(t) = c_n(0)\exp(-iE_nt)$$ where $E_n$ is the eigenvalue. So the time evolution of the state is

$$|\psi(t)\rangle = \sum_n c_n(t)|E_n\rangle$$

is it correct? I understand the math. But physically, how do we understand the initial coefficient $c_n(0)$ evolves in the exponential form? and why the exponential has eigenvalue $E_n$? It looks to me that different eigenvalue plays different role in time evolution, why is that? If we look into the math, seems that the contribution of the eigenstate with higher 'energy' (eigenvalue) is decreasing faster in time, what's the physical reason for that?

The last question is even confusing. The text said the system could has many different states. But when you measure it, only one of the eigenstate will be shown and the measured value will be the corresponding eigenvalue. I don't understand this statement but just assume it is true. So if the state of the system is initially $|\psi(0)\rangle$ and it evolves in time as $|\psi(t)\rangle$. I measure the system at time $t=\tau$, so at this time, is it still true that the outcome must be one of the eigenvalues even the state is time-dependent?

First, you asked if wavefunctions need not be eigenfunctions of the Hamiltonian. That is correct; you can view this through a Fourier decomposition. Each eigenfunction of the Hamiltonian has a different energy, and hence a different frequency (the two are essentially equivalent through $E = \hbar \omega$), and these frequencies determine how each eigenfunction evolves in time. Still, it's perfectly legal to add up linear combinations of eigenfunctions together to construct an arbitrary wavefunction--and we often do this.

I think a good way to visualize it is to look at each eigenfunction of the Hamiltonian as precessing on the complex plane. It is the eigenvalue (the energy, or the frequency) that determines how quickly that eigenfunction precesses over time.

At one point, you mention you thought states with higher energies were decreasing faster in time. Complex exponentials don't go like that. They're like sine and cosine waves, or you can imagine them (as I have) as simply tracing out circles on the complex plane. Higher energy states simply trace out these circles faster.

Your final question gets at a matter that's more one of convention. Remember that a complex multiple of an eigenstate is still an eigenstate, and as such, even if an eigenstate has evolved in time, the evolved state is still an eigenstate of the Hamiltonian. So if an evolved wavefunction is collapsed to an evolved eigenstate, the overall idea that the result is an eigenvalue times an eigenstate still applies.

• Thanks for your clear explanation, it helps me to understand more now. I appreciate your pointing out that the complex exponential not the same as the real exponential, so I think when the eigenvalue appear in the complex exponential, physical, can I say the higher energy (eigenvalue) state is oscillating faster? – user1285419 Apr 7 '13 at 2:58
• By the way, for the last question. What confusing me is this ... let me rephrase it as a new question: if the system is time-dependent so the state evolves in time $|\psi(t)\rangle$, so what can I say about the eigenstates? Will eigenstates as well as eigenvalues always stay unchanged during time evolution? The math is a bit complicate here, when we have $|\psi(t)\rangle = \sum_n c_n(t)|E_n\rangle$, it looks like that eigenstates $|E_n\rangle$ are unchanged but the projection coefficients $c_n$ now changed in time. – user1285419 Apr 7 '13 at 3:03
• You can, at your convenience, say that either (a) the coefficients are constant in time and the eigenstates evolve or (b) the coefficients evolve in time and the eigenstates do not. The two should be mathematically equivalent. It's just a matter of taste and/or mathematical bookkeeping. – Muphrid Apr 7 '13 at 3:33
• oh, I see. Thanks a lot for your help. It really clarify my questions. – user1285419 Apr 7 '13 at 3:57
• @Muphrid regarding the first part of a question, what does it mean to have 'the time evolution of a system not in an energy eigenstate'? It is on the syllabus of my quantum mechanics course, and I can't find any material where it explains how systems evolve in time if they're not in an eigenstate. – inya Dec 25 '16 at 15:21