- #1
cjellison
- 18
- 0
I had thought that the expectation value would be the same...whether you did it in momentum space or position space. Could someone explain what is going on in this particular problem?
[tex]
\psi (x) = \sqrt{b} e^{-b |x| + i p_0 x / \hbar }
[/tex]
Taking the Fourier transform, I can get this function in momentum space.
[tex]
\phi (p) = \sqrt{\frac{2b}{\pi \hbar}} \thickspace \frac{\hbar^2 b}{\hbar^2b^2+(p-p_0)^2}
[/tex]
Now, if I compute [itex] <p^2> [/itex], I should get the same value no matter which space I choose to do it in. However, this does not seem to be the case. I computed these by hand...and by Mathematica...so I am fairly confident that they are correct (just confusing).
[tex]
\begin{align*}
<p^2> &= \int \phi^* p^2 \phi \, dp = b^2 \hbar^2 + p_0^2\\
<p^2> &= \int \psi^* \left(\frac{\hbar}{i}\right)^2 \frac{\partial^2 \psi}{\partial x^2} \, dx = -b^2 \hbar^2 + p_0^2
\end{align*}
[/itex]
As you can see...and please carry out the computations if you doubt them...the expectation value seems to depend on the space in which I computed it. I thought that this was NOT supposed to happen. Any ideas?
[tex]
\psi (x) = \sqrt{b} e^{-b |x| + i p_0 x / \hbar }
[/tex]
Taking the Fourier transform, I can get this function in momentum space.
[tex]
\phi (p) = \sqrt{\frac{2b}{\pi \hbar}} \thickspace \frac{\hbar^2 b}{\hbar^2b^2+(p-p_0)^2}
[/tex]
Now, if I compute [itex] <p^2> [/itex], I should get the same value no matter which space I choose to do it in. However, this does not seem to be the case. I computed these by hand...and by Mathematica...so I am fairly confident that they are correct (just confusing).
[tex]
\begin{align*}
<p^2> &= \int \phi^* p^2 \phi \, dp = b^2 \hbar^2 + p_0^2\\
<p^2> &= \int \psi^* \left(\frac{\hbar}{i}\right)^2 \frac{\partial^2 \psi}{\partial x^2} \, dx = -b^2 \hbar^2 + p_0^2
\end{align*}
[/itex]
As you can see...and please carry out the computations if you doubt them...the expectation value seems to depend on the space in which I computed it. I thought that this was NOT supposed to happen. Any ideas?