91影视

Fundamental quantum mechanical relationships that link successive operations on the wave function or state vector of two operators (L1 and L2) in opposing orders, i.e., L1 L2 and L2 L1. The operators' algebra is defined by the commutation relations.

$[L2,z]=[Lx2\text{鈥�},z]+[Ly2\text{鈥�},z]+[Lz2\text{鈥�},z]=Lx\text{鈥�}[Lx\text{鈥�},z]+[Lx\text{鈥�},z]Lx\text{鈥�}+Ly\text{鈥�}[Ly\text{鈥�},z]+[Ly\text{鈥�},z]Ly\text{鈥�}+Lz\text{鈥�}[Lz\text{鈥�},z]+[Lz\text{鈥�},z]Lz\text{鈥�}.$

$\text{But}\{\begin{array}{l}[L_x,z]=[y{p}_z鈭�z{p}_y,z]=[y{p}_z,z]鈭�[z{p}_y,z]=y[p_z,z]=鈭�i\mathrm{鈩�}y\\ [L_y,z]=[z{p}_x鈭�x{p}_z,z]=[z{p}_x,z]鈭�[x{p}_z,z]=鈭�x[p_z,z]=i\mathrm{鈩�}x\\ [L_z,z]=[x{p}_y鈭�y{p}_x,z]=[x{p}_y,z]鈭�[y{p}_x,z]=0\end{array}$

$\text{厂辞:鈥夆赌夆赌�}[L^2,z]=L_x(鈭�i\mathrm{鈩�}y)+(鈭�i\mathrm{鈩�}y)L_x+L_y\left(i\mathrm{鈩�}x\right)+\left(i\mathrm{鈩�}x\right)L_y=i\mathrm{鈩�}(鈭�L_{x}y鈭�y{L}_x+L_{y}x+x{L}_y).$

$\text{But}\{\begin{array}{l}{L}_{x}y=L_{x}y鈭�y{L}_x+y{L}_x=[L_x,y]+y{L}_x=i\mathrm{鈩�}z+y{L}_x\\ L_{y}x=L_{y}x鈭�x{L}_y+x{L}_y=[L_y,x]+x{L}_y=鈭�i\mathrm{鈩�}z+x{L}_y\end{array}$

$\begin{array}{l}\text{So:}[L^2,z]=i\mathrm{鈩�}(2x{L}_y鈭�i\mathrm{鈩�}z鈭�2y{L}_x鈭�i\mathrm{鈩�}z)鈬�[L^2,z]=2i\mathrm{鈩�}(x{L}_y鈭�y{L}_x鈭�i\mathrm{鈩�}z)\\ [L^2,[L^2,z]]=2i\mathrm{鈩�}\{[L^2,x{L}_y]鈭�[L^2,y{L}_x]鈭�i\mathrm{鈩�}[L^2,z]\}\\ =2i\mathrm{鈩�}\{[L^2,x]L_y+x[L^2,L_y]鈭�[L^2,y]L_x鈭�y[L^2,L_x]鈭�i\mathrm{鈩�}(L^{2}z鈭�z{L}^2)\}\\ [L^2,L_y]=[L^2,L_x]=0\end{array}$

so,

$[L^2,L_x]=0,\text{鈥夆赌夆赌�}[L^2,L_y]=0,\text{鈥夆赌夆赌�}[L^2,L_z]=0$(4.102).

$\begin{array}{l}[L^2,[L^2,z]]=2i\mathrm{鈩�}\{2i\mathrm{鈩�}(y{L}_z鈭�z{L}_y鈭�i\mathrm{鈩�}x)L_y鈭�2i\mathrm{鈩�}(z{L}_x鈭�x{L}_z鈭�i\mathrm{鈩�}y)L_x鈭�i\mathrm{鈩�}(L^{2}z鈭�z{L}^2)\}[L2,[L2,z\left]\right]=2i,\\ or[L^2,[L^2,z]]=鈭�2{\mathrm{鈩�}}^2(2y{L}_z{L}_y\underset{鈭�2z(L_x^2+L_y^2+L_z^2)+2z{L}_z^2}{\underset{铃�}{鈭�2z{L}_y^2鈭�2z{L}_x^2}}鈭�2i\mathrm{鈩�}x{L}_y+2x{L}_z{L}_x+2i\mathrm{鈩�}y{L}_x鈭�L^{2}z+z\end{array}$

$\begin{array}{l}=鈭�2{\mathrm{鈩�}}^2(2y{L}_z{L}_y鈭�2i\mathrm{鈩�}x{L}_y+2x{L}_z{L}_x+2i\mathrm{鈩�}y{L}_x+2z{L}_z^2鈭�2z{L}^2鈭�L^{2}z+z{L}^2)\\ =2{\mathrm{鈩�}}^2(z{L}^2+L^{2}z)鈭�4{\mathrm{鈩�}}^2[\underset{L_{z}y}{\underset{铃�}{(y{L}_z鈭�i\mathrm{鈩�}x)}}{L}_y+\underset{L_{z}x}{\underset{铃�}{(x{L}_z+i\mathrm{鈩�}y)}}{L}_x+z{L}_z{L}_z]\\ =2{\mathrm{鈩�}}^2(z{L}^2+L^{2}z)鈭�4{\mathrm{鈩�}}^2\underset{L_z(r鈰�L)=0}{\underset{铃�}{(L_{z}y{L}_y+L_{z}x{L}_x+L_{z}z{L}_z)}}=2{\mathrm{鈩�}}^2(z{L}^2+L^{2}z)\end{array}$