- #1
unscientific
- 1,734
- 13
Homework Statement
Find ##[L_i,L_j]##.
Homework Equations
[tex][x_i,p_j] = \delta_{ij}i\hbar[/tex]
The Attempt at a Solution
[tex][L_i,L_j] = \epsilon_{ijk}\epsilon_{jlm} [x_jp_k,x_lp_m][/tex]
[tex] = \left( \delta_{jl}\delta_{km} - \delta_{jm}\delta_{kl}\right)[x_jp_k,x_lp_m][/tex]
[tex] = [x_jp_k,x_jp_k] - [x_jp_k,p_kx_j][/tex]
[tex] = p_kx_jx_jp_k - x_jp_kp_kx_j[/tex]
Since ##x_jp_k = p_kx_j## for j≠k, we swap the two in the second term:
[tex] = p_kx_jx_jp_k - x_jp_kx_jp_k[/tex]
[tex] = [p_k,x_j]x_jp_k[/tex]
[tex] = -i\hbar \delta_{jk}x_jp_k[/tex]
I think the answer is something like ##i\hbar L_k##.