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Homework Statement
(a) If a particle is in the spin state ## χ = 1/5 \begin{pmatrix}
i \\
3 \\
\end{pmatrix} ## , calculate the expectation value <Sy>(b) If you measured the observable Sy on the particle in spin state given in (a), what values might you get and what is the probability of each?
Homework Equations
(a)
<Sy> = expectation value
<Sy> = < χ | Sy | χ >
## Sy = ħ/2 \begin{pmatrix}
0 & -i \\
i & 0 \\
\end{pmatrix} ##
|χ> = ##1/5 \begin{pmatrix}
i \\
3 \\
\end{pmatrix} ##
< χ | = ##1/5 \begin{pmatrix}
-i & 3\\
\end{pmatrix} ##
(b)
C = < φ | χ >
C*C (where * = complex conjugate) = probability
The Attempt at a Solution
(a) <Sy> =
## 1/5 \begin{pmatrix}
-i & 3\\
\end{pmatrix}
ħ/2 \begin{pmatrix}
0 & -i \\
i & 0 \\
\end{pmatrix} 1/5 \begin{pmatrix}
i \\
3 \\
\end{pmatrix} ##
## = ħ/50 \begin{pmatrix}
-i & 3\\
\end{pmatrix}
\begin{pmatrix}
-3i \\
-1 \\
\end{pmatrix} ##
<Sy> ## = -6ħ/50 = -3ħ/25 ##
If the particle is in the spin state χ (given above) then the expectation value is = -3ħ/25. I don't understand how the expectation value can be negative though?
(b)
The Sy spin operator has two eigenspinors, corrosponding to two eigenvalues of ±ħ/2
i) For the eigenspinor with the positive eigenvalue of ħ/2
## | φ > = 1/√2 \begin{pmatrix}
1 \\
i \\
\end{pmatrix} ##
## < φ | = 1/√2 \begin{pmatrix}
1 & -i \\
\end{pmatrix} ##
C = ## < φ | χ > = 1/√2 \begin{pmatrix}
1 & -i \\
\end{pmatrix} 1/5 \begin{pmatrix}
i\\
3\\
\end{pmatrix} ##
= ## < φ | χ > = 1/5√2 \begin{pmatrix}
1 & i \\
\end{pmatrix} \begin{pmatrix}
i\\
3\\
\end{pmatrix} ##
C = ## < φ | χ > = 1/5√2 \begin{pmatrix}
-2i \\
\end{pmatrix} ##
## C * C = 1/50 (-2i)(2i) = 4/50 = 2/25 ##
The probability of measuring χ for Sy with the eigenvalue ħ/2 = 2/25
ii) For the eigenspinor with the negative eigenvalue of -ħ/2
## | φ > = 1/√2 \begin{pmatrix}
1 \\
-i \\
\end{pmatrix} ##
## < φ | = 1/√2 \begin{pmatrix}
1 & i \\
\end{pmatrix} ##
C = ## < φ | χ > = 1/√2 \begin{pmatrix}
1 & i \\
\end{pmatrix} 1/5 \begin{pmatrix}
i\\
3\\
\end{pmatrix} ##
C = ## < φ | χ > = 1/5√2 \begin{pmatrix}
-4i \\
\end{pmatrix} ##
## C * C = 1/50 (-4i)(4i) = 16/50 = 8/25 ##
The probability of measuring χ for Sy with the eigenvalue -ħ/2 = 8/25
I think I've answered part (b) correctly, it's just the negative expectation value I'm a bit confused about.