# Homework Help: Angular momentum operator identity J²= J-J+ + J_3 + h*J_3 intermediate step

1. Aug 25, 2012

### xyver

1. The problem statement, all variables and given/known data
I do not understand equal signs 2 and 3 the following Angular momentum operator identity:

2. Relevant equations
$$\hat{J}^2 = \hat{J}_1^2+\hat{J}_2^2 +\hat{J}_3^2$$

$$= \left(\hat{J}_1 +i\hat{J}_2 \right)\left(\hat{J}_1 -i\hat{J}_2 \right) +\hat{J}_3^2 + i \left[ \hat{J}_1, \hat{J}_2 \right]$$

$$= \hat{J}_+\hat{J}_- + \hat{J}_3^2 - \hbar \cdot \hat{J}_3$$

$$= \hat{J}_-\hat{J}_++ \hat{J}_3^2 + \hbar \cdot \hat{J}_3$$

$$\hat{J}_+ = \hat{J}_1 + i\hat{J}_2$$

$$\hat{J}_- = \hat{J}_1 - i\hat{J}_2$$

$$[\hat{J}_i,\hat{J}_j] = i\hbar\epsilon_{ijk}\hat{J}_k$$

3. The attempt at a solution
$$\hat{J}^2= \hat{J}_1^2+\hat{J}_2^2 +\hat{J}_3^2$$

$$= \left(\hat{J}_+ -i \hat{J}_2 \right)^2 \left( \frac{\hat{J}_+ -i \hat{J}_1 }{i}\right)^2 +\hat{J}_3^2$$

$$= \hat{J}_+^2 -i\hat{J}_+ \hat{J}_2 -i\hat{J}_2 \hat{J}_+ +i^2 \hat{J}_2^2 +\hat{J}_3^2 + \frac{\hat{J}_+^2 -i\hat{J}_+ \hat{J}_1 -i\hat{J}_1 \hat{J}_+ + \hat{J}_1^2 } {i^2} +\hat{J}_3^2$$

$$= \hat{J}_+^2 -i\hat{J}_+ \hat{J}_2 -i\hat{J}_2 \hat{J}_+ - \hat{J}_2^2 - \hat{J}_+^2 +\hat{J}_+ \hat{J}_1 +\hat{J}_1 \hat{J}_+ - \hat{J}_1^2 +\hat{J}_3^2$$

$$= -i\hat{J}_+ \hat{J}_2 -i\hat{J}_2 \hat{J}_+ - \hat{J}_2^2 +\hat{J}_+ \hat{J}_1 +\hat{J}_1 \hat{J}_+ - \hat{J}_1^2 +\hat{J}_3^2$$

Unfortunately, this does not lead to the right way. Who can help?

2. Aug 27, 2012

### bigplanet401

Try working backwards. The 1- and 2-components of J are being factored into ladder operators and a commutator is being added on to cancel the extra terms:

\begin{align*} (J_1 + iJ_2)(J_1 - iJ_2) &= J^2_1 + J^2_2 + iJ_2 J_1 - iJ_1 J_2\\ &= J^2_1 + J^2_2 -i [J_1, J_2] \end{align*}

To get to the third expression from the second, replace the ladder operators with their synonyms (J +/-) and use a commutator identity.