Derivation of momentum operator

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Discussion Overview

The discussion centers around the derivation of the momentum operator in quantum mechanics, exploring different methods such as the Fourier transform and the time derivative of the expectation value of position. Participants seek to understand the physical interpretation of the time rate of change of the expectation value of position.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant mentions two methods for deriving the momentum operator: using the Fourier transform and taking the time derivative of the expectation value of position.
  • Another participant references Stone's Theorem and suggests that understanding the momentum operator is linked to the symmetries of Galilean relativity, implying a deeper conceptual framework.
  • Recommendations are made for further reading, including Ballentine's "Quantum Mechanics - A Modern Development" and Landau's classic on Mechanics, emphasizing the importance of working through the details to grasp the underlying principles.

Areas of Agreement / Disagreement

Participants express varying approaches to understanding the momentum operator, with no consensus on a single method or interpretation. The discussion remains open-ended regarding the physical interpretation of the time rate of change of the expectation value of position.

Contextual Notes

Some assumptions about the relationship between the momentum operator and the symmetries of relativity are not fully explored, and the discussion does not resolve the interpretation of the time derivative of the expectation value of position.

pcflores
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hello,

i am trying to learn the derivation of the momentum operator and i found 2 ways of deriving it. one is using Fourier transform and the other is taking the time derivative of the expectation value of x.

i just want to know what is the physical interpretation of the time rate of change of <x>

thank you
 
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Its related to Stones Theorem and infinitesimal generators:
http://en.wikipedia.org/wiki/Stone's_theorem_on_one-parameter_unitary_groups

If you really want to understand the momentum operator get a hold of a copy of Ballentine - Quantum Mechanics - A Modern Development and have a look at chapter 3:
https://www.amazon.com/dp/9814578584/?tag=pfamazon01-20

Its true basis is the symmetries of Galilean relativity - but that revelation is something you need to discover for yourself - its very profound and deep. Revelations like that are best understood by working through the detail.

Added Later:

Then get a hold of Landaus classic on Mechanics:
https://www.amazon.com/dp/0750628960/?tag=pfamazon01-20

It shows classical mechanics has exactly the same basis. The symmetries of Galilean relativity is the key to both just as the symmetries of the Lorentz transformations is the key to relativity.

Thanks
Bill
 
Last edited by a moderator:
bhobba said:
Its related to Stones Theorem and infinitesimal generators:
http://en.wikipedia.org/wiki/Stone's_theorem_on_one-parameter_unitary_groups

If you really want to understand the momentum operator get a hold of a copy of Ballentine - Quantum Mechanics - A Modern Development and have a look at chapter 3:
https://www.amazon.com/dp/9814578584/?tag=pfamazon01-20

Its true basis is the symmetries of Galilean relativity - but that revelation is something you need to discover for yourself - its very profound and deep. Revelations like that are best understood by working through the detail.

Thanks
Bill

thank you
 
Last edited by a moderator:
pcflores said:
thank you

You are most welcome.

Also see if you can have a look at Landau as well - you may have missed that bit since I added it later.

Thanks
Bill
 

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