Quantum Mechanics – Schrödinger Equation in Energy Basis

energyhilbert-spacequantum mechanicsschroedinger equation

Does there exist a Schrödinger equation for the energy space, like for momentum? I would say no, because the energy basis is countable, but are there any other reasons?

Best Answer

The basis-independent form of the Schrödinger equation is $$i\hbar\frac{\mathrm{d}}{\mathrm{d}t}\lvert\alpha\rangle = H\lvert\alpha\rangle.$$

If we wish to express this in a particular basis, we simply multiply by the corresponding basis bras. Thus, in the energy basis, we get \begin{align} \langle n\rvert i\hbar\frac{\mathrm{d}}{\mathrm{d}t}\lvert\alpha\rangle &= \langle n\rvert H\lvert\alpha\rangle\\ i\hbar\dot c_\alpha(n) &= E_n c_\alpha(n), \end{align}

where $c_\alpha(n)$ is the energy-space wavefunction for the state $\lvert\alpha\rangle$, and $\lvert n\rangle$ are the energy eigenstates with eigenvalues $E_n$. Here, we can see that the energy basis is actually quite nice to work with, due to the fact that the time derivative of $c_\alpha(n)$ only depends $c_\alpha(n)$ and not on the value of $c_\alpha$ at other "points" (energy levels).