Conceptual question about wavefunctions/momentum

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Discussion Overview

The discussion revolves around the visualization of the expectation of velocity from a position-space wavefunction in quantum mechanics. Participants explore different approaches to relate the wavefunction to momentum and velocity, considering both real and k-space representations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that the expectation of position can be visualized as the centroid of the distribution and questions how to visualize the expectation of velocity from the position-space wavefunction.
  • Another participant suggests using the expectation value of momentum, indicating familiarity with the momentum operator in real-space representation.
  • A participant confirms the form of the momentum operator as p = -i (d/dx) and emphasizes its role in generating translations in space.
  • Another participant introduces the idea of performing a Fourier transform on the real-space wavefunction to obtain a k-space wavefunction, where the momentum operator has a similar form to the position operator, allowing for visualization of the mean momentum value.
  • One participant agrees with the Fourier transformation approach to obtain the wavefunction in momentum representation.

Areas of Agreement / Disagreement

Participants express differing views on how to approach the visualization of velocity from the position-space wavefunction. While some agree on the utility of the Fourier transform, others emphasize the direct use of the momentum operator. No consensus is reached on a single method.

Contextual Notes

The discussion includes assumptions about the knowledge of operators and the implications of using different representations of the wavefunction. There is an acknowledgment of contradictions in the initial question regarding the use of only the position-space wavefunction.

noospace
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Hi all,

If I have the wave function of a system, then the expectation of position is easily visualized as the centroid of the distribution.

Does anyone know how to visualize the expectation of velocity given just the postion-space wavefunction (real and imaginary parts)
 
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noospace said:
Hi all,

If I have the wave function of a system, then the expectation of position is easily visualized as the centroid of the distribution.

Does anyone know how to visualize the expectation of velocity given just the postion-space wavefunction (real and imaginary parts)

Er... couldn't you just use the expectation value of the momentum, i.e. <p>? I'm assuming that you know what p operator is in the real-space representation.

Zz.
 
ZapperZ said:
Er... couldn't you just use the expectation value of the momentum, i.e. <p>? I'm assuming that you know what p operator is in the real-space representation.

Zz.

I do know what it is!
[tex] p = -i \frac{d}{dx}[/tex]

...always. since p generates translations in space.
 
noospace said:
Hi all,

If I have the wave function of a system, then the expectation of position is easily visualized as the centroid of the distribution.

Does anyone know how to visualize the expectation of velocity given just the postion-space wavefunction (real and imaginary parts)

What I'm going to tell you is in a contradiction with your question, cause you say, you only have realspace wavefunction. However, i find it very instrumental to imagine it in this way:

Take the realspace wavefunction. Do its Fourier transform. You obtain a k-space wavefunction. In this representation, the momentum (~velocity) operator has exactly the same form as position operator in realspace representation. So the centre of this function is the mean momentum value.
 
tomasko789 said:
What I'm going to tell you is in a contradiction with your question, cause you say, you only have realspace wavefunction. However, i find it very instrumental to imagine it in this way:

Take the realspace wavefunction. Do its Fourier transform. You obtain a k-space wavefunction. In this representation, the momentum (~velocity) operator has exactly the same form as position operator in realspace representation. So the centre of this function is the mean momentum value.

yes,I agree this.
Use foiurier transformation to get the wavefunction in the momentum representation.
 

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