# Homework Help: Squared Quantum Runge Lenz Vector

1. Nov 12, 2013

### MisterX

1. The problem statement, all variables and given/known data
$\mathbf{A} = \frac{1}{2}\left(\hbar\mathbf{L} \times \mathbf{p} \right) - \frac{1}{2}\left(\mathbf{p} \times \hbar\mathbf{L} \right) + Ze^2m \frac{\mathbf{r}}{r}$

Derive
$\mathbf{A}^2 = \left(\mathbf{p}^2 - 2m\frac{Ze^2}{r} \right)\left(\hbar^2 \mathbf{L^2} + \hbar^2 \right) + m^2Z^2e^4$

2. Relevant equations
$\left(\mathbf{X} \times \mathbf{Y} \right)_i = \epsilon_{ijk}X_jY_k$
$\left[L_i, p_j \right] = \epsilon_{ijk}p_k$
$\sum_i \epsilon_{ijk}\epsilon_{i\ell m} = \delta_{j\ell } \delta_{km} -\delta_{jm} \delta_{k\ell }$

3. The attempt at a solution
I found that
$-\left(\mathbf{p} \times \mathbf{L} \right)_i = \left(\mathbf{L} \times \mathbf{p} \right)_i^\dagger = \left(\mathbf{L} \times \mathbf{p} \right)_i - 2p_i$

So another way to write A

$\mathbf{A} =-\hbar\mathbf{p} \times \mathbf{L} +\hbar \mathbf{p} + Ze^2m \frac{\mathbf{r}}{r}$

My attempt ( $\left\{\;,\;\right\}$ indicates anti-commuator with dot product)

$\mathbf{A} ^2 = \left(-\hbar\mathbf{p} \times \mathbf{L} + \hbar\mathbf{p} + Ze^2m \frac{\mathbf{r}}{r}\right)\left(-\hbar\mathbf{p} \times \mathbf{L} + \hbar\mathbf{p} + Ze^2m \frac{\mathbf{r}}{r}\right)$
$$=\hbar^2\left(\mathbf{p} \times \mathbf{L}\right)^2 + \hbar^2\mathbf{p}^2 -\hbar^2\left\{\mathbf{p} \times \mathbf{L}, \mathbf{p}\right\} - \hbar Ze^2m\left\{\mathbf{p} \times \mathbf{L}, \frac{\mathbf{r}}{r}\right\} + \hbar Ze^2m\left\{ \mathbf{p}, \frac{\mathbf{r}}{r}\right\} + m^2Z^2e^4$$
Now I get that
\begin{align*}\left(\mathbf{p} \times \mathbf{L}\right)^2 &= \epsilon_{ijk}\epsilon_{i\ell m}p_jL_kp_\ell L_m = \left(\delta_{j\ell} \delta_{km} -\delta_{jm} \delta_{k\ell} \right)p_jL_kp_\ell L_m \\
&= p_jL_kp_j L_k - p_jL_kp_k L_j\end{align*}
using
$\left[L_k, p_j\right] = \epsilon_{kj\ell}p_\ell =\epsilon_{j\ell k}p_\ell$

\begin{align*}\left(\mathbf{p} \times \mathbf{L}\right)^2 &= p_j(p_jL_k + \epsilon_{j\ell k}p_\ell)L_k - p_jp_kL_k L_j\\ &= \mathbf{p}^2 \mathbf{L}^2 + \mathbf{p} \cdot \left(\mathbf{p} \times \mathbf{L} \right) - p_jp_kL_k L_j\end{align*}

So if I do it this way, at least I get the $\mathbf{p}^2 \mathbf{L}^2$ term without too much effort. And I see the second term is going to cancel with part of the anti-commutator $\left\{\mathbf{p} \times \mathbf{L}, \mathbf{p}\right\}$. But I am not sure what to do with the third term, or how the other half of that anti-commutator would go away. I am not sure if this is the right approach and perhaps I have made a mistake. I'd appreciate any help with this problem.

Last edited: Nov 12, 2013
2. Nov 15, 2013

### Oxvillian

Hi MisterX!

You missed an $i$ in one of your commutation relations:

$\left[L_i, p_j \right] = i\epsilon_{ijk}p_k$

Then things should work out ok. Also note that

$\mathbf{p} \cdot \left(\mathbf{p} \times \mathbf{L} \right) = 0$

and

$- p_jp_kL_k L_j = 0.$

3. Nov 16, 2013

### MisterX

Thanks, however I already found out those things (and solved this problem after a significant amount of work).

Perhaps somewhere we should have a catalog of vector calculus identities for vectors of quantum mechanical operators.

$\mathbf{p} \cdot \left(\mathbf{p} \times \mathbf{L} \right) = 0$
and
$\left(\mathbf{p} \times \mathbf{L} \right) \cdot \mathbf{p} =i2\mathbf{p}^2$