- #1
CAF123
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Homework Statement
Consider the Dirac Hamiltonian ##\hat H = c \alpha_i \hat p_i + \beta mc^2## . The operator ##\hat J## is defined as ##\hat J_i = \hat L_i + (\hbar/2) \Sigma_i##, where ##\hat L_i = (r \times p)_i## and ##\Sigma_i = \begin{pmatrix} \sigma_i & 0 \\0 & \sigma_i \end{pmatrix}##.
a) Show that ##[\hat H, \hat L] = -i\hbar c (\alpha \times \hat p)##
b) Evaluate ##[\hat H, \Sigma], [\hat H, J]## and thus ##[\hat H, \hat J^2]##.
c)Evalaute ##[\hat H, \Sigma \cdot \Sigma]##
Homework Equations
##[\hat x, \hat p] = i\hbar \delta_{ij}##
It is also given that ##\alpha_j = \begin{pmatrix} -\sigma_j & 0 \\ 0 & \sigma_j \end{pmatrix}## and ##\beta = \begin{pmatrix} 0 & I \\ I & 0 \end{pmatrix},## I the 2x2 identity.
The Attempt at a Solution
a) is fine, for b) I am getting zero for the first commutator there but I believe that to be wrong. Here is what I have:
$$[\hat H, \Sigma] = [c \alpha_j p_j, \Sigma_i] + mc^2[\beta, \Sigma_i] = c[\alpha_j p_j, \Sigma_i],$$ the second commutation relation there vanishes by expanding out the matrices. Then I also argued this was zero because the ##\alpha_j'##s commute with the ##p_j##'s since they are constant complex matrices. So then the whole expression is zero, again by expanding, but I am not sure what I did wrong. My notes say this shouldn't vanish. Thanks!