QM problem, operators and tensors math

  • Thread starter AHolico
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Main Question or Discussion Point

Let [tex]\mathbf{S} = [/tex][tex]\mbox{$\frac {1}{2}$}[/tex][tex](\sigma_1 + \sigma_2)[/tex] be the total spin of a system of two spin-(1/2) particles.

a) Show that [tex]\mathbf{P} \equiv (\mathbf{S} \cdot \mathbf{r})^2 / {r^2}[/tex] is a projection operator

b) Show that tensor operator [tex]S_1_2[/tex][tex] = 2(3P - \mathbf{S}^2)[/tex] satisfies [tex]S_1_2[/tex][tex] ^2 = 4\mathbf{S}^2 - 2[/tex][tex]S_1_2[/tex]

c) Show that the eigenvalues of [tex]S_1_2[/tex] are 0, 2 and -4

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I'm stuck at the first part, and have no idea on how to proceed. I know that if [tex]P[/tex] is a projection operator, then [tex]P^2 = P[/tex] and [tex]P^+ = P[/tex]. So, the first thing I do is expand P to check these properties. First thing I did is getting that [tex]r^2[/tex] inside of the dot product, leaving me with [tex]\mathbf{P} \equiv (\mathbf{S} \cdot \mathbf{n})^2[/tex], where n is an unitary vector.

[tex]\mathbf{S} = \frac{1}{2} (\sigma_1 + \sigma_2) = \frac{1}{2}\left[ \left(\begin{array}{cc}0 & 1\\1 & 0\end{array}\right) + \left(\begin{array}{cc}0 & -i\\i & 0\end{array}\right)\right] = \frac{1}{2}\left(\begin{array}{cc}0 & 1-i\\1+i & 0\end{array}\right)[/tex]
[tex]\mathbf{n} = \left(\begin{array}{c} cos \theta \\ sin \theta \end{array}\right)[/tex]
[tex]\mathbf{S} \cdot \mathbf{n} = \frac{1}{2}\left(\begin{array}{cc}0 & 1-i\\1+i & 0\end{array}\right)\left(\begin{array}{c} cos \theta \\ sin \theta \end{array}\right) = \frac{1}{2}\left(\begin{array}{c} sin \theta - i sin \theta\\ cos \theta + i cos \theta \end{array}\right)[/tex]

Now, to square it... should I (a) just multiply that vector by itself, or (b) multiply it complex conjungate and the vector???

a)[tex]\frac {1}{4}\left(sin \theta - i sin \theta , cos \theta + i cos \theta \right) \left( \begin{array}{c} sin \theta - i sin \theta\\ cos \theta + i cos \theta \end{array}\right) = \frac{i}{2}\left(\begin{array}{c} -sin \theta \\ cos \theta \end{array}\right)[/tex]

I thiunk this is not a proyection operator because [tex]P^+ = P[/tex].

b)[tex]\frac {1}{4}\left(sin \theta + i sin \theta , cos \theta - i cos \theta \right) \left( \begin{array}{c} sin \theta - i sin \theta\\ cos \theta + i cos \theta \end{array}\right) = \frac{1}{2}\left(\begin{array}{c} sin^2 \theta \\ cos^2 \theta \end{array}\right)[/tex]

But clearly this is not a proyection operator because [tex]P^2 = P[/tex]

In some book I found that [tex]\vec{\sigma} \cdot \vec{n} = \sigma_1 sin \theta cos \phi + \sigma_2 sin \theta sin \phi + \sigma_3 cos \theta[/tex]
If I follow this aproach then
[tex]\mathbf{S} \cdot \mathbf{n} = \frac {1}{2} \sigma_1 cos \theta + \frac {1}{2} \sigma_2 sin \theta[/tex]
[tex](\mathbf{S} \cdot \mathbf{n})^2 = \frac {1}{4} \sigma_1^2 cos^2 \theta + \frac {1}{4} \sigma_2^2 sin^2 \theta + \frac {1}{4} \sigma_1 \sigma_2 sin \theta cos \theta + \frac {1}{4} \sigma_2 \sigma_1 sin \theta cos \theta [/tex]
[tex](\mathbf{S} \cdot \mathbf{n})^2 = \frac {1}{4} \mathbb{I}[/tex]

The identity matrix is a projection operator, but that constant in front of it is giving me problems. What I'm doing wrong?
 
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Answers and Replies

  • #2
Tom Mattson
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Originally posted by AHolico
[tex]\mathbf{S} = \frac{1}{2} (\sigma_1 + \sigma_2) = \frac{1}{2}\left[ \left(\begin{array}{cc}0 & 1\\1 & 0\end{array}\right) + \left(\begin{array}{cc}0 & -i\\i & 0\end{array}\right)\right] = \frac{1}{2}\left(\begin{array}{cc}0 & 1-i\\1+i & 0\end{array}\right)[/tex]
[tex]\mathbf{n} = \left(\begin{array}{c} cos \theta \\ sin \theta \end{array}\right)[/tex]
[tex]\mathbf{S} \cdot \mathbf{n} = \frac{1}{2}\left(\begin{array}{cc}0 & 1-i\\1+i & 0\end{array}\right)\left(\begin{array}{c} cos \theta \\ sin \theta \end{array}\right) = \frac{1}{2}\left(\begin{array}{c} sin \theta - i sin \theta\\ cos \theta + i cos \theta \end{array}\right)[/tex]

Now, to square it... should I (a) just multiply that vector by itself, or (b) multiply it complex conjungate and the vector???
Hold on. It appears that you are not writing P as a vector. If you are working in 2D (are you allowed to assume that?), then you should write P as:

P=(1/2)(Pxi+Pyj)
P=(1/2)[(σ1x+σ2x)i+(σ1y+σ2y)j]

But what you have done here is add the σx and σy without paying attention to the fact that they are components of a vector.

Edit to add: You also seem to be ignoring the fact that you have a 2-particle system. I think you are getting the labels "1" and "2" mixed up with the components of σ, when in fact they are the labels for the spin operators of particles 1 and 2, respectively.
 
Last edited:

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