Agent M27
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Homework Statement
This is problem 2.22 from D.J. Griffiths Introduction to Quantum Mechanics
A free particle has the initial wave function:
\Psi(x,0)=Ae^{-ax^{2}}
Find \Psi(x,t). Hint Integrals of the form:
\int_{-\infty}^{\infty}e^{-(ax^{2}+bx)}dx
can be handled by completing the square: Let y\equiv \sqrt{a}[x+(b/2a)], and note that (ax^{2}+bx)=y^{2}-(b^{2}/4a).
Homework Equations
\Psi(x,t)=\frac{1}{\sqrt{2\pi}}\int_{-\infty}^{\infty} \phi(k)e^{i(kx-\omega t)}dk
\phi(k)=\frac{1}{\sqrt{2\pi}}\int_{-\infty}^{\infty} \Psi(x,0)e^{-ikx}dx
\omega=\frac{\hbar k^{2}}{2m}
The Attempt at a Solution
Homework Statement
So I found \phi(k)=\left(\frac{1}{2\pi a}\right)^{1/4}e^{-k^{2}/4a}.
Plugging this into my eq for \Psi(x,t) I get the following:
\Psi(x,t)=\frac{1}{\sqrt{2\pi}}\left(\frac{1}{2\pi a}\right)^{1/4}\int_{-\infty}^{\infty} e^{-k^{2}/4a}e^{i(kx-(\hbar k^{2}/2m)t)}dk
=\frac{1}{\sqrt{2\pi}}\left(\frac{1}{2\pi a}\right)^{1/4}\int_{-\infty}^{\infty}exp[-\left(\left(\frac{i\hbar t}{2m}+\frac{1}{4a}\right)k^{2}-ikx\right)]dk
Now here is where I get stuck. I feel like I need to do another completing the square manipulation to argument of the exponential,but I am having trouble seeing how the obtained the following solution:
\Psi(x,t)=\left(\frac{2a}{\pi}\right)^{(1/4)}\frac{e^{-ax^{2}}/[1+(i2\hbar at/m}{\sqrt{1+(i2\hbar at/m)}}
Any help would be greatly appreciated. Seems as though Professor Griffiths has some real cute tricks up his sleeve. Thanks in advance.
Joe
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