Finding Eigenvalues of Dirac Matrices with Constraint Conditions

[waht]

I'm stuck on a problem. Given a Hamiltonian

H_{ab} = cP_j(\alpha^{j})_{ab} + mc^{2} (\beta)_{ab} [/itex]<br /> <br /> then<br /> <br /> (H^{2})_{ab} = (\textbf{P}^{2}c^{2} + m^{2}c^{4}) \delta_{ab} [/itex]&lt;br /&gt; &lt;br /&gt; holds if&lt;br /&gt; &lt;br /&gt; \left\{\alpha^j,\alpha^k}\right\}_{ab} = 2 \delta^{jk} \delta_{ab} [/itex]&amp;lt;br /&amp;gt; &amp;lt;br /&amp;gt; \left\{\alpha^j, \beta \right\}_{ab} = 0 [/itex]&amp;amp;lt;br /&amp;amp;gt; &amp;amp;lt;br /&amp;amp;gt; \delta_{ab} = (\beta^2)_{ab} [/itex]&amp;amp;amp;lt;br /&amp;amp;amp;gt; &amp;amp;amp;lt;br /&amp;amp;amp;gt; I&amp;amp;amp;amp;#039;d like to show that Tr (\alpha) = 0 and Tr( \beta) = 0&amp;amp;amp;lt;br /&amp;amp;amp;gt; &amp;amp;amp;lt;br /&amp;amp;amp;gt; My plan is to find the eigenvalues of alpha and beta and add them up. But how could I find the eigenvalues using the constraint conditions?

 

[tim_lou]

you missed one crucial equation,
\alpha_i^2=1

a clever use of
\alpha_i^2=\beta^2=1
and the anti-commutation relation should give you the answer.

also, note that if A,B,C are matrices
Tr(ABC)=Tr(CAB)=Tr(BCA)

(edit) nevermind using the eigenvalues, i was thinking that those are 2D matrices; their dimensions are not given in this case.

 

[waht]

So would

Tr(\alpha) = Tr(\alpha\, (\alpha^j)^2 \,\beta^2) [/itex]