How to Calculate Momentum using QM Completeness Relation?

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Discussion Overview

The discussion revolves around the calculation of momentum using the completeness relation in quantum mechanics (QM). Participants explore the implications of the completeness relation for momentum kets and how to interpret the expression involving momentum operators and eigenvalues.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant states the completeness relation for momentum kets and asks how to calculate an integral involving momentum kets and a momentum variable.
  • Another participant suggests that the integral represents an operator that can be applied to bras and kets.
  • A participant questions the meaning of the momentum variable outside the bra/ket, proposing it could either be a momentum operator or an eigenvalue of the momentum ket.
  • Some participants argue that if the momentum variable is treated as an operator, it leads to an illegal product, while if it is an eigenvalue, the result is simply the identity operator multiplied by the scalar value.
  • There is a suggestion that the result may involve a delta function, indicating uncertainty about the calculation's validity.
  • Participants clarify their positions on whether the momentum variable is an operator or an eigenvalue, leading to further debate about the correct interpretation of the expression.

Areas of Agreement / Disagreement

Participants express differing views on whether the momentum variable should be treated as an operator or an eigenvalue, and there is no consensus on how to proceed with the calculation.

Contextual Notes

Participants highlight potential ambiguities in the expression, particularly regarding the placement and interpretation of the momentum variable, and the implications for the calculation's legality.

luisgml_2000
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Hello!

From the ordinary courses in QM it is known that the momentum kets satisfy the completeness relation

[tex]\int d^3 p \mid p \rangle \langle p \mid = 1[/tex]

Knowing this, how can you calculate

[tex]\int d^3 p \mid p \rangle p \langle p \mid[/tex]

?

Thanks a lot!
 
Last edited:
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What do you want to calculate? It's an operator you can apply to bras and kets

I prefer

[tex]\int d^3p |p\rangle\langle p| p[/tex]
 
Last edited:
What is the p outside the bra/ket exactly? Do you want it to be a momentum operator? or an eigenvalue for the p-ket? If it's the former I think you have an illegal product so there is no such thing as calculating it. If it's a eigenvalue (ie a real number) then it's just the identity operator multiplied by the scalar p.
 
That's why I prefer

[tex]\int d^3p |p\rangle\langle p| \hat{p}^\dagger = \int d^3p |p\rangle\langle p| p[/tex]
 
operator times operator
 
nnnm4 said:
What is the p outside the bra/ket exactly? Do you want it to be a momentum operator?

That p stands for the momentum operator. That's why I'm not sure about how to calculate it. I think the result has something to do with a delta function.
 
your p cannot be an operator as it is in the wrong position, whereas my p (with the hat on top) is an operator; in the second step it is replaced by the eigenvalue as it acts (to the left) on the state vector
 

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