QM: Difference between these Initial Wavefunctions

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The discussion focuses on the differences between two initial wavefunctions, specifically highlighting how the term e^(ipx) in the second wavefunction introduces a nonzero expectation value of momentum, resulting in a wavepacket that moves in a specific direction. The steps for finding the time evolution of both wavefunctions include normalization, calculating their Fourier transforms, and using the inverse Fourier transform. The initial wavefunction with the e^(ipx) term can be interpreted as a Gaussian wavepacket with an average momentum, contrasting with the first wavefunction that spreads equally in both directions. Clarification is sought regarding the relationship between the p terms in the second wavefunction and the Fourier transform, indicating a need for deeper understanding of their implications. The discussion emphasizes the importance of these terms in quantum mechanics.
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Homework Statement



I've been asked as a part of some school project to find the Fourier transform, and time evolution of the following initial wavefunctions:

1. ##\Psi(x,0) = Ae^{\frac{-x^2}{2\sigma ^2}}##
2. ##\Psi(x,0) = Be^{\frac{-x^2}{2\sigma ^2}}e^{\frac{ipx}{\hbar}}##

What physical difference does the ##e^{ipx}## term make?

To find the time evolution of 1 and 2, do I follow the following steps?
1. Normalize them
2. Find their Fourier transform
3. Plug it into their inverse Fourier transform ##\int \frac{\tilde{\psi}}{\sqrt{2\pi}} e^{i(kx - \frac{\hbar k^2}{2m} t)}##

I was told to take ##\hbar = 1## and therefore ##p = k##

Assistance is greatly appreciated

Homework Equations

The Attempt at a Solution

 
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The factor ##e^{ipx}## gives the wavepacket a nonzero expectation value of momentum. So, it's a wavepacket that is "shot" to one direction with a certain velocity, unlike the real-valued Gaussian wavepacket that spreads to both directions equally fast.
 
hilbert2 said:
The factor ##e^{ipx}## gives the wavepacket a nonzero expectation value of momentum. So, it's a wavepacket that is "shot" to one direction with a certain velocity, unlike the real-valued Gaussian wavepacket that spreads to both directions equally fast.

Thanks for the response!

So is it right to think of this as a Gaussian wavepacket with some initial "average" momentum?

Edit:

I'd also like to ask about the ##p## terms in (2) and the Fourier transform ##\frac{1}{\sqrt{2\pi}} \int \Psi(x,0) e^{-ipx}dp##. Are they the same thing? If not, what do they mean?
 
Last edited:
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

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