Bound states as a solution of free particles?

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Discussion Overview

The discussion revolves around the concept of bound states in quantum mechanics and their relationship to free-particle momentum eigenstates, particularly through the lens of Fourier transforms. Participants explore the implications of representing bound states as superpositions of free-particle states, including considerations of potential dependencies.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that bound states can be viewed as superpositions of free-particle momentum eigenstates, using the example of Hermite polynomial eigenfunctions of the harmonic oscillator.
  • Another participant confirms that this perspective leads to the momentum space wave function, although it will be time-dependent in the case of bound states.
  • A question is raised regarding the validity of this approach in the presence of velocity-dependent potentials, with a focus on whether the physical meaning of the momentum space wave function remains unchanged.
  • A later reply discusses the utility of expressing perturbations in momentum space for small perturbations to a free Hamiltonian, referencing the Interaction Picture as a method for analyzing scattering problems.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding and agreement on the mathematical representation of bound states, but there is uncertainty regarding the implications of velocity-dependent potentials and whether the physical meaning of the momentum space wave function holds in those cases.

Contextual Notes

The discussion does not resolve the implications of velocity-dependent potentials on the momentum space wave function, leaving open questions about the physical significance of such representations.

pellman
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It came to me just now that because we can always take the Fourier transform of a well-behaved function, this means we can think of any such state as a superposition of free-particle momentum eigenstates. E.g., the Hermite polynomial eigenfunctions of the harmonic oscillator. They have a Fourier transform (whatever it is) and can therefore be thought of as superpositions of functions e^(ipx) . These are free momentum eigenstates, even though individually they are not solutions to the harmonic oscillator Schrödinger equation.

From a math point of view, this is trivial. But does it have any theoretical significance?
 
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Sure, this gives you the momentum space wave function.
 
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Right. Of course. Thank you! The Fourier transform (momentum space wave function) will be time-dependent in this case. Unlike the free particle case. But it has the same physical meaning.

But does it still work if you have a velocity dependent potential? Classically, that changes the form of the canonical momentum. Mathematically you can still take the Fourier transform. I'm just wondering if the result in that case would have the same physical meaning: momentum space wave function.
 
In the case of small perturbations to a free Hamiltonian (even velocity-dependent ones), it makes sense to cast the perturbation term into this momentum basis, and compute the matrix elements of it between various free states. This is known as the Interaction Picture, and is a convenient way to set up a lot of scattering problems.
 
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